ECMA-262
5th Edition / December 2009

ECMAScript Language
Specification

Reference number
ECMA-123:2009© Ecma International 2009

COPYRIGHT PROTECTED DOCUMENT© Ecma International 2009

Contents

Page

Introduction ....................................................................................................................................................... vii
1

Scope ...................................................................................................................................................... 1

2

Conformance ......................................................................................................................................... 1

3

Normative references ............................................................................................................................ 1

4
4.1
4.2
4.2.1
4.2.2
4.3

Overview ................................................................................................................................................. 1
Web Scripting ........................................................................................................................................ 2
Language Overview ............................................................................................................................... 2
Objects.................................................................................................................................................... 3
The Strict Variant of ECMAScript ........................................................................................................ 4
Definitions .............................................................................................................................................. 4

5
5.1
5.1.1
5.1.2
5.1.3
5.1.4
5.1.5
5.1.6
5.2

Notational Conventions ........................................................................................................................ 7
Syntactic and Lexical Grammars ......................................................................................................... 7Context-Free Grammars ....................................................................................................................... 7
The Lexical and RegExp Grammars .................................................................................................... 8
The Numeric String Grammar .............................................................................................................. 8
The Syntactic Grammar ........................................................................................................................ 8
The JSON Grammar .............................................................................................................................. 9
Grammar Notation ................................................................................................................................. 9
Algorithm Conventions ....................................................................................................................... 11

6

Source Text .......................................................................................................................................... 12

7
7.1
7.2
7.3
7.4
7.5
7.6
7.6.1
7.7
7.8
7.8.1
7.8.2
7.8.3
7.8.4
7.8.5
7.9
7.9.1
7.9.2

Lexical Conventions............................................................................................................................ 13
Unicode Format-Control Characters ................................................................................................. 14
White Space ......................................................................................................................................... 14Line Terminators ................................................................................................................................. 15
Comments ............................................................................................................................................ 16
Tokens .................................................................................................................................................. 17
Identifier Names and Identifiers ......................................................................................................... 17
Reserved Words .................................................................................................................................. 18
Punctuators .......................................................................................................................................... 19
Literals .................................................................................................................................................. 19
Null Literals .......................................................................................................................................... 19
Boolean Literals................................................................................................................................... 20
Numeric Literals .................................................................................................................................. 20
String Literals ...................................................................................................................................... 22
Regular Expression Literals ............................................................................................................... 24
Automatic Semicolon Insertion ......................................................................................................... 25
Rules of Automatic Semicolon Insertion .......................................................................................... 26
Examples of Automatic Semicolon Insertion ................................................................................... 27

8
8.1
8.2
8.3
8.4
8.5
8.6
8.6.1
8.6.2
8.7

Types .................................................................................................................................................... 28
The Undefined Type ............................................................................................................................ 28
The Null Type ....................................................................................................................................... 28
The Boolean Type................................................................................................................................ 28
The String Type ................................................................................................................................... 28
The Number Type ................................................................................................................................ 29
The Object Type ................................................................................................................................... 30
Property Attributes .............................................................................................................................. 30
Object Internal Properties and Methods ........................................................................................... 31
The Reference Specification Type ..................................................................................................... 34© Ecma International 2009i

8.7.1
8.7.2
8.8
8.9
8.10
8.10.18.10.28.10.38.10.48.10.5
8.11
8.12
8.12.18.12.28.12.38.12.48.12.58.12.68.12.78.12.88.12.9

GetValue (V)..........................................................................................................................................35
PutValue (V, W) ....................................................................................................................................35
The List Specification Type ................................................................................................................36
The Completion Specification Type ...................................................................................................36
The Property Descriptor and Property Identifier Specification Types ...........................................37
IsAccessorDescriptor ( Desc )............................................................................................................37
IsDataDescriptor ( Desc ) ....................................................................................................................37
IsGenericDescriptor ( Desc ) ..............................................................................................................37
FromPropertyDescriptor ( Desc ) .......................................................................................................38
ToPropertyDescriptor ( Obj ) ..............................................................................................................38
The Lexical Environment and Environment Record Specification Types .....................................39
Algorithms for Object Internal Methods ............................................................................................39[[GetOwnProperty]] (P) ........................................................................................................................39[[GetProperty]] (P)................................................................................................................................39[[Get]] (P) ..............................................................................................................................................39[[CanPut]] (P) ........................................................................................................................................39[[Put]] ( P, V, Throw ) ...........................................................................................................................40[[HasProperty]] (P) ...............................................................................................................................40[[Delete]] (P, Throw) .............................................................................................................................40[[DefaultValue]] (hint) ..........................................................................................................................41[[DefineOwnProperty]] (P, Desc, Throw) ...........................................................................................41

9
9.1
9.2
9.3
9.3.1
9.4
9.5
9.6
9.7
9.8
9.8.1
9.9
9.10
9.11
9.12

Type Conversion and Testing ............................................................................................................42
ToPrimitive ...........................................................................................................................................43
ToBoolean ............................................................................................................................................43
ToNumber .............................................................................................................................................43
ToNumber Applied to the String Type ...............................................................................................44
ToInteger...............................................................................................................................................46
ToInt32: (Signed 32 Bit Integer)..........................................................................................................46
ToUint32: (Unsigned 32 Bit Integer)...................................................................................................47
ToUint16: (Unsigned 16 Bit Integer)...................................................................................................47
ToString ................................................................................................................................................47
ToString Applied to the Number Type ...............................................................................................48
ToObject ...............................................................................................................................................49
CheckObjectCoercible ........................................................................................................................49
IsCallable ..............................................................................................................................................49
The SameValue Algorithm ..................................................................................................................50

10
10.1
10.1.1
10.2
10.2.110.2.210.2.310.3
10.3.1
10.4
10.4.1
10.4.210.4.3
10.5
10.6

Executable Code and Execution Contexts ........................................................................................50
Types of Executable Code ..................................................................................................................50
Strict Mode Code .................................................................................................................................51
Lexical Environments ..........................................................................................................................51
Environment Records .........................................................................................................................51
Lexical Environment Operations........................................................................................................56
The Global Environment .....................................................................................................................56
Execution Contexts .............................................................................................................................56
Identifier Resolution ............................................................................................................................57
Establishing an Execution Context....................................................................................................57
Entering Global Code ..........................................................................................................................58
Entering Eval Code ..............................................................................................................................58
Entering Function Code ......................................................................................................................58
Declaration Binding Instantiation ......................................................................................................59
Arguments Object ................................................................................................................................60

11
11.1
11.1.111.1.2
11.1.311.1.411.1.5

Expressions..........................................................................................................................................62
Primary Expressions ...........................................................................................................................62
The this Keyword ...............................................................................................................................62
Identifier Reference .............................................................................................................................63
Literal Reference ..................................................................................................................................63
Array Initialiser .....................................................................................................................................63
Object Initialiser ...................................................................................................................................64ii© Ecma International 2009

11.1.6 The Grouping Operator ....................................................................................................................... 66
11.2
Left-Hand-Side Expressions .............................................................................................................. 66
11.2.1 Property Accessors............................................................................................................................. 67
11.2.2 The new Operator................................................................................................................................. 68
11.2.3 Function Calls ...................................................................................................................................... 68
11.2.4 Argument Lists .................................................................................................................................... 68
11.2.5 Function Expressions ......................................................................................................................... 69
11.3
Postfix Expressions ............................................................................................................................ 69
11.3.1 Postfix Increment Operator ................................................................................................................ 69
11.3.2 Postfix Decrement Operator ............................................................................................................... 69
11.4
Unary Operators .................................................................................................................................. 70
11.4.1 The delete Operator .......................................................................................................................... 70
11.4.2 The void Operator .............................................................................................................................. 70
11.4.3 The typeof Operator .......................................................................................................................... 71
11.4.4 Prefix Increment Operator .................................................................................................................. 71
11.4.5 Prefix Decrement Operator ................................................................................................................. 71
11.4.6 Unary + Operator ................................................................................................................................. 72
11.4.7 Unary - Operator ................................................................................................................................. 72
11.4.8 Bitwise NOT Operator ( ~ ).................................................................................................................. 72
11.4.9 Logical NOT Operator ( ! ).................................................................................................................. 72
11.5
Multiplicative Operators...................................................................................................................... 72
11.5.1 Applying the * Operator ..................................................................................................................... 73
11.5.2 Applying the / Operator ..................................................................................................................... 73
11.5.3 Applying the % Operator ..................................................................................................................... 74
11.6
Additive Operators .............................................................................................................................. 74
11.6.1 The Addition operator ( + ) ................................................................................................................. 74
11.6.2 The Subtraction Operator ( - ) ........................................................................................................... 75
11.6.3 Applying the Additive Operators to Numbers .................................................................................. 75
11.7
Bitwise Shift Operators ....................................................................................................................... 75
11.7.1 The Left Shift Operator ( << ).............................................................................................................. 75
11.7.2 The Signed Right Shift Operator ( >> ) .............................................................................................. 76
11.7.3 The Unsigned Right Shift Operator ( >>> ) ....................................................................................... 76
11.8
Relational Operators ........................................................................................................................... 77
11.8.1 The Less-than Operator ( < ) .............................................................................................................. 77
11.8.2 The Greater-than Operator ( > ) .......................................................................................................... 77
11.8.3 The Less-than-or-equal Operator ( <= ) ............................................................................................. 78
11.8.4 The Greater-than-or-equal Operator ( >= ) ........................................................................................ 78
11.8.5 The Abstract Relational Comparison Algorithm .............................................................................. 78
11.8.6 The instanceof operator...................................................................................................................... 79
11.8.7 The in operator .................................................................................................................................... 79
11.9
Equality Operators .............................................................................................................................. 79
11.9.1 The Equals Operator ( == ) ................................................................................................................. 80
11.9.2 The Does-not-equals Operator ( != ) ................................................................................................. 80
11.9.3 The Abstract Equality Comparison Algorithm ................................................................................. 80
11.9.4 The Strict Equals Operator ( === ) ..................................................................................................... 81
11.9.5 The Strict Does-not-equal Operator ( !== ) ...................................................................................... 81
11.9.6 The Strict Equality Comparison Algorithm ....................................................................................... 81
11.10 Binary Bitwise Operators.................................................................................................................... 82
11.11 Binary Logical Operators.................................................................................................................... 82
11.12 Conditional Operator ( ? : )............................................................................................................. 83
11.13 Assignment Operators ........................................................................................................................ 84
11.13.1 Simple Assignment ( = )...................................................................................................................... 84
11.13.2 Compound Assignment ( op= ) .......................................................................................................... 84
11.14 Comma Operator ( , ) ....................................................................................................................... 85
12
12.1
12.2

Statements ........................................................................................................................................... 86
Block ..................................................................................................................................................... 86
Variable Statement .............................................................................................................................. 87© Ecma International 2009iii

12.2.1 Strict Mode Restrictions .....................................................................................................................88
12.3
Empty Statement..................................................................................................................................88
12.4
Expression Statement .........................................................................................................................88
12.5
The if Statement .................................................................................................................................89
12.6
Iteration Statements ............................................................................................................................89
12.6.1 The do-while Statement .....................................................................................................................89
12.6.2 The while Statement ..........................................................................................................................90
12.6.3 The for Statement...............................................................................................................................90
12.6.4 The for-in Statement .........................................................................................................................91
12.7
The continue Statement ....................................................................................................................92
12.8
The break Statement ..........................................................................................................................92
12.9
The return Statement ........................................................................................................................92
12.10 The with Statement ............................................................................................................................93
12.10.1 Strict Mode Restrictions .....................................................................................................................93
12.11 The switch Statement ........................................................................................................................93
12.12 Labelled Statements ............................................................................................................................95
12.13 The throw Statement ..........................................................................................................................95
12.14 The try Statement...............................................................................................................................96
12.14.1 Strict Mode Restrictions .....................................................................................................................97
12.15 The debugger statement ....................................................................................................................97
13
13.1
13.2
13.2.113.2.213.2.3Function Definition ..............................................................................................................................97
Strict Mode Restrictions .....................................................................................................................98
Creating Function Objects ..................................................................................................................99[[Call]] ....................................................................................................................................................99[[Construct]] ...................................................................................................................................... 100
The [[ThrowTypeError]] Function Object ....................................................................................... 100

14
14.1

Program ............................................................................................................................................. 100
Directive Prologues and the Use Strict Directive .......................................................................... 101

15
15.1
15.1.115.1.215.1.315.1.415.1.515.2
15.2.115.2.215.2.315.2.415.2.515.3
15.3.1
15.3.215.3.315.3.415.3.515.4
15.4.1
15.4.215.4.315.4.415.4.515.5
15.5.1
15.5.215.5.3Standard Built-in ECMAScript Objects ........................................................................................... 101
The Global Object ............................................................................................................................. 102
Value Properties of the Global Object ............................................................................................ 103
Function Properties of the Global Object ...................................................................................... 103
URI Handling Function Properties .................................................................................................. 105
Constructor Properties of the Global Object ................................................................................. 109
Other Properties of the Global Object ............................................................................................ 110
Object Objects................................................................................................................................... 111
The Object Constructor Called as a Function ............................................................................... 111
The Object Constructor .................................................................................................................... 111
Properties of the Object Constructor ............................................................................................. 111
Properties of the Object Prototype Object ..................................................................................... 114
Properties of Object Instances ........................................................................................................ 116
Function Objects............................................................................................................................... 116
The Function Constructor Called as a Function ........................................................................... 116
The Function Constructor ................................................................................................................ 116
Properties of the Function Constructor ......................................................................................... 117
Properties of the Function Prototype Object ................................................................................. 117
Properties of Function Instances .................................................................................................... 120
Array Objects .................................................................................................................................... 121
The Array Constructor Called as a Function ................................................................................. 121
The Array Constructor ..................................................................................................................... 122
Properties of the Array Constructor ............................................................................................... 122
Properties of the Array Prototype Object ....................................................................................... 123
Properties of Array Instances.......................................................................................................... 139
String Objects ................................................................................................................................... 140
The String Constructor Called as a Function ................................................................................ 140
The String Constructor .................................................................................................................... 141
Properties of the String Constructor .............................................................................................. 141

iv© Ecma International 2009

15.5.4 Properties of the String Prototype Object ...................................................................................... 141
15.5.5 Properties of String Instances ......................................................................................................... 150
15.6
Boolean Objects ................................................................................................................................ 151
15.6.1 The Boolean Constructor Called as a Function ............................................................................. 151
15.6.2 The Boolean Constructor ................................................................................................................. 151
15.6.3 Properties of the Boolean Constructor ........................................................................................... 152
15.6.4 Properties of the Boolean Prototype Object................................................................................... 152
15.6.5 Properties of Boolean Instances ..................................................................................................... 152
15.7
Number Objects ................................................................................................................................. 153
15.7.1 The Number Constructor Called as a Function .............................................................................. 153
15.7.2 The Number Constructor .................................................................................................................. 153
15.7.3 Properties of the Number Constructor............................................................................................ 153
15.7.4 Properties of the Number Prototype Object ................................................................................... 154
15.7.5 Properties of Number Instances ...................................................................................................... 158
15.8
The Math Object................................................................................................................................. 158
15.8.1 Value Properties of the Math Object................................................................................................ 158
15.8.2 Function Properties of the Math Object .......................................................................................... 159
15.9
Date Objects ....................................................................................................................................... 164
15.9.1 Overview of Date Objects and Definitions of Abstract Operators ............................................... 164
15.9.2 The Date Constructor Called as a Function.................................................................................... 169
15.9.3 The Date Constructor ........................................................................................................................ 169
15.9.4 Properties of the Date Constructor ................................................................................................. 170
15.9.5 Properties of the Date Prototype Object ......................................................................................... 171
15.9.6 Properties of Date Instances ............................................................................................................ 178
15.10 RegExp (Regular Expression) Objects............................................................................................ 179
15.10.1 Patterns .............................................................................................................................................. 179
15.10.2 Pattern Semantics ............................................................................................................................. 181
15.10.3 The RegExp Constructor Called as a Function .............................................................................. 193
15.10.4 The RegExp Constructor .................................................................................................................. 193
15.10.5 Properties of the RegExp Constructor ............................................................................................ 194
15.10.6 Properties of the RegExp Prototype Object ................................................................................... 194
15.10.7 Properties of RegExp Instances ...................................................................................................... 195
15.11 Error Objects ...................................................................................................................................... 196
15.11.1 The Error Constructor Called as a Function .................................................................................. 196
15.11.2 The Error Constructor ....................................................................................................................... 197
15.11.3 Properties of the Error Constructor ................................................................................................ 197
15.11.4 Properties of the Error Prototype Object ........................................................................................ 197
15.11.5 Properties of Error Instances ........................................................................................................... 198
15.11.6 Native Error Types Used in This Standard ..................................................................................... 198
15.11.7 NativeError Object Structure ............................................................................................................ 199
15.12 The JSON Object ............................................................................................................................... 200
15.12.1 The JSON Grammar .......................................................................................................................... 201
15.12.2 parse ( text [ , reviver ] ) .................................................................................................................... 202
15.12.3 stringify ( value [ , replacer [ , space ] ] )......................................................................................... 203
16

Errors .................................................................................................................................................. 207

Annex A (informative) Grammar Summary................................................................................................. 209
Annex B (informative) Compatibility ........................................................................................................... 229
Annex C (informative) The Strict Mode of ECMAScript ............................................................................. 233
Annex D (informative) Corrections and Clarifications in the 5th Edition with Possible 3rd Edition
Compatibility Impact ......................................................................................................................... 235
Annex E (informative) Additions and Changes in the 5th Edition that Introduce Incompatibilities
with the 3rd Edition ............................................................................................................................ 237© Ecma International 2009v

vi

© Ecma International 2009

Introduction
This Ecma Standard is based on several originating technologies, the most well known being JavaScript(Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first
appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscapeand in all browsers from Microsoft starting with Internet Explorer 3.0.
The development of this Standard started in November 1996. The first edition of this Ecma Standard was
adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, andapproved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998
approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between thefirst and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new control
statements, try/catch exception handling, tighter definition of errors, formatting for numeric output and minorchanges in anticipation of forthcoming internationalisation facilities and future language growth. The third
edition of the ECMAScript standard was adopted by the Ecma General Assembly of December 1999 andpublished as ISO/IEC 16262:2002 in June 2002.
Since publication of the third edition, ECMAScript has achieved massive adoption in conjunction with theWorld Wide Web where it has become the programming language that is supported by essentially all web
browsers. Significant work was done to develop a fourth edition of ECMAScript. Although that work was notcompleted and not published 1 as the fourth edition of ECMAScript, it informs continuing evolution of thelanguage. The present fifth edition of ECMAScript (published as ECMA-262 5th edition) codifies de facto
interpretations of the language specification that have become common among browser implementations andadds support for new features that have emerged since the publication of the third edition. Such features
include accessor properties, reflective creation and inspection of objects, program control of property
attributes, additional array manipulation functions, support for the JSON object encoding format, and a strictmode that provides enhanced error checking and program security.
ECMAScript is a vibrant language and the evolution of the language is not complete. Significant technicalenhancement will continue with future editions of this specification.

This Ecma Standard has been adopted by the General Assembly of December 2009.

1 Note: Please note that for ECMAScript Edition 4 the Ecma standard number “ECMA-262 Edition 4” was reserved but not

used in the Ecma publication process. Therefore “ECMA-262 Edition 4” as an Ecma International publication does notexist.

© Ecma International 2009vii

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LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGEANY OWNERSHIP RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR
A PARTICULAR PURPOSE."

viii© Ecma International 2009

ECMAScript Language Specification

1

Scope

This Standard defines the ECMAScript scripting language.

2

Conformance

A conforming implementation of ECMAScript must provide and support all the types, values, objects,
properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of this International standard shall interpret characters in conformance with theUnicode Standard, Version 3.0 or later and ISO/IEC 10646-1 with either UCS-2 or UTF-16 as the adopted
encoding form, implementation level 3. If the adopted ISO/IEC 10646-1 subset is not otherwise specified, it ispresumed to be the BMP subset, collection 300. If the adopted encoding form is not otherwise specified, it
presumed to be the UTF-16 encoding form.
A conforming implementation of ECMAScript is permitted to provide additional types, values, objects,
properties, and functions beyond those described in this specification. In particular, a conforming
implementation of ECMAScript is permitted to provide properties not described in this specification, andvalues for those properties, for objects that are described in this specification.
A conforming implementation of ECMAScript is permitted to support program and regular expression syntax
not described in this specification. In particular, a conforming implementation of ECMAScript is permitted tosupport program syntax that makes use of the “future reserved words” listed in 7.6.1.2 of this specification.

3

Normative references

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO/IEC 9899:1996, Programming Languages – C, including amendment 1 and technical corrigenda 1 and 2
ISO/IEC 10646-1:1993, Information Technology – Universal Multiple-Octet Coded Character Set (UCS) plus
its amendments and corrigenda

4

Overview

This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating
computational objects within a host environment. ECMAScript as defined here is not intended to becomputationally self-sufficient; indeed, there are no provisions in this specification for input of external data or
output of computed results. Instead, it is expected that the computational environment of an ECMAScript
program will provide not only the objects and other facilities described in this specification but also certain
environment-specific host objects, whose description and behaviour are beyond the scope of this specification
except to indicate that they may provide certain properties that can be accessed and certain functions that canbe called from an ECMAScript program.

© Ecma International 20091

A scripting language is a programming language that is used to manipulate, customise, and automate thefacilities of an existing system. In such systems, useful functionality is already available through a userinterface, and the scripting language is a mechanism for exposing that functionality to program control. In thisway, the existing system is said to provide a host environment of objects and facilities, which completes thecapabilities of the scripting language. A scripting language is intended for use by both professional and nonprofessional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven Web
pages in browsers and to perform server computation as part of a Web-based client-server architecture.
ECMAScript can provide core scripting capabilities for a variety of host environments, and therefore the core
scripting language is specified in this document apart from any particular host environment.
Some of the facilities of ECMAScript are similar to those used in other programming languages; in particular
Java™, Self, and Scheme as described in:
Gosling, James, Bill Joy and Guy Steele. The Java™ Language Specification. Addison Wesley Publishing Co.,
1996.
Ungar, David, and Smith, Randall B. Self: The Power of Simplicity. OOPSLA '87 Conference Proceedings, pp.
227–241, Orlando, FL, October 1987.
IEEE Standard for the Scheme Programming Language. IEEE Std 1178-1990.

4.1

Web Scripting

A web browser provides an ECMAScript host environment for client-side computation including, for instance,
objects that represent windows, menus, pop-ups, dialog boxes, text areas, anchors, frames, history, cookies,
and input/output. Further, the host environment provides a means to attach scripting code to events such aschange of focus, page and image loading, unloading, error and abort, selection, form submission, and mouseactions. Scripting code appears within the HTML and the displayed page is a combination of user interface
elements and fixed and computed text and images. The scripting code is reactive to user interaction and there
is no need for a main program.
A web server provides a different host environment for server-side computation including objects representing
requests, clients, and files; and mechanisms to lock and share data. By using browser-side and server-side
scripting together, it is possible to distribute computation between the client and server while providing acustomised user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host environment, completing the
ECMAScript execution environment.

4.2

Language Overview

The following is an informal overview of ECMAScript—not all parts of the language are described. This
overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript
program is a cluster of communicating objects. An ECMAScript object is a collection of properties each withzero or more attributes that determine how each property can be used—for example, when the Writable
attribute for a property is set to false, any attempt by executed ECMAScript code to change the value of theproperty fails. Properties are containers that hold other objects, primitive values, or functions. A primitive
value is a member of one of the following built-in types: Undefined, Null, Boolean, Number, and String; anobject is a member of the remaining built-in type Object; and a function is a callable object. A function that is
associated with an object via a property is a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript entities. Thesebuilt-in objects include the global object, the Object object, the Function object, the Array object, the String
object, the Boolean object, the Number object, the Math object, the Date object, the RegExp object, the2© Ecma International 2009

JSON object, and the Error objects Error, EvalError, RangeError, ReferenceError, SyntaxError,
TypeError and URIError.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary operations,multiplicative operators, additive operators, bitwise shift operators, relational operators, equality operators,
binary bitwise operators, binary logical operators, assignment operators, and the comma operator.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as
an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are
types associated with properties, and defined functions are not required to have their declarations appear
textually before calls to them.
4.2.1

Objects

ECMAScript does not use classes such as those in C++, Smalltalk, or Java. Instead objects may be created invarious ways including via a literal notation or via constructors which create objects and then execute codethat initialises all or part of them by assigning initial values to their properties. Each constructor is a functionthat has a property named “prototype” that is used to implement prototype-based inheritance and sharedproperties. Objects are created by using constructors in new expressions; for example, newDate(2009,11) creates a new Date object. Invoking a constructor without using new has consequences that
depend on the constructor. For example, Date() produces a string representation of the current date andtime rather than an object.
Every object created by a constructor has an implicit reference (called the object’s prototype) to the value of
its constructor’s “prototype” property. Furthermore, a prototype may have a non-null implicit reference to its
prototype, and so on; this is called the prototype chain. When a reference is made to a property in an object,that reference is to the property of that name in the first object in the prototype chain that contains a propertyof that name. In other words, first the object mentioned directly is examined for such a property; if that object
contains the named property, that is the property to which the reference refers; if that object does not containthe named property, the prototype for that object is examined next; and so on.

implicit prototype linkCFprototype
P1

CFp

P2

CFP1

explicit prototype property

cf1

cf2

cf3

cf4

cf5

q1

q1

q1

q1

q1

q2

q2

q2

q2

q2

Figure 1 — Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried byclasses, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are carriedby objects, and structure, behaviour, and state are all inherited.

© Ecma International 20093

All objects that do not directly contain a particular property that their prototype contains share that property
and its value. Figure 1 illustrates this:CF is a constructor (and also an object). Five objects have been created by using new expressions: cf1, cf2,
cf3, cf4, and cf5. Each of these objects contains properties named q1 and q2. The dashed lines represent the
implicit prototype relationship; so, for example, cf3’s prototype is CFp. The constructor, CF, has two properties
itself, named P1 and P2, which are not visible to CFp, cf1, cf2, cf3, cf4, or cf5. The property named CFP1 in CFp
is shared by cf1, cf2, cf3, cf4, and cf5 (but not by CF), as are any properties found in CFp’s implicit prototype
chain that are not named q1, q2, or CFP1. Notice that there is no implicit prototype link between CF and CFp.
Unlike class-based object languages, properties can be added to objects dynamically by assigning values tothem. That is, constructors are not required to name or assign values to all or any of the constructed object’s
properties. In the above diagram, one could add a new shared property for cf1, cf2, cf3, cf4, and cf5 byassigning a new value to the property in CFp.
4.2.2

The Strict Variant of ECMAScript

The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict
their usage of some features available in the language. They might do so in the interests of security, to avoid
what they consider to be error-prone features, to get enhanced error checking, or for other reasons of theirchoosing. In support of this possibility, ECMAScript defines a strict variant of the language. The strict variantof the language excludes some specific syntactic and semantic features of the regular ECMAScript language
and modifies the detailed semantics of some features. The strict variant also specifies additional error
conditions that must be reported by throwing error exceptions in situations that are not specified as errors by
the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language. Strict mode
selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at the level ofindividual ECMAScript code units. Because strict mode is selected at the level of a syntactic code unit, strictmode only imposes restrictions that have local effect within such a code unit. Strict mode does not restrict ormodify any aspect of the ECMAScript semantics that must operate consistently across multiple code units. A
complete ECMAScript program may be composed for both strict mode and non-strict mode ECMAScript code
units. In this case, strict mode only applies when actually executing code that is defined within a strict mode
code unit.
In order to conform to this specification, an ECMAScript implementation must implement both the fullunrestricted ECMAScript language and the strict mode variant of the ECMAScript language as defined by thisspecification. In addition, an implementation must support the combination of unrestricted and strict mode
code units into a single composite program.

4.3

Definitions

For the purposes of this document, the following terms and definitions apply.
4.3.1
type
set of data values as defined in Clause 8 of this specification.
4.3.2
primitive value
member of one of the types Undefined, Null, Boolean, Number, or String as defined in Clause 8.
NOTE

A primitive value is a datum that is represented directly at the lowest level of the language implementation.

4.3.3
objectmember of the type Object.
NOTE

4

An object is a collection of properties and has a single prototype object. The prototype may be the null value.

© Ecma International 2009

4.3.4
constructor
Function object that creates and initialises objects.
NOTE
The value of a constructor’s “prototype” property is a prototype object that is used to implement inheritanceand shared properties.

4.3.5
prototypeobject that provides shared properties for other objects.
NOTE
When a constructor creates an object, that object implicitly references the constructor’s “prototype” property
for the purpose of resolving property references. The constructor’s “prototype” property can be referenced by the
program expression constructor.prototype, and properties added to an object’s prototype are shared, throughinheritance, by all objects sharing the prototype. Alternatively, a new object may be created with an explicitly specified
prototype by using the Object.create built-in function.

4.3.6
native object
object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by
the host environment.
NOTE
Standard native objects are defined in this specification. Some native objects are built-in; others may beconstructed during the course of execution of an ECMAScript program.

4.3.7
built-in object
object supplied by an ECMAScript implementation, independent of the host environment, that is present at thestart of the execution of an ECMAScript program.
NOTE
Standard built-in objects are defined in this specification, and an ECMAScript implementation may specify anddefine others. Every built-in object is a native object. A built-in constructor is a built-in object that is also a constructor.

4.3.8
host object
object supplied by the host environment to complete the execution environment of ECMAScript.
NOTE

Any object that is not native is a host object.

4.3.9
undefined value
primitive value used when a variable has not been assigned a value.
4.3.10Undefined type
type whose sole value is the undefined value.
4.3.11null value
primitive value that represents the intentional absence of any object value.
4.3.12
Null type
type whose sole value is the null value.
4.3.13
Boolean value
member of the Boolean type.
NOTE

There are only two Boolean values, true and false.

© Ecma International 20095

4.3.14
Boolean type
type consisting of the primitive values true and false.
4.3.15
Boolean object
member of the Object type that is an instance of the standard built-in Boolean constructor.
NOTE
A Boolean object is created by using the Boolean constructor in a new expression, supplying a Boolean
value as an argument. The resulting object has an internal property whose value is the Boolean value. A Boolean object
can be coerced to a Boolean value.

4.3.16
String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer.
NOTE
A String value is a member of the String type. Each integer value in the sequence usually represents a single
16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or requirements on the values except thatthey must be 16-bit unsigned integers.

4.3.17
String typeset of all possible String values.
4.3.18String object
member of the Object type that is an instance of the standard built-in String constructor.
NOTE
A String object is created by using the String constructor in a new expression, supplying a String value as
an argument. The resulting object has an internal property whose value is the String value. A String object can be coerced
to a String value by calling the String constructor as a function (15.5.1).

4.3.19Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754 value.
NOTE

A Number value is a member of the Number type and is a direct representation of a number.

4.3.20
Number typeset of all possible Number values including the special “Not-a-Number” (NaN) values, positive infinity, andnegative infinity.
4.3.21
Number object
member of the Object type that is an instance of the standard built-in Number constructor.
NOTE
A Number object is created by using the Number constructor in a new expression, supplying a Number value
as an argument. The resulting object has an internal property whose value is the Number value. A Number object can becoerced to a Number value by calling the Number constructor as a function (15.7.1).

4.3.22Infinity
Number value that is the positive infinite Number value.
4.3.23
NaN
Number value that is a IEEE 754 “Not-a-Number” value.

6© Ecma International 2009

4.3.24
function
member of the Object type that is an instance of the standard built-in Function constructor and that may be
invoked as a subroutine.
NOTE
In addition to its named properties, a function contains executable code and state that determine how it
behaves when invoked. A function’s code may or may not be written in ECMAScript.

4.3.25
built-in function
built-in object that is a function.
NOTE
Examples of built-in functions include parseInt and Math.exp. An implementation may provide
implementation-dependent built-in functions that are not described in this specification.

4.3.26
property
association between a name and a value that is a part of an object.
NOTE
Depending upon the form of the property the value may be represented either directly as a data value (aprimitive value, an object, or a function object) or indirectly by a pair of accessor functions.

4.3.27methodfunction that is the value of a property.
NOTE

When a function is called as a method of an object, the object is passed to the function as its this value.

4.3.28
built-in method
method that is a built-in function.
NOTE
Standard built-in methods are defined in this specification, and an ECMAScript implementation may specify
and provide other additional built-in methods.

4.3.29attributeinternal value that defines some characteristic of a property.
4.3.30own property
property that is directly contained by its object.
4.3.31
inherited property
property of an object that is not an own property but is a property (either own or inherited) of the object’s
prototype.

5

Notational Conventions

5.1

Syntactic and Lexical Grammars

5.1.1

Context-Free Grammars

A context-free grammar consists of a number of productions. Each production has an abstract symbol called anonterminal as its left-hand side, and a sequence of zero or more nonterminal and terminal symbols as its
right-hand side. For each grammar, the terminal symbols are drawn from a specified alphabet.

© Ecma International 20097

Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given
context-free grammar specifies a language, namely, the (perhaps infinite) set of possible sequences ofterminal symbols that can result from repeatedly replacing any nonterminal in the sequence with a right-handside of a production for which the nonterminal is the left-hand side.
5.1.2

The Lexical and RegExp Grammars

A lexical grammar for ECMAScript is given in clause 7. This grammar has as its terminal symbols characters(Unicode code units) that conform to the rules for SourceCharacter defined in Clause 6. It defines a set of
productions, starting from the goal symbol InputElementDiv or InputElementRegExp, that describe how
sequences of such characters are translated into a sequence of input elements.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for
ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers, literals, andpunctuators of the ECMAScript language. Moreover, line terminators, although not considered to be tokens,
also become part of the stream of input elements and guide the process of automatic semicolon insertion (7.9).
Simple white space and single-line comments are discarded and do not appear in the stream of input
elements for the syntactic grammar. A MultiLineComment (that is, a comment of the form “/*…*/” regardlessof whether it spans more than one line) is likewise simply discarded if it contains no line terminator; but if a
MultiLineComment contains one or more line terminators, then it is replaced by a single line terminator, whichbecomes part of the stream of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 15.10. This grammar also has as its terminal symbols thecharacters as defined by SourceCharacter. It defines a set of productions, starting from the goal symbol Pattern,that describe how sequences of characters are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as separating
punctuation. The lexical and RegExp grammars share some productions.
5.1.3

The Numeric String Grammar

Another grammar is used for translating Strings into numeric values. This grammar is similar to the part of thelexical grammar having to do with numeric literals and has as its terminal symbols SourceCharacter. This
grammar appears in 9.3.1.
Productions of the numeric string grammar are distinguished by having three colons “:::” as punctuation.
5.1.4

The Syntactic Grammar

The syntactic grammar for ECMAScript is given in clauses 11, 12, 13 and 14. This grammar has ECMAScript
tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of productions, startingfrom the goal symbol Program, that describe how sequences of tokens can form syntactically correct
ECMAScript programs.
When a stream of characters is to be parsed as an ECMAScript program, it is first converted to a stream of
input elements by repeated application of the lexical grammar; this stream of input elements is then parsed bya single application of the syntactic grammar. The program is syntactically in error if the tokens in the streamof input elements cannot be parsed as a single instance of the goal nonterminal Program, with no tokens left
over.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 11, 12, 13 and 14 is actually not a complete account of which
token sequences are accepted as correct ECMAScript programs. Certain additional token sequences are also
accepted, namely, those that would be described by the grammar if only semicolons were added to thesequence in certain places (such as before line terminator characters). Furthermore, certain token sequencesthat are described by the grammar are not considered acceptable if a terminator character appears in certain“awkward” places.

8© Ecma International 2009

5.1.5The JSON Grammar

The JSON grammar is used to translate a String describing a set of ECMAScript objects into actual objects.
The JSON grammar is given in 15.12.1.
The JSON grammar consists of the JSON lexical grammar and the JSON syntactic grammar. The JSON
lexical grammar is used to translate character sequences into tokens and is similar to parts of the ECMAScript
lexical grammar. The JSON syntactic grammar describes how sequences of tokens from the JSON lexical
grammar can form syntactically correct JSON object descriptions.
Productions of the JSON lexical grammar are distinguished by having two colons “::” as separating
punctuation. The JSON lexical grammar uses some productions from the ECMAScript lexical grammar. The
JSON syntactic grammar is similar to parts of the ECMAScript syntactic grammar. Productions of the JSON
syntactic grammar are distinguished by using one colon “:” as separating punctuation.
5.1.6

Grammar Notation

Terminal symbols of the lexical and string grammars, and some of the terminal symbols of the syntactic
grammar, are shown in fixed width font, both in the productions of the grammars and throughout thisspecification whenever the text directly refers to such a terminal symbol. These are to appear in a program
exactly as written. All terminal symbol characters specified in this way are to be understood as the appropriate
Unicode character from the ASCII range, as opposed to any similar-looking characters from other Unicode
ranges.
Nonterminal symbols are shown in italic type. The definition of a nonterminal is introduced by the name of thenonterminal being defined followed by one or more colons. (The number of colons indicates to which grammar
the production belongs.) One or more alternative right-hand sides for the nonterminal then follow on
succeeding lines. For example, the syntactic definition:
WhileStatement :while ( Expression ) Statement
states that the nonterminal WhileStatement represents the token while, followed by a left parenthesis token,
followed by an Expression, followed by a right parenthesis token, followed by a Statement. The occurrences ofExpression and Statement are themselves nonterminals. As another example, the syntactic definition:
ArgumentList :AssignmentExpression
ArgumentList , AssignmentExpression
states that an ArgumentList may represent either a single AssignmentExpression or an ArgumentList, followed by
a comma, followed by an AssignmentExpression. This definition of ArgumentList is recursive, that is, it is definedin terms of itself. The result is that an ArgumentList may contain any positive number of arguments, separated
by commas, where each argument expression is an AssignmentExpression. Such recursive definitions ofnonterminals are common.
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an optional symbol.
The alternative containing the optional symbol actually specifies two right-hand sides, one that omits theoptional element and one that includes it. This means that:
VariableDeclaration :Identifier Initialiseropt
is a convenient abbreviation for:
VariableDeclaration :Identifier
Identifier Initialiser© Ecma International 20099

and that:
IterationStatement :for ( ExpressionNoInopt ; Expressionopt ; Expressionopt ) Statement
is a convenient abbreviation for:
IterationStatement :for ( ; Expressionopt ; Expressionopt ) Statementfor ( ExpressionNoIn ; Expressionopt ; Expressionopt ) Statement
which in turn is an abbreviation for:
IterationStatement :for ( ; ; Expressionopt ) Statementfor ( ; Expression ; Expressionopt ) Statementfor ( ExpressionNoIn ; ; Expressionopt ) Statementfor ( ExpressionNoIn ; Expression ; Expressionopt ) Statement
which in turn is an abbreviation for:
IterationStatement :for ( ; ; ) Statementfor ( ; ; Expression ) Statementfor ( ; Expression ; ) Statementfor ( ; Expression ; Expression ) Statementfor ( ExpressionNoIn ; ; ) Statementfor ( ExpressionNoIn ; ; Expression ) Statementfor ( ExpressionNoIn ; Expression ; ) Statementfor ( ExpressionNoIn ; Expression ; Expression ) Statement
so the nonterminal IterationStatement actually has eight alternative right-hand sides.
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the production's righthand side contains no terminals or nonterminals.
If the phrase “[lookahead ∉ set]” appears in the right-hand side of a production, it indicates that the production
may not be used if the immediately following input token is a member of the given set. The set can be written
as a list of terminals enclosed in curly braces. For convenience, the set can also be written as a nonterminal,
in which case it represents the set of all terminals to which that nonterminal could expand. For example, given
the definitions
DecimalDigit :: one of
0 1 2 3 4

5

6

7

8

9

DecimalDigits ::DecimalDigit
DecimalDigits DecimalDigit
the definition
LookaheadExample ::n [lookahead ∉ {1, 3, 5, 7, 9}] DecimalDigits
DecimalDigit [lookahead ∉ DecimalDigit ]matches either the letter n followed by one or more decimal digits the first of which is even, or a decimal digit
not followed by another decimal digit.

10© Ecma International 2009

If the phrase “[no LineTerminator here]” appears in the right-hand side of a production of the syntactic grammar, it
indicates that the production is a restricted production: it may not be used if a LineTerminator occurs in the
input stream at the indicated position. For example, the production:
ReturnStatement :return [no LineTerminator here] Expressionopt ;indicates that the production may not be used if a LineTerminator occurs in the program between the return
token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted production, any number of occurrences of
LineTerminator may appear between any two consecutive tokens in the stream of input elements without
affecting the syntactic acceptability of the program.
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the terminal
symbols on the following line or lines is an alternative definition. For example, the lexical grammar for
ECMAScript contains the production:
NonZeroDigit :: one of
1 2 3 4 5

6

7

8

9

which is merely a convenient abbreviation for:NonZeroDigit ::
1
2
3
4
5
6
7
8
9
When an alternative in a production of the lexical grammar or the numeric string grammar appears to be amulti-character token, it represents the sequence of characters that would make up such a token.
The right-hand side of a production may specify that certain expansions are not permitted by using the phrase“but not” and then indicating the expansions to be excluded. For example, the production:
Identifier ::IdentifierName but not ReservedWordmeans that the nonterminal Identifier may be replaced by any sequence of characters that could replace
IdentifierName provided that the same sequence of characters could not replace ReservedWord.
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases where it
would be impractical to list all the alternatives:
SourceCharacter ::any Unicode code unit

5.2

Algorithm Conventions

The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used toprecisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended
to imply the use of any specific implementation technique. In practice, there may be more efficient algorithms
available to implement a given feature.

© Ecma International 200911

In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations,
are named and written in parameterized functional form so that they may be referenced by name from within
other algorithms.
When an algorithm is to produce a value as a result, the directive “return x” is used to indicate that the result of
the algorithm is the value of x and that the algorithm should terminate. The notation Result(n) is used as
shorthand for “the result of step n”.
For clarity of expression, algorithm steps may be subdivided into sequential substeps. Substeps are indented
and may themselves be further divided into indented substeps. Outline numbering conventions are used to
identify substeps with the first level of substeps labelled with lower case alphabetic characters and the second
level of substeps labelled with lower case roman numerals. If more than three levels are required these rules
repeat with the fourth level using numeric labels. For example:
1.

Top-level stepa. Substep.
b. Substep
i.
Subsubstep.
ii.
Subsubstep.
1. Subsubsubstep
a Subsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the substeps
are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a predicate that is
the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
Mathematical operations such as addition, subtraction, negation, multiplication, division, and the mathematical
functions defined later in this clause should always be understood as computing exact mathematical results
on mathematical real numbers, which do not include infinities and do not include a negative zero that is
distinguished from positive zero. Algorithms in this standard that model floating-point arithmetic include explicit
steps, where necessary, to handle infinities and signed zero and to perform rounding. If a mathematicaloperation or function is applied to a floating-point number, it should be understood as being applied to theexact mathematical value represented by that floating-point number; such a floating-point number must be
finite, and if it is +0 or −0 then the corresponding mathematical value is simply 0.
The mathematical function abs(x) yields the absolute value of x, which is −x if x is negative (less than zero) andotherwise is x itself.
The mathematical function sign(x) yields 1 if x is positive and −1 if x is negative. The sign function is not used inthis standard for cases when x is zero.
The notation “x modulo y” (y must be finite and nonzero) computes a value k of the same sign as y (or zero)such that abs(k) < abs(y) and x−k = q × y for some integer q.
The mathematical function floor(x) yields the largest integer (closest to positive infinity) that is not larger than x.
NOTE

floor(x) = x−(x modulo 1).

If an algorithm is defined to “throw an exception”, execution of the algorithm is terminated and no result isreturned. The calling algorithms are also terminated, until an algorithm step is reached that explicitly deals
with the exception, using terminology such as “If an exception was thrown…”. Once such an algorithm stephas been encountered the exception is no longer considered to have occurred.

6

Source Text

ECMAScript source text is represented as a sequence of characters in the Unicode character encoding,version 3.0 or later. The text is expected to have been normalised to Unicode Normalised Form C (canonical12© Ecma International 2009

composition), as described in Unicode Technical Report #15. Conforming ECMAScript implementations are
not required to perform any normalisation of text, or behave as though they were performing normalisation oftext, themselves. ECMAScript source text is assumed to be a sequence of 16-bit code units for the purposesof this specification. Such a source text may include sequences of 16-bit code units that are not valid UTF-16
character encodings. If an actual source text is encoded in a form other than 16-bit code units it must beprocessed as if it was first convert to UTF-16.
SourceCharacter ::any Unicode code unit
Throughout the rest of this document, the phrase “code unit” and the word “character” will be used to refer to a
16-bit unsigned value used to represent a single 16-bit unit of text. The phrase “Unicode character” will be
used to refer to the abstract linguistic or typographical unit represented by a single Unicode scalar value(which may be longer than 16 bits and thus may be represented by more than one code unit). The phrase“code point” refers to such a Unicode scalar value. “Unicode character” only refers to entities represented bysingle Unicode scalar values: the components of a combining character sequence are still individual “Unicodecharacters,” even though a user might think of the whole sequence as a single character.
In string literals, regular expression literals, and identifiers, any character (code unit) may also be expressed
as a Unicode escape sequence consisting of six characters, namely \u plus four hexadecimal digits. Within acomment, such an escape sequence is effectively ignored as part of the comment. Within a string literal or
regular expression literal, the Unicode escape sequence contributes one character to the value of the literal.
Within an identifier, the escape sequence contributes one character to the identifier.
NOTE
Although this document sometimes refers to a “transformation” between a “character” within a “string” and the
16-bit unsigned integer that is the code unit of that character, there is actually no transformation because a “character”
within a “string” is actually represented using that 16-bit unsigned value.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a Java
program, if the Unicode escape sequence \u000A, for example, occurs within a single-line comment, it is interpreted as aline terminator (Unicode character 000A is line feed) and therefore the next character is not part of the comment. Similarly,
if the Unicode escape sequence \u000A occurs within a string literal in a Java program, it is likewise interpreted as a line
terminator, which is not allowed within a string literal—one must write \n instead of \u000A to cause a line feed to be partof the string value of a string literal. In an ECMAScript program, a Unicode escape sequence occurring within a commentis never interpreted and therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequenceoccurring within a string literal in an ECMAScript program always contributes a character to the String value of the literaland is never interpreted as a line terminator or as a quote mark that might terminate the string literal.

7

Lexical Conventions

The source text of an ECMAScript program is first converted into a sequence of input elements, which aretokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly
taking the longest possible sequence of characters as the next input element.
There are two goal symbols for the lexical grammar. The InputElementDiv symbol is used in those syntactic
grammar contexts where a leading division (/) or division-assignment (/=) operator is permitted. The
InputElementRegExp symbol is used in other syntactic grammar contexts.
NOTE

There are no syntactic grammar contexts where both a leading division or division-assignment, and a leading

RegularExpressionLiteral are permitted. This is not affected by semicolon insertion (see 7.9); in examples such as thefollowing:a = b/hi/g.exec(c).map(d);where the first non-whitespace, non-comment character after a LineTerminator is slash (/) and the syntactic context allows
division or division-assignment, no semicolon is inserted at the LineTerminator. That is, the above example is interpreted in
the same way as:a = b / hi / g.exec(c).map(d);© Ecma International 200913

Syntax
InputElementDiv ::WhiteSpace
LineTerminator
Comment
Token
DivPunctuator
InputElementRegExp ::WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

7.1

Unicode Format-Control Characters

The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character
Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formattingof a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format control
characters may be used within comments, and within string literals and regular expression literals.
<ZWNJ> and <ZWJ> are format-control characters that are used to make necessary distinctions when forming
words or phrases in certain languages. In ECMAScript source text, <ZWNJ> and <ZWJ> may also be used in
an identifier after the first character.
<BOM> is a format-control character used primarily at the start of a text to mark it as Unicode and to allow
detection of the text's encoding and byte order. <BOM> characters intended for this purpose can sometimes
also appear after the start of a text, for example as a result of concatenating files. <BOM> characters aretreated as white space characters (see 7.2).
The special treatment of certain format-control characters outside of comments, string literals, and regularexpression literals is summarized in Table 1.
Table 1 — Format-Control Character Usage

7.2

Code Unit Value

Name

Formal Name

Usage

\u200C\u200C\uFEFF

Zero width non-joiner
Zero width joiner
Byte Order Mark<ZWNJ><ZWJ><BOM>

IdentifierPart
IdentifierPart
Whitespace

White SpaceWhite space characters are used to improve source text readability and to separate tokens (indivisible lexicalunits) from each other, but are otherwise insignificant. White space characters may occur between any two
tokens and at the start or end of input. White space characters may also occur within a StringLiteral or a
RegularExpressionLiteral (where they are considered significant characters forming part of the literal value) orwithin a Comment, but cannot appear within any other kind of token.
The ECMAScript white space characters are listed in Table 2.

14© Ecma International 2009

Table 2 — Whitespace Characters
Code Unit Value\u0009\u000B\u000C\u0020\u00A0\uFEFF
Other category “Zs”

Name
Tab
Vertical Tab
Form FeedSpace
No-break space
Byte Order MarkAny other Unicode“space separator”

Formal Name
<TAB>
<VT>
<FF>
<SP>
<NBSP><BOM><USP>

ECMAScript implementations must recognize all of the white space characters defined in Unicode 3.0. Later
editions of the Unicode Standard may define other white space characters. ECMAScript implementations may
recognize white space characters from later editions of the Unicode Standard.
SyntaxWhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP>
<BOM>
<USP>

7.3

Line Terminators

Like white space characters, line terminator characters are used to improve source text readability and to
separate tokens (indivisible lexical units) from each other. However, unlike white space characters, line
terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators
may occur between any two tokens, but there are a few places where they are forbidden by the syntactic
grammar. Line terminators also affect the process of automatic semicolon insertion (7.9). A line terminatorcannot occur within any token except a StringLiteral. Line terminators may only occur within a StringLiteral
token as part of a LineContinuation.
A line terminator can occur within a MultiLineComment (7.4) but cannot occur within a SingleLineComment.
Line terminators are included in the set of white space characters that are matched by the \s class in regularexpressions.
The ECMAScript line terminator characters are listed in Table 3.
Table 3 — Line Terminator Characters
Code Unit Value\u000A\u000D\u2028\u2029Name

Formal NameLine Feed
Carriage Return
Line separator
Paragraph separator

<LF>
<CR>
<LS>
<PS>

Only the characters in Table 3 are treated as line terminators. Other new line or line breaking characters aretreated as white space but not as line terminators. The character sequence <CR><LF> is commonly used asa line terminator. It should be considered a single character for the purpose of reporting line numbers.

© Ecma International 200915

Syntax
LineTerminator ::
<LF>
<CR>
<LS>
<PS>
LineTerminatorSequence ::<LF><CR> [lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

7.4

Comments

Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any character except a LineTerminator character, and because of
the general rule that a token is always as long as possible, a single-line comment always consists of all
characters from the // marker to the end of the line. However, the LineTerminator at the end of the line is notconsidered to be part of the single-line comment; it is recognised separately by the lexical grammar andbecomes part of the stream of input elements for the syntactic grammar. This point is very important, because
it implies that the presence or absence of single-line comments does not affect the process of automatic
semicolon insertion (see 7.9).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line
terminator character, then the entire comment is considered to be a LineTerminator for purposes of parsing by
the syntactic grammar.
SyntaxComment ::MultiLineComment
SingleLineComment
MultiLineComment ::/* MultiLineCommentCharsopt */MultiLineCommentChars ::MultiLineNotAsteriskChar MultiLineCommentCharsopt* PostAsteriskCommentCharsopt
PostAsteriskCommentChars ::MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt* PostAsteriskCommentCharsopt
MultiLineNotAsteriskChar ::SourceCharacter but not asterisk *MultiLineNotForwardSlashOrAsteriskChar ::SourceCharacter but not forward-slash / or asterisk *SingleLineComment ::// SingleLineCommentCharsopt
SingleLineCommentChars ::SingleLineCommentChar SingleLineCommentCharsopt

16© Ecma International 2009

SingleLineCommentChar ::SourceCharacter but not LineTerminator

7.5

TokensSyntaxToken ::
IdentifierName
Punctuator
NumericLiteralStringLiteral
NOTE
The DivPunctuator and RegularExpressionLiteral productions define tokens, but are not included in the Token
production.

7.6

Identifier Names and Identifiers

Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section ofchapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is nota ReservedWord (see 7.6.1). The Unicode identifier grammar is based on both normative and informative
character categories specified by the Unicode Standard. The characters in the specified categories in version
3.0 of the Unicode standard must be treated as in those categories by all conforming ECMAScript
implementations.
This standard specifies specific character additions: The dollar sign ($) and the underscore (_) are permitted
anywhere in an IdentifierName.
Unicode escape sequences are also permitted in an IdentifierName, where they contribute a single character tothe IdentifierName, as computed by the CV of the UnicodeEscapeSequence (see 7.8.4). The \ preceding theUnicodeEscapeSequence does not contribute a character to the IdentifierName. A UnicodeEscapeSequence cannot
be used to put a character into an IdentifierName that would otherwise be illegal. In other words, if a
\ UnicodeEscapeSequence sequence were replaced by its UnicodeEscapeSequence's CV, the result must still bea valid IdentifierName that has the exact same sequence of characters as the original IdentifierName. Allinterpretations of identifiers within this specification are based upon their actual characters regardless ofwhether or not an escape sequence was used to contribute any particular characters.
Two IdentifierName that are canonically equivalent according to the Unicode standard are not equal unlessthey are represented by the exact same sequence of code units (in other words, conforming ECMAScript
implementations are only required to do bitwise comparison on IdentifierName values). The intent is that theincoming source text has been converted to normalised form C before it reaches the compiler.
ECMAScript implementations may recognize identifier characters defined in later editions of the UnicodeStandard. If portability is a concern, programmers should only employ identifier characters defined in Unicode
3.0.
SyntaxIdentifier ::IdentifierName but not ReservedWord
IdentifierName ::IdentifierStart
IdentifierName IdentifierPart© Ecma International 200917

IdentifierStart ::UnicodeLetter$_\ UnicodeEscapeSequence
IdentifierPart ::IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation<ZWNJ><ZWJ>
UnicodeLetterany character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter (Ll)”, “Titlecase letter(Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter number (Nl)”.
UnicodeCombiningMark
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining spacing mark (Mc)”UnicodeDigit
any character in the Unicode category “Decimal number (Nd)”UnicodeConnectorPunctuation
any character in the Unicode category “Connector punctuation (Pc)”
UnicodeEscapeSequence
see 7.8.4.
7.6.1

Reserved Words

A reserved word is an IdentifierName that cannot be used as an Identifier.
SyntaxReservedWord ::Keyword
FutureReservedWord
NullLiteral
BooleanLiteral
7.6.1.1

Keywords

The following tokens are ECMAScript keywords and may not be used as Identifiers in ECMAScript programs.
Syntax
Keyword :: one of
break
case
catch
continue
debuggerdefault
delete18

doelse
finallyforfunction
ifininstanceof
newreturnswitch
this
throw
try

typeof
var
void
while
with

© Ecma International 2009

7.6.1.2

Future Reserved Words

The following words are used as keywords in proposed extensions and are therefore reserved to allow for the
possibility of future adoption of those extensions.
SyntaxFutureReservedWord :: one of
class
const

enum
exportextends
importsuper

The following tokens are also considered to be FutureReservedWords when they occur within strict mode code
(see 10.1.1). The occurrence of any of these tokens within strict mode code in any context where theoccurrence of a FutureReservedWord would produce an error must also produce an equivalent error:
implementsinterface

7.7

letpackageprivateprotected

publicstaticyield

Punctuators

Syntax
Punctuator :: one of
{
}

(

)

[

]
<=

.

;

,

<

>

>=

==

!=

===

!==

+

-

*

%

++

--

<<

>>

>>>

&

|

^

!

~

&&

||

?

:

=

+=

-=

*=

%=

<<=

>>=

>>>=

&=

|=

^=

DivPunctuator :: one of
/
/=

7.8

Literals

Syntax
Literal ::NullLiteral
BooleanLiteral
NumericLiteralStringLiteral
RegularExpressionLiteral
7.8.1

Null Literals

SyntaxNullLiteral ::null© Ecma International 200919

Semantics
The value of the null literal null is the sole value of the Null type, namely null.
7.8.2

Boolean Literals

Syntax
BooleanLiteral ::
true
false
Semantics
The value of the Boolean literal true is a value of the Boolean type, namely true.
The value of the Boolean literal false is a value of the Boolean type, namely false.
7.8.3

Numeric Literals

SyntaxNumericLiteral ::DecimalLiteral
HexIntegerLiteral
DecimalLiteral ::DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt
DecimalIntegerLiteral ::0
NonZeroDigit DecimalDigitsopt
DecimalDigits ::DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit :: one of
0 1 2 3 4

5

6

7

8

NonZeroDigit :: one of
1 2 3 4 5

6

7

8

9

9

ExponentPart ::ExponentIndicator SignedInteger
ExponentIndicator :: one ofe E
SignedInteger ::DecimalDigits+ DecimalDigits- DecimalDigits
HexIntegerLiteral ::0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit

20© Ecma International 2009

HexDigit :: one of
0 1 2 3

4

5

6

7

8

9

a

b

c

d

e

f

A

B

C

D

E

F

The source character immediately following a NumericLiteral must not be an IdentifierStart or DecimalDigit.
NOTE

For example:
3in

is an error and not the two input elements 3 and in.

Semantics
A numeric literal stands for a value of the Number type. This value is determined in two steps: first, amathematical value (MV) is derived from the literal; second, this mathematical value is rounded as described
below.
•
•
•
•
•
•

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

The MV of NumericLiteral :: DecimalLiteral is the MV of DecimalLiteral.
The MV of NumericLiteral :: HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral . is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits is the MV of DecimalIntegerLiteral plus
(the MV of DecimalDigits times 10–n), where n is the number of characters in DecimalDigits.
The MV of DecimalLiteral :: DecimalIntegerLiteral . ExponentPart is the MV of DecimalIntegerLiteral times
10e, where e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral . DecimalDigits ExponentPart is (the MV of
DecimalIntegerLiteral plus (the MV of DecimalDigits times 10–n)) times 10e, where n is the number ofcharacters in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral ::. DecimalDigits is the MV of DecimalDigits times 10–n, where n is the number ofcharacters in DecimalDigits.
The MV of DecimalLiteral ::. DecimalDigits ExponentPart is the MV of DecimalDigits times 10e–n, where n is
the number of characters in DecimalDigits and e is the MV of ExponentPart.
The MV of DecimalLiteral :: DecimalIntegerLiteral is the MV of DecimalIntegerLiteral.
The MV of DecimalLiteral :: DecimalIntegerLiteral ExponentPart is the MV of DecimalIntegerLiteral times 10e,where e is the MV of ExponentPart.
The MV of DecimalIntegerLiteral :: 0 is 0.
The MV of DecimalIntegerLiteral :: NonZeroDigit DecimalDigits is (the MV of NonZeroDigit times 10n) plus
the MV of DecimalDigits, where n is the number of characters in DecimalDigits.
The MV of DecimalDigits :: DecimalDigit is the MV of DecimalDigit.
The MV of DecimalDigits :: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of
DecimalDigit.
The MV of ExponentPart :: ExponentIndicator SignedInteger is the MV of SignedInteger.
The MV of SignedInteger :: DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger :: + DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger :: - DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit :: 0 or of HexDigit :: 0 is 0.
The MV of DecimalDigit :: 1 or of NonZeroDigit :: 1 or of HexDigit :: 1 is 1.
The MV of DecimalDigit :: 2 or of NonZeroDigit :: 2 or of HexDigit :: 2 is 2.
The MV of DecimalDigit :: 3 or of NonZeroDigit :: 3 or of HexDigit :: 3 is 3.
The MV of DecimalDigit :: 4 or of NonZeroDigit :: 4 or of HexDigit :: 4 is 4.
The MV of DecimalDigit :: 5 or of NonZeroDigit :: 5 or of HexDigit :: 5 is 5.
The MV of DecimalDigit :: 6 or of NonZeroDigit :: 6 or of HexDigit :: 6 is 6.
The MV of DecimalDigit :: 7 or of NonZeroDigit :: 7 or of HexDigit :: 7 is 7.
The MV of DecimalDigit :: 8 or of NonZeroDigit :: 8 or of HexDigit :: 8 is 8.
The MV of DecimalDigit :: 9 or of NonZeroDigit :: 9 or of HexDigit :: 9 is 9.
The MV of HexDigit :: a or of HexDigit :: A is 10.
The MV of HexDigit :: b or of HexDigit :: B is 11.

© Ecma International 2009

21

•
•
•
•
•
•
•

The MV of HexDigit :: c or of HexDigit :: C is 12.
The MV of HexDigit :: d or of HexDigit :: D is 13.
The MV of HexDigit :: e or of HexDigit :: E is 14.
The MV of HexDigit :: f or of HexDigit :: F is 15.
The MV of HexIntegerLiteral :: 0x HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral :: 0X HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral :: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the
MV of HexDigit.

Once the exact MV for a numeric literal has been determined, it is then rounded to a value of the Number type.
If the MV is 0, then the rounded value is +0; otherwise, the rounded value must be the Number value for the
MV (as specified in 8.5), unless the literal is a DecimalLiteral and the literal has more than 20 significant digits,
in which case the Number value may be either the Number value for the MV of a literal produced by replacing
each significant digit after the 20th with a 0 digit or the Number value for the MV of a literal produced byreplacing each significant digit after the 20th with a 0 digit and then incrementing the literal at the 20th
significant digit position. A digit is significant if it is not part of an ExponentPart and••it is not 0; orthere is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

A conforming implementation, when processing strict mode code (see 10.1.1), must not extend the syntax of
NumericLiteral to include OctalIntegerLiteral as described in B.1.1.
7.8.4

String Literals

A string literal is zero or more characters enclosed in single or double quotes. Each character may be
represented by an escape sequence. All characters may appear literally in a string literal except for the closing
quote character, backslash, carriage return, line separator, paragraph separator, and line feed. Any character
may appear in the form of an escape sequence.
SyntaxStringLiteral ::" DoubleStringCharactersopt "' SingleStringCharactersopt 'DoubleStringCharacters ::DoubleStringCharacter DoubleStringCharactersopt
SingleStringCharacters ::SingleStringCharacter SingleStringCharactersopt
DoubleStringCharacter ::SourceCharacter but not double-quote " or backslash \ or LineTerminator\ EscapeSequence
LineContinuation
SingleStringCharacter ::SourceCharacter but not single-quote ' or backslash \ or LineTerminator\ EscapeSequence
LineContinuation
LineContinuation ::\ LineTerminatorSequence22© Ecma International 2009

EscapeSequence ::CharacterEscapeSequence
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence
CharacterEscapeSequence ::SingleEscapeCharacter
NonEscapeCharacter
SingleEscapeCharacter :: one of' " \ b f n rtv

NonEscapeCharacter ::SourceCharacter but not EscapeCharacter or LineTerminator
EscapeCharacter ::SingleEscapeCharacter
DecimalDigitxu
HexEscapeSequence ::x HexDigit HexDigit
UnicodeEscapeSequence ::u HexDigit HexDigit HexDigit HexDigit
The definitions of the nonterminal HexDigit is given in 7.6. SourceCharacter is defined in clause 6.
Semantics
A string literal stands for a value of the String type. The String value (SV) of the literal is described in terms ofcharacter values (CV) contributed by the various parts of the string literal. As part of this process, somecharacters within the string literal are interpreted as having a mathematical value (MV), as described below orin 7.8.3.
•
•
•
•
•
•
•
•
•
•
•
•
•
•

The SV of StringLiteral :: "" is the empty character sequence.
The SV of StringLiteral :: '' is the empty character sequence.
The SV of StringLiteral :: " DoubleStringCharacters " is the SV of DoubleStringCharacters.
The SV of StringLiteral :: ' SingleStringCharacters ' is the SV of SingleStringCharacters.
The SV of DoubleStringCharacters :: DoubleStringCharacter is a sequence of one character, the CV of
DoubleStringCharacter.
The SV of DoubleStringCharacters :: DoubleStringCharacter DoubleStringCharacters is a sequence of the CV
of DoubleStringCharacter followed by all the characters in the SV of DoubleStringCharacters in order.
The SV of SingleStringCharacters :: SingleStringCharacter is a sequence of one character, the CV ofSingleStringCharacter.
The SV of SingleStringCharacters :: SingleStringCharacter SingleStringCharacters is a sequence of the CV
of SingleStringCharacter followed by all the characters in the SV of SingleStringCharacters in order.
The SV of LineContinuation :: \ LineTerminatorSequence is the empty character sequence.
The CV of DoubleStringCharacter :: SourceCharacter but not double-quote " or backslash \ or LineTerminatoris the SourceCharacter character itself.
The CV of DoubleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.
The CV of SingleStringCharacter :: SourceCharacter but not single-quote ' or backslash \ or LineTerminatoris the SourceCharacter character itself.
The CV of SingleStringCharacter :: \ EscapeSequence is the CV of the EscapeSequence.
The CV of EscapeSequence :: CharacterEscapeSequence is the CV of the CharacterEscapeSequence.

© Ecma International 2009

23

•
•
•
•

The CV of EscapeSequence :: 0 [lookahead ∉ DecimalDigit] is a <NUL> character (Unicode value 0000).
The CV of EscapeSequence :: HexEscapeSequence is the CV of the HexEscapeSequence.
The CV of EscapeSequence :: UnicodeEscapeSequence is the CV of the UnicodeEscapeSequence.
The CV of CharacterEscapeSequence :: SingleEscapeCharacter is the character whose code unit value isdetermined by the SingleEscapeCharacter according to Table 4:
Table 4 — String Single Character Escape Sequences
Escape Sequence
\b
\t
\n
\v
\f
\r
\"
\'
\\

•
•
•
•

Code Unit Value

Name

Symbol\u0008\u0009\u000A\u000B\u000C\u000D\u0022\u0027\u005C

backspacehorizontal tab
line feed (new line)
vertical tab
form feed
carriage returndouble quote
single quotebackslash

<BS>
<HT>
<LF>
<VT>
<FF>
<CR>
"
'
\

The CV of CharacterEscapeSequence :: NonEscapeCharacter is the CV of the NonEscapeCharacter.
The CV of NonEscapeCharacter :: SourceCharacter but not EscapeCharacter or LineTerminator is the
SourceCharacter character itself.
The CV of HexEscapeSequence :: x HexDigit HexDigit is the character whose code unit value is (16 times
the MV of the first HexDigit) plus the MV of the second HexDigit.
The CV of UnicodeEscapeSequence :: u HexDigit HexDigit HexDigit HexDigit is the character whose code
unit value is (4096 times the MV of the first HexDigit) plus (256 times the MV of the second HexDigit) plus
(16 times the MV of the third HexDigit) plus the MV of the fourth HexDigit.

A conforming implementation, when processing strict mode code (see 10.1.1), may not extend the syntax ofEscapeSequence to include OctalEscapeSequence as described in B.1.2.
NOTE
A line terminator character cannot appear in a string literal, except as part of a LineContinuation to produce theempty character sequence. The correct way to cause a line terminator character to be part of the String value of a string
literal is to use an escape sequence such as \n or \u000A.

7.8.5

Regular Expression Literals

A regular expression literal is an input element that is converted to a RegExp object (see 15.10) each time theliteral is evaluated. Two regular expression literals in a program evaluate to regular expression objects thatnever compare as === to each other even if the two literals' contents are identical. A RegExp object may also
be created at runtime by new RegExp (see 15.10.4) or calling the RegExp constructor as a function (15.10.3).
The productions below describe the syntax for a regular expression literal and are used by the input element
scanner to find the end of the regular expression literal. The Strings of characters comprising theRegularExpressionBody and the RegularExpressionFlags are passed uninterpreted to the regular expression
constructor, which interprets them according to its own, more stringent grammar. An implementation may
extend the regular expression constructor's grammar, but it must not extend the RegularExpressionBody andRegularExpressionFlags productions or the productions used by these productions.
Syntax
RegularExpressionLiteral ::/ RegularExpressionBody / RegularExpressionFlags

24© Ecma International 2009

RegularExpressionBody ::RegularExpressionFirstChar RegularExpressionChars
RegularExpressionChars ::[empty]RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::RegularExpressionNonTerminator but not * or \ or / or [RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionChar ::RegularExpressionNonTerminator but not \ or / or [RegularExpressionBackslashSequence
RegularExpressionClass
RegularExpressionBackslashSequence ::\ RegularExpressionNonTerminator
RegularExpressionNonTerminator ::SourceCharacter but not LineTerminator
RegularExpressionClass ::[ RegularExpressionClassChars ]RegularExpressionClassChars ::[empty]RegularExpressionClassChars RegularExpressionClassChar
RegularExpressionClassChar ::RegularExpressionNonTerminator but not ] or \RegularExpressionBackslashSequence
RegularExpressionFlags ::[empty]RegularExpressionFlags IdentifierPart
NOTE
Regular expression literals may not be empty; instead of representing an empty regular expression literal, thecharacters // start a single-line comment. To specify an empty regular expression, use: /(?:)/.

Semantics
A regular expression literal evaluates to a value of the Object type that is an instance of the standard built-in
constructor RegExp. This value is determined in two steps: first, the characters comprising the regularexpression's RegularExpressionBody and RegularExpressionFlags production expansions are collected
uninterpreted into two Strings Pattern and Flags, respectively. Then each time the literal is evaluated, a new
object is created as if by the expression new RegExp(Pattern, Flags) where RegExp is the standard
built-in constructor with that name. The newly constructed object becomes the value of theRegularExpressionLiteral. If the call to new RegExp would generate an error as specified in 15.10.4.1, the error
must be treated as an early error (Clause 16).

7.9

Automatic Semicolon Insertion

Certain ECMAScript statements (empty statement, variable statement, expression statement, do-whilestatement, continue statement, break statement, return statement, and throw statement) must be
terminated with semicolons. Such semicolons may always appear explicitly in the source text. For
convenience, however, such semicolons may be omitted from the source text in certain situations. Thesesituations are described by saying that semicolons are automatically inserted into the source code token
stream in those situations.

© Ecma International 200925

7.9.1

Rules of Automatic Semicolon Insertion

There are three basic rules of semicolon insertion:
1.

When, as the program is parsed from left to right, a token (called the offending token) is encountered thatis not allowed by any production of the grammar, then a semicolon is automatically inserted before theoffending token if one or more of the following conditions is true:••

The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.

2.

When, as the program is parsed from left to right, the end of the input stream of tokens is encounteredand the parser is unable to parse the input token stream as a single complete ECMAScript Program, thena semicolon is automatically inserted at the end of the input stream.

3.

When, as the program is parsed from left to right, a token is encountered that is allowed by someproduction of the grammar, but the production is a restricted production and the token would be the firsttoken for a terminal or nonterminal immediately following the annotation “[no LineTerminator here]” within the
restricted production (and therefore such a token is called a restricted token), and the restricted token isseparated from the previous token by at least one LineTerminator, then a semicolon is automaticallyinserted before the restricted token.

However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted
automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become
one of the two semicolons in the header of a for statement (see 12.6.3).
NOTE

The following are the only restricted productions in the grammar:
PostfixExpression :LeftHandSideExpression [no LineTerminator here] ++LeftHandSideExpression [no LineTerminator here] -ContinueStatement :continue [no LineTerminator here] Identifieropt ;BreakStatement :break [no LineTerminator here] Identifieropt ;ReturnStatement :return [no LineTerminator here] Expressionopt ;ThrowStatement :throw [no LineTerminator here] Expression ;The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a postfix operator, and at least one

LineTerminator occurred between the preceding token and the ++ or -- token, then a semicolon is automatically inserted
before the ++ or -- token.
When a continue, break, return, or throw token is encountered and a LineTerminator is encountered before thenext token, a semicolon is automatically inserted after the continue, break, return, or throw token.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should appear on the same line as its operand.
An Expression in a return or throw statement should start on the same line as the return or throw token.
A Identifier in a break or continue statement should be on the same line as the break or continue token.

26© Ecma International 2009

7.9.2

Examples of Automatic Semicolon Insertion

The source{ 1 2 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In
contrast, the source
{ 1
2 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into thefollowing:{ 1;2 ;} 3;
which is a valid ECMAScript sentence.
The sourcefor (a; b)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because thesemicolon is needed for the header of a for statement. Automatic semicolon insertion never inserts one of
the two semicolons in the header of a for statement.
The source
returna + b
is transformed by automatic semicolon insertion into the following:
return;a + b;NOTE
The expression a + b is not treated as a value to be returned by the return statement, because a
LineTerminator separates it from the token return.

The source
a = b
++c
is transformed by automatic semicolon insertion into the following:a = b;++c;
NOTE
The token ++ is not treated as a postfix operator applying to the variable b, because a LineTerminator occurs
between b and ++.

The source
if (a > b)else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the else token,
even though no production of the grammar applies at that point, because an automatically inserted semicolon
would then be parsed as an empty statement.
The sourcea = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesised expression that begins thesecond line can be interpreted as an argument list for a function call:a = b + c(d + e).print()© Ecma International 200927

In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for the
programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on
automatic semicolon insertion.

8

Types

Algorithms within this specification manipulate values each of which has an associated type. The possible
value types are exactly those defined in this clause. Types are further subclassified into ECMAScript language
types and specification types.
An ECMAScript language type corresponds to values that are directly manipulated by an ECMAScript
programmer using the ECMAScript language. The ECMAScript language types are Undefined, Null, Boolean,
String, Number, and Object.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of
ECMAScript language constructs and ECMAScript language types. The specification types are Reference,
List, Completion, Property Descriptor, Property Identifier, Lexical Environment, and Environment Record.
Specification type values are specification artefacts that do not necessarily correspond to any specific entity
within an ECMAScript implementation. Specification type values may be used to describe intermediate resultsof ECMAScript expression evaluation but such values cannot be stored as properties of objects or values of
ECMAScript language variables.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of x” where “type” refers to the
ECMAScript language and specification types defined in this clause.

8.1

The Undefined TypeThe Undefined type has exactly one value, called undefined. Any variable that has not been assigned a value
has the value undefined.

8.2

The Null TypeThe Null type has exactly one value, called null.

8.3

The Boolean TypeThe Boolean type represents a logical entity having two values, called true and false.

8.4

The String TypeThe String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values(“elements”). The String type is generally used to represent textual data in a running ECMAScript program, inwhich case each element in the String is treated as a code unit value (see Clause 6). Each element isregarded as occupying a position within the sequence. These positions are indexed with nonnegative integers.
The first element (if any) is at position 0, the next element (if any) at position 1, and so on. The length of aString is the number of elements (i.e., 16-bit values) within it. The empty String has length zero and therefore
contains no elements.
When a String contains actual textual data, each element is considered to be a single UTF-16 code unit.
Whether or not this is the actual storage format of a String, the characters within a String are numbered bytheir initial code unit element position as though they were represented using UTF-16. All operations on
Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers;
they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.
NOTE
The rationale behind this design was to keep the implementation of Strings as simple and high-performing as
possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text readfrom a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program28© Ecma International 2009

sees it. Usually this would occur at the same time incoming text is converted from its original character encoding toUnicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be inNormalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as longas they do not contain any Unicode escape sequences.

8.5

The Number TypeThe Number type has exactly 18437736874454810627 (that is, 264−253+3) values, representing the doubleprecision 64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic,
except that the 9007199254740990 (that is, 253−2) distinct “Not-a-Number” values of the IEEE Standard are
represented in ECMAScript as a single special NaN value. (Note that the NaN value is produced by the
program expression NaN.) In some implementations, external code might be able to detect a difference
between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code,
all NaN values are indistinguishable from each other.
There are two other special values, called positive Infinity and negative Infinity. For brevity, these values
are also referred to for expository purposes by the symbols +∞ and −∞, respectively. (Note that these two
infinite Number values are produced by the program expressions +Infinity (or simply Infinity) and Infinity.)
The other 18437736874454810624 (that is, 264−253) values are called the finite numbers. Half of these arepositive numbers and half are negative numbers; for every finite positive Number value there is acorresponding negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For brevity, these values are also referred to for
expository purposes by the symbols +0 and −0, respectively. (Note that these two different zero Number
values are produced by the program expressions +0 (or simply 0) and -0.)
The 18437736874454810622 (that is, 264−253−2) finite nonzero values are of two kinds:
18428729675200069632 (that is, 264−254) of them are normalised, having the forms × m × 2e
where s is +1 or −1, m is a positive integer less than 253 but not less than 252, and e is an integer ranging from−1074 to 971, inclusive.
The remaining 9007199254740990 (that is, 253−2) values are denormalised, having the forms × m × 2e
where s is +1 or −1, m is a positive integer less than 252, and e is −1074.
Note that all the positive and negative integers whose magnitude is no greater than 253 are representable in
the Number type (indeed, the integer 0 has two representations, +0 and -0).
A finite number has an odd significand if it is nonzero and the integer m used to express it (in one of the twoforms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x represents an exact nonzero realmathematical quantity (which might even be an irrational number such as π) means a Number value chosen in
the following manner. Consider the set of all finite values of the Number type, with −0 removed and with two
additional values added to it that are not representable in the Number type, namely 21024 (which is +1 × 253 ×2971) and −21024 (which is −1 × 253 × 2971). Choose the member of this set that is closest in value to x. If twovalues of the set are equally close, then the one with an even significand is chosen; for this purpose, the two
extra values 21024 and −21024 are considered to have even significands. Finally, if 21024 was chosen, replace itwith +∞; if −21024 was chosen, replace it with −∞; if +0 was chosen, replace it with −0 if and only if x is less thanzero; any other chosen value is used unchanged. The result is the Number value for x. (This procedurecorresponds exactly to the behaviour of the IEEE 754 “round to nearest” mode.)© Ecma International 200929

Some ECMAScript operators deal only with integers in the range −231 through 231−1, inclusive, or in the range
0 through 232−1, inclusive. These operators accept any value of the Number type but first convert each suchvalue to one of 232 integer values. See the descriptions of the ToInt32 and ToUint32 operators in 9.5 and 9.6,
respectively.

8.6

The Object TypeAn Object is a collection of properties. Each property is either a named data property, a named accessor
property, or an internal property:•

A named data property associates a name with an ECMAScript language value and a set of Boolean
attributes.

•

A named accessor property associates a name with one or two accessor functions, and a set of Boolean
attributes. The accessor functions are used to store or retrieve an ECMAScript language value that is
associated with the property.

•

An internal property has no name and is not directly accessible via ECMAScript language operators.
Internal properties exist purely for specification purposes.

There are two kinds of access for named (non-internal) properties: get and put, corresponding to retrieval andassignment, respectively.
8.6.1

Property Attributes

Attributes are used in this specification to define and explain the state of named properties. A named data
property associates a name with the attributes listed in Table 5
Table 5 — Attributes of a Named Data Property
Attribute Name[[Value]][[Writable]]

Value Domain
Any ECMAScript
language type
Boolean[[Enumerable]]

Boolean[[Configurable]]

BooleanDescription
The value retrieved by reading the property.
If false, attempts by ECMAScript code to change theproperty’s [[Value]] attribute using [[Put]] will not succeed.
If true, the property will be enumerated by a for-in
enumeration (see 12.6.4). Otherwise, the property is said
to be non-enumerable.
If false, attempts to delete the property, change theproperty to be an accessor property, or change its
attributes (other than [[Value]]) will fail.

A named accessor property associates a name with the attributes listed in Table 6.

30© Ecma International 2009

Table 6 — Attributes of a Named Accessor Property
Attribute Name[[Get]]

Value Domain
Object or
Undefined[[Set]]

Object or
Undefined[[Enumerable]]

Boolean[[Configurable]]

BooleanDescription
If the value is an Object it must be a function Object. The
function’s [[Call]] internal method (8.6.2) is called with anempty arguments list to return the property value each timea get access of the property is performed.
If the value is an Object it must be a function Object. The
function’s [[Call]] internal method (8.6.2) is called with anarguments list containing the assigned value as its soleargument each time a set access of the property is
performed. The effect of a property's [[Set]] internal method
may, but is not required to, have an effect on the value
returned by subsequent calls to the property's [[Get]]
internal method.
If true, the property is to be enumerated by a for-in
enumeration (see 12.6.4). Otherwise, the property is said tobe non-enumerable.
If false, attempts to delete the property, change theproperty to be a data property, or change its attributes willfail.

If the value of an attribute is not explicitly specified by this specification for a named property, the default value
defined in Table 7 is used.
Table 7 — Default Attribute Values
Attribute Name[[Value]][[Get]][[Set]][[Writable]][[Enumerable]][[Configurable]]

8.6.2

Default Value
undefined
undefined
undefined
false
false
false

Object Internal Properties and Methods

This specification uses various internal properties to define the semantics of object values. These internal
properties are not part of the ECMAScript language. They are defined by this specification purely for
expository purposes. An implementation of ECMAScript must behave as if it produced and operated uponinternal properties in the manner described here. The names of internal properties are enclosed in double
square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not
implement the indicated internal property, a TypeError exception is thrown.
The Table 8 summarises the internal properties used by this specification that are applicable to all
ECMAScript objects. The Table 9 summarises the internal properties used by this specification that are only
applicable to some ECMAScript objects. The descriptions in these tables indicates their behaviour for native
ECMAScript objects, unless stated otherwise in this document for particular kinds of native ECMAScript
objects. Host objects may support these internal properties with any implementation-dependent behaviour aslong as it is consistent with the specific host object restrictions stated in this document.
The “Value Type Domain” columns of the following tables define the types of values associated with internal
properties. The type names refer to the types defined in Clause 8 augmented by the following additional
names. “any” means the value may be any ECMAScript language type. “primitive” means Undefined, Null,
Boolean, String, or Number. “SpecOp” means the internal property is an internal method, an implementation
provided procedure defined by an abstract operation specification. “SpecOp” is followed by a list of descriptive© Ecma International 200931

parameter names. If a parameter name is the same as a type name then the name describes the type of theparameter. If a “SpecOp” returns a value, its parameter list is followed by the symbol “→” and the type of the
returned value.
Table 8 — Internal Properties Common to All Objects
Internal Property[[Prototype]][[Class]]

Value Type Domain
Object or Null
String[[Extensible]]

Boolean[[Get]]

SpecOp(propertyName) →any
SpecOp (propertyName) →Undefined or Property
Descriptor
SpecOp (propertyName) →Undefined or Property
Descriptor
SpecOp (propertyName,any, Boolean)[[GetOwnProperty]][[GetProperty]][[Put]][[CanPut]]

SpecOp (propertyName) →Boolean[[HasProperty]]

SpecOp (propertyName) →Boolean[[Delete]]

SpecOp (propertyName,
Boolean) → Boolean[[DefaultValue]]

SpecOp (Hint) → primitive[[DefineOwnProperty]]

SpecOp (propertyName,
PropertyDescriptor,
Boolean) → Boolean

Description
The prototype of this object.
A String value indicating a specification definedclassification of objects.
If true, own properties may be added to theobject.
Returns the value of the named property.
Returns the Property Descriptor of the named
own property of this object, or undefined if
absent.
Returns the fully populated Property Descriptorof the named property of this object, or
undefined if absent.
Sets the specified named property to the valueof the second parameter. The flag controls
failure handling.
Returns a Boolean value indicating whether a[[Put]] operation with PropertyName can be
performed.
Returns a Boolean value indicating whether theobject already has a property with the given
name.
Removes the specified named own property
from the object. The flag controls failure
handling.
Hint is a String. Returns a default value for theobject.
Creates or alters the named own property tohave the state described by a Property
Descriptor. The flag controls failure handling.

Every object (including host objects) must implement all of the internal properties listed in Table 8. However,the [[DefaultValue]] internal method may, for some objects, simply throw a TypeError exception.
All objects have an internal property called [[Prototype]]. The value of this property is either null or an object
and is used for implementing inheritance. Whether or not a native object can have a host object as its[[Prototype]] depends on the implementation. Every [[Prototype]] chain must have finite length (that is, startingfrom any object, recursively accessing the [[Prototype]] internal property must eventually lead to a null value).
Named data properties of the [[Prototype]] object are inherited (are visible as properties of the child object) forthe purposes of get access, but not for put access. Named accessor properties are inherited for both getaccess and put access.
Every ECMAScript object has a Boolean-valued [[Extensible]] internal property that controls whether or notnamed properties may be added to the object. If the value of the [[Extensible]] internal property is false thenadditional named properties may not be added to the object. In addition, if [[Extensible]] is false the value ofthe [[Class]] and [[Prototype]] internal properties of the object may not be modified. Once the value of an[[Extensible]] internal property has been set to false it may not be subsequently changed to true.
NOTE
This specification defines no ECMAScript language operators or built-in functions that permit a program tomodify an object’s [[Class]] or [[Prototype]] internal properties or to change the value of [[Extensible]] from false to true.

32© Ecma International 2009

Implementation specific extensions that modify [[Class]], [[Prototype]] or [[Extensible]] must not violate the invariants
defined in the preceding paragraph.

The value of the [[Class]] internal property is defined by this specification for every kind of built-in object. The
value of the [[Class]] internal property of a host object may be any String value except one of "Arguments","Array", "Boolean", "Date", "Error", "Function", "JSON", "Math", "Number", "Object","RegExp", and "String". The value of a [[Class]] internal property is used internally to distinguish different
kinds of objects. Note that this specification does not provide any means for a program to access that value
except through Object.prototype.toString (see 15.2.4.2).
Unless otherwise specified, the common internal methods of native ECMAScript objects behave as describedin 8.12. Array objects have a slightly different implementation of the [[DefineOwnProperty]] internal method(see 15.4.5.1) and String objects have a slightly different implementation of the [[GetOwnProperty]] internal
method (see 15.5.5.2). Arguments objects (10.6) have different implementations of [[Get]], [[GetOwnProperty]],[[DefineOwnProperty]], and [[Delete]]. Function objects (15.3) have a different implementation of [[Get]].
Host objects may implement these internal methods in any manner unless specified otherwise; for example,
one possibility is that [[Get]] and [[Put]] for a particular host object indeed fetch and store property values but[[HasProperty]] always generates false. However, if any specified manipulation of a host object's internal
properties is not supported by an implementation, that manipulation must throw a TypeError exception whenattempted.
The [[GetOwnProperty]] internal method of a host object must conform to the following invariants for each
property of the host object:•

If a property is described as a data property and it may return different values over time, then either orboth of the [[Writable]] and [[Configurable] attributes must be true even if no mechanism to change thevalue is exposed via the other internal methods.

•

If a property is described as a data property and its [[Writable]] and [[Configurable]] are both false, thenthe SameValue (according to 9.12) must be returned for the [[Value]] attribute of the property on all calls
to [[GetOwnProperty]].

•

If the attributes other than [[Writable]] may change over time or if the property might disappear, then the[[Configurable]] attribute must be true.

•

If the [[Writable]] attribute may change from false to true, then the [[Configurable]] attribute must be true.

•

If the value of the host object’s [[Extensible]] internal property is has been observed by ECMAScript code
to be false, then if a call to [[GetOwnProperty]] describes a property as non-existent all subsequent calls
must also describe that property as non-existent.

The [[DefineOwnProperty]] internal method of a host object must not permit the addition of a new property to ahost object if the [[Extensible]] internal property of that host object has been observed by ECMAScript code to
be false.
If the [[Extensible]] internal property of that host object has been observed by ECMAScript code to be false
then it must not subsequently become true.

© Ecma International 200933

Table 9 — Internal Properties Only Defined for Some Objects
Internal Property[[PrimitiveValue]]

Value Type
Domainprimitive[[Construct]]

SpecOp(a List ofany) → Object[[Call]]SpecOp(any, a Listof any) → any orReference[[HasInstance]]

SpecOp(any) →Boolean[[Scope]]

Lexical Environment[[FormalParameters]]

List of Strings[[Code]]ECMAScript code[[TargetFunction]]Object[[BoundThis]]any[[BoundArguments]]

List of any[[Match]]SpecOp(String,index) →MatchResult[[ParameterMap]]

Object8.7

Description
Internal state information associated with this object. Of thestandard built-in ECMAScript objects, only Boolean, Date,
Number, and String objects implement [[PrimitiveValue]].
Creates an object. Invoked via the new operator. The
arguments to the SpecOp are the arguments passed to thenew operator. Objects that implement this internal method
are called constructors.
Executes code associated with the object. Invoked via afunction call expression. The arguments to the SpecOp area this object and a list containing the arguments passed to
the function call expression. Objects that implement thisinternal method are callable. Only callable objects that are
host objects may return Reference values.
Returns a Boolean value indicating whether the argument islikely an Object that was constructed by this object. Of thestandard built-in ECMAScript objects, only Function objects
implement [[HasInstance]].
A lexical environment that defines the environment in whicha Function object is executed. Of the standard built-in
ECMAScript objects, only Function objects implement[[Scope]].
A possibly empty List containing the identifier Strings of a
Function’s FormalParameterList. Of the standard built-in
ECMAScript objects, only Function objects implement[[FormalParameterList]].
The ECMAScript code of a function. Of the standard built-in
ECMAScript objects, only Function objects implement[[Code]].
The target function of a function object created using thestandard built-in Function.prototype.bind method. Only
ECMAScript objects created using Function.prototype.bindhave a [[TargetFunction]] internal property.
The pre-bound this value of a function Object created using
the standard built-in Function.prototype.bind method. Only
ECMAScript objects created using Function.prototype.bindhave a [[BoundThis]] internal property.
The pre-bound argument values of a function Object createdusing the standard built-in Function.prototype.bind method.
Only
ECMAScript
objects
createdusing
Function.prototype.bind have a [[BoundArguments]] internal
property.
Tests for a regular expression match and returns a
MatchResult value (see 15.10.2.1). Of the standard built-in
ECMAScript objects, only RegExp objects implement[[Match]].
Provides a mapping between the properties of an arguments
object (see 10.6) and the formal parameters of theassociated function. Only ECMAScript objects that arearguments objects have a [[ParameterMap]] internal
property.

The Reference Specification TypeThe Reference type is used to explain the behaviour of such operators as delete, typeof, and theassignment operators. For example, the left-hand operand of an assignment is expected to produce a34© Ecma International 2009

reference. The behaviour of assignment could, instead, be explained entirely in terms of a case analysis on
the syntactic form of the left-hand operand of an assignment operator, but for one difficulty: function calls are
permitted to return references. This possibility is admitted purely for the sake of host objects. No built-in
ECMAScript function defined by this specification returns a reference and there is no provision for a userdefined function to return a reference. (Another reason not to use a syntactic case analysis is that it would belengthy and awkward, affecting many parts of the specification.)
A Reference is a resolved name binding. A Reference consists of three components, the base value, thereferenced name and the Boolean valued strict reference flag. The base value is either undefined, an Object, a
Boolean, a String, a Number, or an environment record (10.2.1). A base value of undefined indicates that thereference could not be resolved to a binding. The referenced name is a String.
The following abstract operations are used in this specification to access the components of references:•

GetBase(V). Returns the base value component of the reference V.

•

GetReferencedName(V). Returns the referenced name component of the reference V.

•

IsStrictReference(V). Returns the strict reference component of the reference V.

•

HasPrimitiveBase(V). Returns true if the base value is a Boolean, String, or Number.

•

IsPropertyReference(V). Returns true if either the base value is an object or HasPrimitiveBase(V) is true;
otherwise returns false.

•

IsUnresolvableReference(V). Returns true if the base value is undefined and false otherwise.

The following abstract operations are used in this specification to operate on references:
8.7.1
1.
2.
3.
4.

5.

GetValue (V)If Type(V) is not Reference, return V.
Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), throw a ReferenceError exception.
If IsPropertyReference(V), thena. If HasPrimitiveBase(V) is false, then let get be the [[Get]] internal method of base, otherwise let get
be the special [[Get]] internal method defined below.
b. Return the result of calling the get internal method using base as its this value, and passing
GetReferencedName(V) for the argument.
Else, base must be an environment record.
a. Return the result of calling the GetBindingValue (see 10.2.1) concrete method of base passing
GetReferencedName(V) and IsStrictReference(V) as arguments.

The following [[Get]] internal method is used by GetValue when V is a property reference with a primitive basevalue. It is called using base as its this value and with property P as its argument. The following steps aretaken:
1.
2.
3.
4.
5.
6.
7.

Let O be ToObject(base).
Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
If desc is undefined, return undefined.
If IsDataDescriptor(desc) is true, return desc.[[Value]].
Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
If getter is undefined, return undefined.
Return the result calling the [[Call]] internal method of getter providing base as the this value and providing
no arguments.

NOTE
The object that may be created in step 1 is not accessible outside of the above method. An implementation
might choose to avoid the actual creation of the object. The only situation where such an actual property access that uses
this internal method can have visible effect is when it invokes an accessor function.

8.7.2
1.

PutValue (V, W)

If Type(V) is not Reference, throw a ReferenceError exception.

© Ecma International 2009

35

2.
3.

4.

5.
6.

Let base be the result of calling GetBase(V).
If IsUnresolvableReference(V), thena. If IsStrictReference(V) is true, theni.
Throw ReferenceError exception.
b. Call the [[Put]] internal method of the global object, passing GetReferencedName(V) for theproperty name, W for the value, and false for the Throw flag.
Else if IsPropertyReference(V), thena. If HasPrimitiveBase(V) is false, then let put be the [[Put]] internal method of base, otherwise let put
be the special [[Put]] internal method defined below.
b. Call the put internal method using base as its this value, and passing GetReferencedName(V) for theproperty name, W for the value, and IsStrictReference(V) for the Throw flag.
Else base must be a reference whose base is an environment record. So,
a. Call the SetMutableBinding (10.2.1) concrete method of base, passing GetReferencedName(V), W,
and IsStrictReference(V) as arguments.
Return.

The following [[Put]] internal method is used by PutValue when V is a property reference with a primitive basevalue. It is called using base as its this value and with property P, value W, and Boolean flag Throw asarguments. The following steps are taken:
1.
2.
3.
4.
5.
6.

7.
8.

Let O be ToObject(base).
If the result of calling the [[CanPut]] internal method of O with argument P is false, thena. If Throw is true, then throw a TypeError exception.
b. Else return.
Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
If IsDataDescriptor(ownDesc) is true, thena. If Throw is true, then throw a TypeError exception.
b. Else Return.
Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This may beeither an own or inherited accessor property descriptor or an inherited data property descriptor.
If IsAccessorDescriptor(desc) is true, thena. Let setter be desc.[[Set]] which cannot be undefined.
b. Call the [[Call]] internal method of setter providing base as the this value and an argument list
containing only W.
Else, this is a request to create an own property on the transient object O
a. If Throw is true, then throw a TypeError exception.
Return.

NOTE
The object that may be created in step 1 is not accessible outside of the above method. An implementation
might choose to avoid the actual creation of that transient object. The only situations where such an actual property
assignment that uses this internal method can have visible effect are when it either invokes an accessor function or is inviolation of a Throw predicated error check. When Throw is true any property assignment that would create a new propertyon the transient object throws an error.

8.8

The List Specification TypeThe List type is used to explain the evaluation of argument lists (see 11.2.4) in new expressions, in function
calls, and in other algorithms where a simple list of values is needed. Values of the List type are simply
ordered sequences of values. These sequences may be of any length.

8.9

The Completion Specification TypeThe Completion type is used to explain the behaviour of statements (break, continue, return and throw)that perform nonlocal transfers of control. Values of the Completion type are triples of the form (type, value,
target), where type is one of normal, break, continue, return, or throw, value is any ECMAScript language
value or empty, and target is any ECMAScript identifier or empty.
The term “abrupt completion” refers to any completion with a type other than normal.

36© Ecma International 2009

8.10 The Property Descriptor and Property Identifier Specification Types
The Property Descriptor type is used to explain the manipulation and reification of named property attributes.
Values of the Property Descriptor type are records composed of named fields where each field’s name is an
attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may
be present or absent.
Property Descriptor values may be further classified as data property descriptors and accessor property
descriptors based upon the existence or use of certain fields. A data property descriptor is one that includes
any fields named either [[Value]] or [[Writable]]. An accessor property descriptor is one that includes any fields
named either [[Get]] or [[Set]]. Any property descriptor may have fields named [[Enumerable]] and[[Configurable]]. A Property Descriptor value may not be both a data property descriptor and an accessor
property descriptor; however, it may be neither. A generic property descriptor is a Property Descriptor valuethat is neither a data property descriptor nor an accessor property descriptor. A fully populated property
descriptor is one that is either an accessor property descriptor or a data property descriptor and that has all of
the fields that correspond to the property attributes defined in either 8.6.1 Table 5 or Table 6.
For notational convenience within this specification, an object literal-like syntax can be used to define aproperty descriptor value. For example, Property Descriptor {[[Value]]: 42, [[Writable]]: false, [[Configurable]]:true} defines a data property descriptor. Field name order is not significant. Any fields that are not explicitly
listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Property
Descriptor. For example, if D is a property descriptor then D.[[Value]] is shorthand for “the field of D named[[Value]]”.
The Property Identifier type is used to associate a property name with a Property Descriptor. Values of theProperty Identifier type are pairs of the form (name, descriptor), where name is a String and descriptor is a
Property Descriptor value.
The following abstract operations are used in this specification to operate upon Property Descriptor values:
8.10.1 IsAccessorDescriptor ( Desc )When the abstract operation IsAccessorDescriptor is called with property descriptor Desc, the following steps
are taken:
1.
2.
3.

If Desc is undefined, then return false.
If both Desc.[[Get]] and Desc.[[Set]] are absent, then return false.
Return true.

8.10.2 IsDataDescriptor ( Desc )When the abstract operation IsDataDescriptor is called with property descriptor Desc, the following steps aretaken:
1.
2.
3.

If Desc is undefined, then return false.
If both Desc.[[Value]] and Desc.[[Writable]] are absent, then return false.
Return true.

8.10.3 IsGenericDescriptor ( Desc )When the abstract operation IsGenericDescriptor is called with property descriptor Desc, the following steps
are taken:
1.
2.
3.

If Desc is undefined, then return false.
If IsAccessorDescriptor(Desc) and IsDataDescriptor(Desc) are both false, then return true.
Return false.

© Ecma International 200937

8.10.4 FromPropertyDescriptor ( Desc )When the abstract operation FromPropertyDescriptor is called with property descriptor Desc, the following
steps are taken:
The following algorithm assumes that Desc is a fully populated Property Descriptor, such as that returned from[[GetOwnProperty]] (see 8.12.1).
1.
2.
3.

4.

5.
6.
7.

If Desc is undefined, then return undefined.
Let obj be the result of creating a new object as if by the expression new Object() where Object is the standard
built-in constructor with that name.
If IsDataDescriptor(Desc) is true, thena. Call the [[DefineOwnProperty]] internal method of obj with arguments "value", Property Descriptor{[[Value]]: Desc.[[Value]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
b. Call the [[DefineOwnProperty]] internal method of obj with arguments "writable", Property Descriptor{[[Value]]: Desc.[[Writable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Else, IsAccessorDescriptor(Desc) must be true, so
a. Call the [[DefineOwnProperty]] internal method of obj with arguments "get", Property Descriptor{[[Value]]: Desc.[[Get]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
b. Call the [[DefineOwnProperty]] internal method of obj with arguments "set", Property Descriptor{[[Value]]: Desc.[[Set]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Call the [[DefineOwnProperty]] internal method of obj with arguments "enumerable", Property Descriptor{[[Value]]: Desc.[[Enumerable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Call the [[DefineOwnProperty]] internal method of obj with arguments "configurable", Property Descriptor{[[Value]]: Desc.[[Configurable]], [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
Return obj.

8.10.5 ToPropertyDescriptor ( Obj )When the abstract operation ToPropertyDescriptor is called with object Desc, the following steps are taken:
1.
2.
3.

If Type(Obj) is not Object throw a TypeError exception.
Let desc be the result of creating a new Property Descriptor that initially has no fields.
If the result of calling the [[HasProperty]] internal method of Obj with argument "enumerable" is true,
thena. Let enum be the result of calling the [[Get]] internal method of Obj with "enumerable".
b. Set the [[Enumerable]] field of desc to ToBoolean(enum).
4. If the result of calling the [[HasProperty]] internal method of Obj with argument "configurable" is true,
thena. Let conf be the result of calling the [[Get]] internal method of Obj with argument"configurable".
b. Set the [[Configurable]] field of desc to ToBoolean(conf).
5. If the result of calling the [[HasProperty]] internal method of Obj with argument "value" is true, thena. Let value be the result of calling the [[Get]] internal method of Obj with argument “value”.
b. Set the [[Value]] field of desc to value.
6. If the result of calling the [[HasProperty]] internal method of Obj with argument "writable" is true, thena. Let writable be the result of calling the [[Get]] internal method of Obj with argument "writable".
b. Set the [[Writable]] field of desc to ToBoolean(writable).
7. If the result of calling the [[HasProperty]] internal method of Obj with argument "get" is true, thena. Let getter be the result of calling the [[Get]] internal method of Obj with argument "get".
b. If IsCallable(getter) is false and getter is not undefined, then throw a TypeError exception.
c. Set the [[Get]] field of desc to getter.
8. If the result of calling the [[HasProperty]] internal method of Obj with argument "set" is true, thena. Let setter be the result of calling the [[Get]] internal method of Obj with argument "set".
b. If IsCallable(setter) is false and setter is not undefined, then throw a TypeError exception.
c. Set the [[Set]] field of desc to setter.
9. If either desc.[[Get]] or desc.[[Set]] are present, thena. If either desc.[[Value]] or desc.[[Writable]] are present, then throw a TypeError exception.
10. Return desc.

38© Ecma International 2009

8.11 The Lexical Environment and Environment Record Specification Types
The Lexical Environment and Environment Record types are used to explain the behaviour of name resolution
in nested functions and blocks. These types and the operations upon them are defined in Clause 10.

8.12 Algorithms for Object Internal Methods
In the following algorithm descriptions, assume O is a native ECMAScript object, P is a String, Desc is a
Property Description record, and Throw is a Boolean flag.
8.12.1 [[GetOwnProperty]] (P)When the [[GetOwnProperty]] internal method of O is called with property name P, the following steps aretaken:
1.
2.
3.
4.
5.
6.
7.
8.

If O doesn’t have an own property with name P, return undefined.
Let D be a newly created Property Descriptor with no fields.
Let X be O’s own property named P.
If X is a data property, thena. Set D.[[Value]] to the value of X’s [[Value]] attribute.
b. Set D.[[Writable]] to the value of X’s [[Writable]] attribute
Else X is an accessor property, so
a. Set D.[[Get]] to the value of X’s [[Get]] attribute.
b. Set D.[[Set]] to the value of X’s [[Set]] attribute.
Set D.[[Enumerable]] to the value of X’s [[Enumerable]] attribute.
Set D.[[Configurable]] to the value of X’s [[Configurable]] attribute.
Return D.

However, if O is a String object it has a more elaborate [[GetOwnProperty]] internal method defined in 15.5.5.2.
8.12.2 [[GetProperty]] (P)When the [[GetProperty]] internal method of O is called with property name P, the following steps are taken:
1.
2.
3.
4.
5.

Let prop be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
If prop is not undefined, return prop.
Let proto be the value of the [[Prototype]] internal property of O.
If proto is null, return undefined.
Return the result of calling the [[GetProperty]] internal method of proto with argument P.

8.12.3 [[Get]] (P)When the [[Get]] internal method of O is called with property name P, the following steps are taken:
8. Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
9. If desc is undefined, return undefined.
10. If IsDataDescriptor(desc) is true, return desc.[[Value]].
11. Otherwise, IsAccessorDescriptor(desc) must be true so, let getter be desc.[[Get]].
12. If getter is undefined, return undefined.
13. Return the result calling the [[Call]] internal method of getter providing O as the this value and providing noarguments.
8.12.4 [[CanPut]] (P)When the [[CanPut]] internal method of O is called with property name P, the following steps are taken:
1.
2.

Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
If desc is not undefined, thena. If IsAccessorDescriptor(desc) is true, then© Ecma International 2009

39

3.
4.
5.
6.
7.
8.

i.
If desc.[[Set]] is undefined, then return false.
ii.
Else return true.
b. Else, desc must be a DataDescriptor so return the value of desc.[[Writable]].
Let proto be the [[Prototype]] internal property of O.
If proto is null, then return the value of the [[Extensible]] internal property of O.
Let inherited be the result of calling the [[GetProperty]] internal method of proto with property name P.
If inherited is undefined, return the value of the [[Extensible]] internal property of O.
If IsAccessorDescriptor(inherited) is true, thena. If inherited.[[Set]] is undefined, then return false.
b. Else return true.
Else, inherited must be a DataDescriptor
a. If the [[Extensible]] internal property of O is false, return false.
b. Else return the value of inherited.[[Writable]].

Host objects may define additional constraints upon [[Put]] operations. If possible, host objects should notallow [[Put]] operations in situations where this definition of [[CanPut]] returns false.
8.12.5 [[Put]] ( P, V, Throw )When the [[Put]] internal method of O is called with property P, value V, and Boolean flag Throw, the following
steps are taken:
1.
2.
3.

4.
5.

6.

7.

If the result of calling the [[CanPut]] internal method of O with argument P is false, thena. If Throw is true, then throw a TypeError exception.
b. Else return.
Let ownDesc be the result of calling the [[GetOwnProperty]] internal method of O with argument P.
If IsDataDescriptor(ownDesc) is true, thena. Let valueDesc be the Property Descriptor {[[Value]]: V}.
b. Call the [[DefineOwnProperty]] internal method of O passing P, valueDesc, and Throw asarguments.
c. Return.
Let desc be the result of calling the [[GetProperty]] internal method of O with argument P. This may beeither an own or inherited accessor property descriptor or an inherited data property descriptor.
If IsAccessorDescriptor(desc) is true, thena. Let setter be desc.[[Set]] which cannot be undefined.
b. Call the [[Call]] internal method of setter providing O as the this value and providing V as the soleargument.
Else, create a named data property named P on object O as follows
a. Let newDesc be the Property Descriptor{[[Value]]: V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}.
b. Call the [[DefineOwnProperty]] internal method of O passing P, newDesc, and Throw as arguments.
Return.

8.12.6 [[HasProperty]] (P)When the [[HasProperty]] internal method of O is called with property name P, the following steps are taken:
1.
2.
3.

Let desc be the result of calling the [[GetProperty]] internal method of O with property name P.
If desc is undefined, then return false.
Else return true.

8.12.7 [[Delete]] (P, Throw)
When the [[Delete]] internal method of O is called with property name P and the Boolean flag Throw, thefollowing steps are taken:
1.
2.
3.

40

Let desc be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
If desc is undefined, then return true.
If desc.[[Configurable]] is true, thena. Remove the own property with name P from O.

© Ecma International 2009

4.
5.

b. Return true.
Else if Throw, then throw a TypeError exception.
Return false.

8.12.8 [[DefaultValue]] (hint)
When the [[DefaultValue]] internal method of O is called with hint String, the following steps are taken:
1.
2.

3.
4.

5.

Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
If IsCallable(toString) is true then,a. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value andan empty argument list.
b. If str is a primitive value, return str.
Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
If IsCallable(valueOf) is true then,a. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value andan empty argument list.
b. If val is a primitive value, return val.
Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:
1.
2.

3.
4.

5.

Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
If IsCallable(valueOf) is true then,a. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value andan empty argument list.
b. If val is a primitive value, return val.
Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
If IsCallable(toString) is true then,a. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value andan empty argument list.
b. If str is a primitive value, return str.
Throw a TypeError exception.

When the [[DefaultValue]] internal method of O is called with no hint, then it behaves as if the hint were
Number, unless O is a Date object (see 15.9.6), in which case it behaves as if the hint were String.
The above specification of [[DefaultValue]] for native objects can return only primitive values. If a host object
implements its own [[DefaultValue]] internal method, it must ensure that its [[DefaultValue]] internal method
can return only primitive values.
8.12.9 [[DefineOwnProperty]] (P, Desc, Throw)
In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError exception,otherwise return false”. The algorithm contains steps that test various fields of the Property Descriptor Desc for
specific values. The fields that are tested in this manner need not actually exist in Desc. If a field is absent
then its value is considered to be false.
When the [[DefineOwnProperty]] internal method of O is called with property name P, property descriptor Desc,
and Boolean flag Throw, the following steps are taken:
1.
2.
3.
4.

Let current be the result of calling the [[GetOwnProperty]] internal method of O with property name P.
Let extensible be the value of the [[Extensible]] internal property of O.
If current is undefined and extensible is false, then Reject.
If current is undefined and extensible is true, thena. If IsGenericDescriptor(Desc) or IsDataDescriptor(Desc) is true, theni.
Create an own data property named P of object O whose [[Value]], [[Writable]],[[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value ofan attribute field of Desc is absent, the attribute of the newly created property is set to its
default value.

© Ecma International 2009

41

b.

Else, Desc must be an accessor Property Descriptor so,i.
Create an own accessor property named P of object O whose [[Get]], [[Set]],[[Enumerable]] and [[Configurable]] attribute values are described by Desc. If the value ofan attribute field of Desc is absent, the attribute of the newly created property is set to its
default value.
c. Return true.
5. Return true, if every field in Desc is absent.
6. Return true, if every field in Desc also occurs in current and the value of every field in Desc is the samevalue as the corresponding field in current when compared using the SameValue algorithm (9.12).
7. If the [[Configurable]] field of current is false thena. Reject, if the [[Configurable]] field of Desc is true.
b. Reject, if the [[Enumerable]] field of Desc is present and the [[Enumerable]] fields of current andDesc are the Boolean negation of each other.
8. If IsGenericDescriptor(Desc) is true, then no further validation is required.
9. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) have different results, thena. Reject, if the [[Configurable]] field of current is false.
b. If IsDataDescriptor(current) is true, theni.
Convert the property named P of object O from a data property to an accessor property.
Preserve the existing values of the converted property’s [[Configurable]] and[[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
c. Else,i.
Convert the property named P of object O from an accessor property to a data property.
Preserve the existing values of the converted property’s [[Configurable]] and[[Enumerable]] attributes and set the rest of the property’s attributes to their default values.
10. Else, if IsDataDescriptor(current) and IsDataDescriptor(Desc) are both true, thena. If the [[Configurable]] field of current is false, theni.
Reject, if the [[Writable]] field of current is false and the [[Writable]] field of Desc is true.
ii.
If the [[Writable]] field of current is false, then
1. Reject, if the [[Value]] field of Desc is present and SameValue(Desc.[[Value]],current.[[Value]]) is false.
b. else, the [[Configurable]] field of current is true, so any change is acceptable.
11. Else, IsAccessorDescriptor(current) and IsAccessorDescriptor(Desc) are both true so,a. If the [[Configurable]] field of current is false, theni.
Reject, if the [[Set]] field of Desc is present and SameValue(Desc.[[Set]], current.[[Set]]) is
false.
ii.
Reject, if the [[Get]] field of Desc is present and SameValue(Desc.[[Get]], current.[[Get]])is false.
12. For each attribute field of Desc that is present, set the correspondingly named attribute of the property
named P of object O to the value of the field.
13. Return true.
However, if O is an Array object, it has a more elaborate [[DefineOwnProperty]] internal method defined in
15.4.5.1.
NOTE
Step 10.b allows any field of Desc to be different from the corresponding field of current if current’s[[Configurable]] field is true. This even permits changing the [[Value]] of a property whose [[Writable]] attribute is false.
This is allowed because a true [[Configurable]] attribute would permit an equivalent sequence of calls where [[Writable]] isfirst set to true, a new [[Value]] is set, and then [[Writable]] is set to false.

9Type Conversion and Testing

The ECMAScript runtime system performs automatic type conversion as needed. To clarify the semantics of
certain constructs it is useful to define a set of conversion abstract operations. These abstract operations are
not a part of the language; they are defined here to aid the specification of the semantics of the language. The
conversion abstract operations are polymorphic; that is, they can accept a value of any ECMAScript language
type, but not of specification types.

42© Ecma International 2009

9.1

ToPrimitive

The abstract operation ToPrimitive takes an input argument and an optional argument PreferredType. The
abstract operation ToPrimitive converts its input argument to a non-Object type. If an object is capable ofconverting to more than one primitive type, it may use the optional hint PreferredType to favour that type.
Conversion occurs according to Table 10:
Table 10 — ToPrimitive Conversions

9.2

Input Type

ResultUndefined

The result equals the input argument (no conversion).

Null

The result equals the input argument (no conversion).

Boolean

The result equals the input argument (no conversion).

Number

The result equals the input argument (no conversion).

String

The result equals the input argument (no conversion).

ObjectReturn a default value for the Object. The default value of an object isretrieved by calling the [[DefaultValue]] internal method of the object,passing the optional hint PreferredType. The behaviour of the[[DefaultValue]] internal method is defined by this specification for all native
ECMAScript objects in 8.12.8.

ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:
Table 11 — ToBoolean Conversions

9.3

Argument Type

ResultUndefined

false

Null

false

Boolean

The result equals the input argument (no conversion).

Number

The result is false if the argument is +0, −0, or NaN; otherwise the result is
true.

String

The result is false if the argument is the empty String (its length is zero);otherwise the result is true.

Objecttrue

ToNumber

The abstract operation ToNumber converts its argument to a value of type Number according to Table 12:© Ecma International 200943

Table 12 — To Number Conversions

9.3.1

Argument Type

ResultUndefined

NaN

Null

+0

Boolean

The result is 1 if the argument is true. The result is +0 if the argument isfalse.

Number

The result equals the input argument (no conversion).

StringSee grammar and note below.

ObjectApply the following steps:
1. Let primValue be ToPrimitive(input argument, hint Number).
2. Return ToNumber(primValue).

ToNumber Applied to the String Type

ToNumber applied to Strings applies the following grammar to the input String. If the grammar cannot interpret
the String as an expansion of StringNumericLiteral, then the result of ToNumber is NaN.
StringNumericLiteral :::StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt
StrWhiteSpace :::StrWhiteSpaceChar StrWhiteSpaceopt
StrWhiteSpaceChar :::WhiteSpace
LineTerminator
StrNumericLiteral :::StrDecimalLiteral
HexIntegerLiteral
StrDecimalLiteral :::StrUnsignedDecimalLiteral+ StrUnsignedDecimalLiteral- StrUnsignedDecimalLiteral
StrUnsignedDecimalLiteral :::Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt
DecimalDigits :::DecimalDigit
DecimalDigits DecimalDigit
DecimalDigit ::: one of
0 1 2 3 4

5

6

7

8

9

ExponentPart :::ExponentIndicator SignedInteger
ExponentIndicator ::: one ofe E

44© Ecma International 2009

SignedInteger :::DecimalDigits+ DecimalDigits- DecimalDigits
HexIntegerLiteral :::0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit
HexDigit ::: one of
0 1 2 3

4

5

6

7

8

9

a

b

c

d

e

f

A

B

C

D

E

F

Some differences should be noted between the syntax of a StringNumericLiteral and a NumericLiteral (see7.8.3):••••A StringNumericLiteral may be preceded and/or followed by white space and/or line terminators.
A StringNumericLiteral that is decimal may have any number of leading 0 digits.
A StringNumericLiteral that is decimal may be preceded by + or - to indicate its sign.
A StringNumericLiteral that is empty or contains only white space is converted to +0.

The conversion of a String to a Number value is similar overall to the determination of the Number value for anumeric literal (see 7.8.3), but some of the details are different, so the process for converting a String numeric
literal to a value of Number type is given here in full. This value is determined in two steps: first, amathematical value (MV) is derived from the String numeric literal; second, this mathematical value is roundedas described below.
•
•
•
•
•
•
•
•

•
•
•

•
•

•
•
•
•
•

The MV of StringNumericLiteral ::: [empty] is 0.
The MV of StringNumericLiteral ::: StrWhiteSpace is 0.
The MV of StringNumericLiteral ::: StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt is the MV of
StrNumericLiteral, no matter whether white space is present or not.
The MV of StrNumericLiteral ::: StrDecimalLiteral is the MV of StrDecimalLiteral.
The MV of StrNumericLiteral ::: HexIntegerLiteral is the MV of HexIntegerLiteral.
The MV of StrDecimalLiteral ::: StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: + StrUnsignedDecimalLiteral is the MV of StrUnsignedDecimalLiteral.
The MV of StrDecimalLiteral ::: - StrUnsignedDecimalLiteral is the negative of the MV of
StrUnsignedDecimalLiteral. (Note that if the MV of StrUnsignedDecimalLiteral is 0, the negative of this MV is
also 0. The rounding rule described below handles the conversion of this signless mathematical zero to afloating-point +0 or −0 as appropriate.)
The MV of StrUnsignedDecimalLiteral::: Infinity is 1010000 (a value so large that it will round to +∞).
The MV of StrUnsignedDecimalLiteral::: DecimalDigits. is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral::: DecimalDigits . DecimalDigits is the MV of the first DecimalDigits
plus (the MV of the second DecimalDigits times 10−n), where n is the number of characters in the second
DecimalDigits.
The MV of StrUnsignedDecimalLiteral::: DecimalDigits. ExponentPart is the MV of DecimalDigits times 10e,where e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral::: DecimalDigits. DecimalDigits ExponentPart is (the MV of the first
DecimalDigits plus (the MV of the second DecimalDigits times 10−n)) times 10e, where n is the number of charactersin the second DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral:::. DecimalDigits is the MV of DecimalDigits times 10−n, where n is thenumber of characters in DecimalDigits.
The MV of StrUnsignedDecimalLiteral:::. DecimalDigits ExponentPart is the MV of DecimalDigits times 10e−n,where n is the number of characters in DecimalDigits and e is the MV of ExponentPart.
The MV of StrUnsignedDecimalLiteral::: DecimalDigits is the MV of DecimalDigits.
The MV of StrUnsignedDecimalLiteral::: DecimalDigits ExponentPart is the MV of DecimalDigits times 10e,where e is the MV of ExponentPart.
The MV of DecimalDigits ::: DecimalDigit is the MV of DecimalDigit.

© Ecma International 2009

45

•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

The MV of DecimalDigits ::: DecimalDigits DecimalDigit is (the MV of DecimalDigits times 10) plus the MV of
DecimalDigit.
The MV of ExponentPart ::: ExponentIndicator SignedInteger is the MV of SignedInteger.
The MV of SignedInteger ::: DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger ::: + DecimalDigits is the MV of DecimalDigits.
The MV of SignedInteger ::: - DecimalDigits is the negative of the MV of DecimalDigits.
The MV of DecimalDigit ::: 0 or of HexDigit ::: 0 is 0.
The MV of DecimalDigit ::: 1 or of HexDigit ::: 1 is 1.
The MV of DecimalDigit ::: 2 or of HexDigit ::: 2 is 2.
The MV of DecimalDigit ::: 3 or of HexDigit ::: 3 is 3.
The MV of DecimalDigit ::: 4 or of HexDigit ::: 4 is 4.
The MV of DecimalDigit ::: 5 or of HexDigit ::: 5 is 5.
The MV of DecimalDigit ::: 6 or of HexDigit ::: 6 is 6.
The MV of DecimalDigit ::: 7 or of HexDigit ::: 7 is 7.
The MV of DecimalDigit ::: 8 or of HexDigit ::: 8 is 8.
The MV of DecimalDigit ::: 9 or of HexDigit ::: 9 is 9.
The MV of HexDigit ::: a or of HexDigit ::: A is 10.
The MV of HexDigit ::: b or of HexDigit ::: B is 11.
The MV of HexDigit ::: c or of HexDigit ::: C is 12.
The MV of HexDigit ::: d or of HexDigit ::: D is 13.
The MV of HexDigit ::: e or of HexDigit ::: E is 14.
The MV of HexDigit ::: f or of HexDigit ::: F is 15.
The MV of HexIntegerLiteral ::: 0x HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: 0X HexDigit is the MV of HexDigit.
The MV of HexIntegerLiteral ::: HexIntegerLiteral HexDigit is (the MV of HexIntegerLiteral times 16) plus the
MV of HexDigit.

Once the exact MV for a String numeric literal has been determined, it is then rounded to a value of the
Number type. If the MV is 0, then the rounded value is +0 unless the first non white space character in theString numeric literal is ‘-’, in which case the rounded value is −0. Otherwise, the rounded value must be the
Number value for the MV (in the sense defined in 8.5), unless the literal includes a StrUnsignedDecimalLiteraland the literal has more than 20 significant digits, in which case the Number value may be either the Number
value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit or the
Number value for the MV of a literal produced by replacing each significant digit after the 20th with a 0 digit
and then incrementing the literal at the 20th digit position. A digit is significant if it is not part of an ExponentPart
and• it is not 0; or• there is a nonzero digit to its left and there is a nonzero digit, not in the ExponentPart, to its right.

9.4

ToInteger

The abstract operation ToInteger converts its argument to an integral numeric value. This abstract operation
functions as follows:
1.
2.
3.
4.

9.5

Let number be the result of calling ToNumber on the input argument.
If number is NaN, return +0.
If number is +0, −0, +∞, or −∞, return number.
Return the result of computing sign(number) * floor(abs(number)).

ToInt32: (Signed 32 Bit Integer)

The abstract operation ToInt32 converts its argument to one of 232 integer values in the range −231 through231−1, inclusive. This abstract operation functions as follows:46© Ecma International 2009

1.
2.
3.
4.
5.

Let number be the result of calling ToNumber on the input argument.
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let posInt be sign(number) * floor(abs(number)).
Let int32bit be posInt modulo 2 32; that is, a finite integer value k of Number type with positive sign and lessthan 2 32 in magnitude such that the mathematical difference of posInt and k is mathematically an integer
multiple of 2 32.
If int32bit is greater than or equal to 2 31, return int32bit − 2 32, otherwise return int32bit.

NOTE
•
•
•

Given the above definition of ToInt32:

The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves thatvalue unchanged.
ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are
mapped to +0.)
ToInt32 maps −0 to +0.

9.6

ToUint32: (Unsigned 32 Bit Integer)

The abstract operation ToUint32 converts its argument to one of 232 integer values in the range 0 through 232−1,
inclusive. This abstraction operation functions as follows:
1.
2.
3.
4.
5.

Let number be the result of calling ToNumber on the input argument.
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let posInt be sign(number) * floor(abs(number)).
Let int32bit be posInt modulo 2 32; that is, a finite integer value k of Number type with positive sign and lessthan 2 32 in magnitude such that the mathematical difference of posInt and k is mathematically an integer
multiple of 2 32.
Return int32bit.

NOTE
•
•
•
•

Given the above definition of ToUInt32:

Step 5 is the only difference between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves
that value unchanged.
ToUint32(ToInt32(x)) is equal to ToUint32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are
mapped to +0.)
ToUint32 maps −0 to +0.

9.7

ToUint16: (Unsigned 16 Bit Integer)

The abstract operation ToUint16 converts its argument to one of 216 integer values in the range 0 through 216−1,
inclusive. This abstract operation functions as follows:
1.
2.
3.
4.
5.

Let number be the result of calling ToNumber on the input argument.
If number is NaN, +0, −0, +∞, or −∞, return +0.
Let posInt be sign(number) * floor(abs(number)).
Let int16bit be posInt modulo 2 16; that is, a finite integer value k of Number type with positive sign and lessthan 2 16 in magnitude such that the mathematical difference of posInt and k is mathematically an integer
multiple of 2 16.
Return int16bit.

NOTE
•
•

9.8

Given the above definition of ToUint16:

The substitution of 216 for 232 in step 4 is the only difference between ToUint32 and ToUint16.
ToUint16 maps −0 to +0.

ToString

The abstract operation ToString converts its argument to a value of type String according to Table 13:© Ecma International 200947

Table 13 — ToString Conversions
Argument Type

ResultUndefined"undefined"

Null"null"

BooleanIf the argument is true, then the result is "true".
If the argument is false, then the result is "false".

9.8.1

NumberSee 9.8.1.

StringReturn the input argument (no conversion)

ObjectApply the following steps:
1. Let primValue be ToPrimitive(input argument, hint String).
2. Return ToString(primValue).

ToString Applied to the Number TypeThe abstract operation ToString converts a Number m to String format as follows:
1.
2.
3.
4.
5.

If m is NaN, return the String "NaN".
If m is +0 or −0, return the String "0".
If m is less than zero, return the String concatenation of the String "-" and ToString(−m).
If m is infinity, return the String "Infinity".
Otherwise, let n, k, and s be integers such that k ≥ 1, 10 k−1 ≤ s < 10 k , the Number value for s × 10 n−k is m, andk is as small as possible. Note that k is the number of digits in the decimal representation of s, that s is notdivisible by 10, and that the least significant digit of s is not necessarily uniquely determined by thesecriteria.
6. If k ≤ n ≤ 21, return the String consisting of the k digits of the decimal representation of s (in order, with noleading zeroes), followed by n − k occurrences of the character ‘0’.
7. If 0 < n ≤ 21, return the String consisting of the most significant n digits of the decimal representation of s,followed by a decimal point ‘.’, followed by the remaining k− n digits of the decimal representation of s.
8. If −6 < n ≤ 0, return the String consisting of the character ‘0’, followed by a decimal point ‘.’, followed by−n occurrences of the character ‘0’, followed by the k digits of the decimal representation of s.
9. Otherwise, if k = 1, return the String consisting of the single digit of s, followed by lowercase character ‘e’,followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or negative, followed by
the decimal representation of the integer abs(n−1) (with no leading zeros).
10. Return the String consisting of the most significant digit of the decimal representation of s, followed by adecimal point ‘.’, followed by the remaining k−1 digits of the decimal representation of s, followed by thelowercase character ‘e’, followed by a plus sign ‘+’ or minus sign ‘−’ according to whether n−1 is positive or
negative, followed by the decimal representation of the integer abs(n−1) (with no leading zeros).
NOTE 1
The following observations may be useful as guidelines for implementations, but are not part of the normativerequirements of this Standard:••

If x is any Number value other than −0, then ToNumber(ToString(x)) is exactly the same Number value as x.
The least significant digit of s is not always uniquely determined by the requirements listed in step 5.

NOTE 2 For implementations that provide more accurate conversions than required by the rules above, it isrecommended that the following alternative version of step 5 be used as a guideline:
Otherwise, let n, k, and s be integers such that k ≥ 1, 10k−1 ≤ s < 10k, the Number value for s × 10n−k is m, and k is as small as
possible. If there are multiple possibilities for s, choose the value of s for which s × 10n−k is closest in value to m. If there are
two such possible values of s, choose the one that is even. Note that k is the number of digits in the decimal representation ofs and that s is not divisible by 10.
NOTE 3 Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal
conversion of floating-point numbers:

48© Ecma International 2009

Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis,
Manuscript 90-10. AT&T Bell Laboratories (Murray Hill, New Jersey). November 30, 1990. Available as
http://cm.bell-labs.com/cm/cs/doc/90/4-10.ps.gz. Associated code available as
http://cm.bell-labs.com/netlib/fp/dtoa.c.gz and as
http://cm.bell-labs.com/netlib/fp/g_fmt.c.gz and may also be found at the various netlib mirror sites.

9.9

ToObject

The abstract operation ToObject converts its argument to a value of type Object according to Table 14:
Table 14 — ToObject
Argument Type

ResultUndefined

Throw a TypeError exception.

Null

Throw a TypeError exception.

BooleanCreate a new Boolean object whose [[PrimitiveValue]] internal property is
set to the value of the argument. See 15.6 for a description of Boolean
objects.

NumberCreate a new Number object whose [[PrimitiveValue]] internal property is
set to the value of the argument. See 15.7 for a description of Number
objects.

StringCreate a new String object whose [[PrimitiveValue]] internal property is set
to the value of the argument. See 15.5 for a description of String objects.

Object

The result is the input argument (no conversion).

9.10 CheckObjectCoercible
The abstract operation CheckObjectCoercible throws an error if its argument is a value that cannot be
converted to an Object using ToObject. It is defined by Table 15:
Table 15 — CheckObjectCoercible Results
Argument Type

ResultUndefined

Throw a TypeError exception.

Null

Throw a TypeError exception.

BooleanReturnNumberReturnStringReturnObjectReturn9.11 IsCallable
The abstract operation IsCallable determines if its argument, which must be an ECMAScript language value,
is a callable function Object according to Table 16:© Ecma International 200949

Table 16 — IsCallable Results
Argument Type

ResultUndefined

Return false.

Null

Return false.

BooleanReturn false.

NumberReturn false.

StringReturn false.

ObjectIf the argument object has an [[Call]] internal method, then return true,
otherwise return false.

9.12 The SameValue Algorithm
The internal comparison abstract operation SameValue(x, y), where x and y are ECMAScript language values,
produces true or false. Such a comparison is performed as follows:
1.
2.
3.
4.

5.
6.
7.

If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, then.
a. If x is NaN and y is NaN, return true.
b. If x is +0 and y is -0, return false.
c. If x is -0 and y is +0, return false.
d. If x is the same Number value as y, return true.
e. Return false.
If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length andsame characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.

10 Executable Code and Execution Contexts
10.1 Types of Executable Code
There are three types of ECMAScript executable code:•

Global code is source text that is treated as an ECMAScript Program. The global code of aparticular Program does not include any source text that is parsed as part of a FunctionBody.

•

Eval code is the source text supplied to the built-in eval function. More precisely, if the parameterto the built-in eval function is a String, it is treated as an ECMAScript Program. The eval code for aparticular invocation of eval is the global code portion of that Program.

•

Function code is source text that is parsed as part of a FunctionBody. The function code of aparticular FunctionBody does not include any source text that is parsed as part of a nested
FunctionBody. Function code also denotes the source text supplied when using the built-in
Function object as a constructor. More precisely, the last parameter provided to the Function
constructor is converted to a String and treated as the FunctionBody. If more than one parameter isprovided to the Function constructor, all parameters except the last one are converted to Stringsand concatenated together, separated by commas. The resulting String is interpreted as theFormalParameterList for the FunctionBody defined by the last parameter. The function code for aparticular instantiation of a Function does not include any source text that is parsed as part of anested FunctionBody.

50© Ecma International 2009

10.1.1 Strict Mode CodeAn ECMAScript Program syntactic unit may be processed using either unrestricted or strict mode syntax andsemantics. When processed using strict mode the three types of ECMAScript code are referred to as strict
global code, strict eval code, and strict function code. Code is interpreted as strict mode code in the following
situations:•Global code is strict global code if it begins with a Directive Prologue that contains a Use Strict Directive(see 14.1).

•

Eval code is strict eval code if it begins with a Directive Prologue that contains a Use Strict Directive or if
the call to eval is a direct call (see 15.1.2.1.1) to the eval function that is contained in strict mode code.

•

Function code that is part of a FunctionDeclaration, FunctionExpression, or accessor PropertyAssignment isstrict function code if its FunctionDeclaration, FunctionExpression, or PropertyAssignment is contained in strictmode code or if the function code begins with a Directive Prologue that contains a Use Strict Directive.

•

Function code that is supplied as the last argument to the built-in Function constructor is strict function
code if the last argument is a String that when processed as a FunctionBody begins with a Directive
Prologue that contains a Use Strict Directive.

10.2 Lexical Environments
A Lexical Environment is a specification type used to define the association of Identifiers to specific variables
and functions based upon the lexical nesting structure of ECMAScript code. A Lexical Environment consists ofan Environment Record and a possibly null reference to an outer Lexical Environment. Usually a Lexical
Environment is associated with some specific syntactic structure of ECMAScript code such as a
FunctionDeclaration, a WithStatement, or a Catch clause of a TryStatement and a new Lexical Environment iscreated each time such code is evaluated.
An Environment Record records the identifier bindings that are created within the scope of its associatedLexical Environment.
The outer environment reference is used to model the logical nesting of Lexical Environment values. The
outer reference of a (inner) Lexical Environment is a reference to the Lexical Environment that logicallysurrounds the inner Lexical Environment. An outer Lexical Environment may, of course, have its own outer
Lexical Environment. A Lexical Environment may serve as the outer environment for multiple inner Lexical
Environments. For example, if a FunctionDeclaration contains two nested FunctionDeclarations then the Lexical
Environments of each of the nested functions will have as their outer Lexical Environment the Lexical
Environment of the current execution of the surrounding function.
Lexical Environments and Environment Record values are purely specification mechanisms and need notcorrespond to any specific artefact of an ECMAScript implementation. It is impossible for an ECMAScript
program to directly access or manipulate such values.
10.2.1 Environment Records
There are two kinds of Environment Record values used in this specification: declarative environment records
and object environment records. Declarative environment records are used to define the effect of ECMAScript
language syntactic elements such as FunctionDeclarations, VariableDeclarations, and Catch clauses that directly
associate identifier bindings with ECMAScript language values. Object environment records are used todefine the effect of ECMAScript elements such as Program and WithStatement that associate identifierbindings with the properties of some object.
For specification purposes Environment Record values can be thought of as existing in a simple objectoriented hierarchy where Environment Record is an abstract class with two concrete subclasses, declarativeenvironment record and object environment record. The abstract class includes the abstract specification© Ecma International 200951

methods defined in Table 17. These abstract methods have distinct concrete algorithms for each of theconcrete subclasses.
Table 17 — Abstract Methods of Environment Records

10.2.1.1

MethodPurposeHasBinding(N)Determine if an environment record has a binding for an
identifier. Return true if it does and false if it does not. The
String value N is the text of the identifier.

CreateMutableBinding(N, D)

Create a new mutable binding in an environment record. The
String value N is the text of the bound name. If the optional
Boolean argument D is true the binding is may be subsequently
deleted.

SetMutableBinding(N,V, S)

Set the value of an already existing mutable binding in anenvironment record. The String value N is the text of the boundname. V is the value for the binding and may be a value of any
ECMAScript language type. S is a Boolean flag. If S is true andthe binding cannot be set throw a TypeError exception. S is
used to identify strict mode references.

GetBindingValue(N,S)

Returns the value of an already existing binding from anenvironment record. The String value N is the text of the boundname. S is used to identify strict mode references. If S is true
and the binding does not exist or is uninitialized throw a
ReferenceError exception.

DeleteBinding(N)Delete a binding from an environment record. The String value N
is the text of the bound name If a binding for N exists, remove
the binding and return true. If the binding exists but cannot be
removed return false. If the binding does not exist return true.

ImplicitThisValue()

Returns the value to use as the this value on calls to function
objects that are obtained as binding values from thisenvironment record.

Declarative Environment Records

Each declarative environment record is associated with an ECMAScript program scope containing variable
and/or function declarations. A declarative environment record binds the set of identifiers defined by the
declarations contained within its scope.
In addition to the mutable bindings supported by all Environment Records, declarative environment records
also provide for immutable bindings. An immutable binding is one where the association between an identifier
and a value may not be modified once it has been established. Creation and initialization of immutable binding
are distinct steps so it is possible for such bindings to exist in either an initialized or uninitialized state.
Declarative environment records support the methods listed in Table 18 in addition to the Environment Record
abstract specification methods:
Table 18 — Additional Methods of Declarative Environment Records

52

MethodPurposeCreateImmutableBinding(N)Create a new but uninitialized immutable binding in anenvironment record. The String value N is the text of the boundname.

InitializeImmutableBinding(N,V)

Set the value of an already existing but uninitialized immutablebinding in an environment record. The String value N is the textof the bound name. V is the value for the binding and is a valueof any ECMAScript language type.

© Ecma International 2009

The behaviour of the concrete specification methods for Declarative Environment Records are defined by thefollowing algorithms.
10.2.1.1.1

HasBinding(N)The concrete environment record method HasBinding for declarative environment records simply determines
if the argument identifier is one of the identifiers bound by the record:
1.
2.
3.

Let envRec be the declarative environment record for which the method was invoked.
If envRec has a binding for the name that is the value of N, return true.
If it does not have such a binding, return false

10.2.1.1.2

CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for declarative environment records createsa new mutable binding for the name N that is initialized to the value undefined. A binding must not alreadyexist in this Environment Record for N. If Boolean argument D is provided and has the value true the newbinding is marked as being subject to deletion.
1.
2.
3.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create a mutable binding in envRec for N and set its bound value to undefined. If D is true record that the
newly created binding may be deleted by a subsequent DeleteBinding call.

10.2.1.1.3

SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for declarative environment records attempts tochange the bound value of the current binding of the identifier whose name is the value of the argument N to
the value of argument V. A binding for N must already exist. If the binding is an immutable binding, a
TypeError is always thrown. The S argument is ignored because strict mode does not change the meaning ofsetting bindings in declarative environment records.
1.
2.
3.
4.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec must have a binding for N.
If the binding for N in envRec is a mutable binding, change its bound value to V.
Else this must be an attempt to change the value of an immutable binding so throw a TypeError exception.

10.2.1.1.4

GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for declarative environment records simply
returns the value of its bound identifier whose name is the value of the argument N. The binding must alreadyexist. If S is true and the binding is an uninitialized immutable binding throw a ReferenceError exception.
1.
2.
3.
4.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec has a binding for N.
If the binding for N in envRec is an uninitialized immutable binding, thena. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Else, return the value currently bound to N in envRec.

10.2.1.1.5

DeleteBinding (N)The concrete Environment Record method DeleteBinding for declarative environment records can only deletebindings that have been explicitly designated as being subject to deletion.
1.
2.
3.

Let envRec be the declarative environment record for which the method was invoked.
If envRec does not have a binding for the name that is the value of N, return true.
If the binding for N in envRec is cannot be deleted, return false.

© Ecma International 2009

53

4.
5.

Remove the binding for N from envRec.
Return true.

10.2.1.1.6

ImplicitThisValue()

Declarative Environment Records always return undefined as their ImplicitThisValue.
1.

Return undefined.

10.2.1.1.7

CreateImmutableBinding (N)The concrete Environment Record method CreateImmutableBinding for declarative environment records
creates a new immutable binding for the name N that is initialized to the value undefined. A binding must notalready exist in this environment record for N.
1.
2.
3.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record that it is uninitialized.

10.2.1.1.8

InitializeImmutableBinding (N,V)

The concrete Environment Record method InitializeImmutableBinding for declarative environment records isused to set the bound value of the current binding of the identifier whose name is the value of the argument N
to the value of argument V. An uninitialized immutable binding for N must already exist.
1.
2.
3.
4.

Let envRec be the declarative environment record for which the method was invoked.
Assert: envRec must have an uninitialized immutable binding for N.
Set the bound value for N in envRec to V.
Record that the immutable binding for N in envRec has been initialized.

10.2.1.2

Object Environment Records

Each object environment record is associated with an object called its binding object. An object environment
record binds the set of identifier names that directly correspond to the property names of its binding object.
Property names that are not an IdentifierName are not included in the set of bound identifiers. Both own andinherited properties are included in the set regardless of the setting of their [[Enumerable]] attribute. Because
properties can be dynamically added and deleted from objects, the set of identifiers bound by an object
environment record may potentially change as a side-effect of any operation that adds or deletes properties.
Any bindings that are created as a result of such a side-effect are considered to be a mutable binding even if
the Writable attribute of the corresponding property has the value false. Immutable bindings do not exist for
object environment records.
Object environment records can be configured to provide their binding object as an implicit this value for use
in function calls. This capability is used to specify the behaviour of With Statement (12.10) induced bindings.
The capability is controlled by a provideThis Boolean value that is associated with each object environment
record. By default, the value of provideThis is false for any object environment record.
The behaviour of the concrete specification methods for Object Environment Records is defined by thefollowing algorithms.
10.2.1.2.1

HasBinding(N)The concrete Environment Record method HasBinding for object environment records determines if its
associated binding object has a property whose name is the value of the argument N:
1.
2.
3.

Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Return the result of calling the [[HasProperty]] internal method of bindings, passing N as the property name.

54© Ecma International 2009

10.2.1.2.2

CreateMutableBinding (N, D)

The concrete Environment Record method CreateMutableBinding for object environment records creates inan environment record’s associated binding object a property whose name is the String value and initializes itto the value undefined. A property named N must not already exist in the binding object. If Boolean argument
D is provided and has the value true the new property’s [[Configurable]] attribute is set to true, otherwise it is
set to false.
1.
2.
3.
4.
5.

Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Assert: The result of calling the [[HasProperty]] internal method of bindings, passing N as the property
name, is false.
If D is true then let configValue be true otherwise let configValue be false.
Call the [[DefineOwnProperty]] internal method of bindings, passing N, Property Descriptor{[[Value]]:undefined, [[Writable]]: true, [[Enumerable]]: true , [[Configurable]]: configValue}, and false
as arguments.

10.2.1.2.3

SetMutableBinding (N,V,S)

The concrete Environment Record method SetMutableBinding for object environment records attempts to set
the value of the environment record’s associated binding object’s property whose name is the value of theargument N to the value of argument V. A property named N should already exist but if it does not or is not
currently writable, error handling is determined by the value of the Boolean argument S.
1.
2.
3.

Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Call the [[Put]] internal method of bindings with arguments N, V, and S.

10.2.1.2.4

GetBindingValue(N,S)

The concrete Environment Record method GetBindingValue for object environment records returns the valueof its associated binding object’s property whose name is the String value of the argument identifier N. The
property should already exist but if it does not the result depends upon the value of the S argument:
1.
2.
3.
4.
5.

Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Let value be the result of calling the [[HasProperty]] internal method of bindings, passing N as the property
name.
If value is false, thena. If S is false, return the value undefined, otherwise throw a ReferenceError exception.
Return the result of calling the [[Get]] internal method of bindings, passing N for the argument.

10.2.1.2.5

DeleteBinding (N)The concrete Environment Record method DeleteBinding for object environment records can only deletebindings that correspond to properties of the environment object whose [[Configurable]] attribute have thevalue true.
1.
2.
3.

Let envRec be the object environment record for which the method was invoked.
Let bindings be the binding object for envRec.
Return the result of calling the [[Delete]] internal method of bindings, passing N and false as arguments.

10.2.1.2.6

ImplicitThisValue()

Object Environment Records return undefined as their ImplicitThisValue unless their provideThis flag is true.
1.
2.
3.

Let envRec be the object environment record for which the method was invoked.
If the provideThis flag of envRec is true, return the binding object for envRec.
Otherwise, return undefined.

© Ecma International 200955

10.2.2 Lexical Environment Operations
The following abstract operations are used in this specification to operate upon lexical environments:
10.2.2.1

GetIdentifierReference (lex, name, strict)

The abstract operation GetIdentifierReference is called with a Lexical Environment lex, an identifier String
name, and a Boolean flag strict. The value of lex may be null. When called, the following steps are performed:
1.
2.
3.
4.
5.

If lex is the value null, thena. Return a value of type Reference whose base value is undefined, whose referenced name is name,and whose strict mode flag is strict.
Let envRec be lex’s environment record.
Let exists be the result of calling the HasBinding(N) concrete method of envRec passing name as theargument N.
If exists is true, thena. Return a value of type Reference whose base value is envRec, whose referenced name is name, andwhose strict mode flag is strict.
Elsea. Let outer be the value of lex’s outer environment reference.
b. Return the result of calling GetIdentifierReference passing outer, name, and strict as arguments.

10.2.2.2

NewDeclarativeEnvironment (E)When the abstract operation NewDeclarativeEnvironment is called with either a Lexical Environment or nullas argument E the following steps are performed:
1.
2.
3.
4.
5.

Let env be a new Lexical Environment.
Let envRec be a new declarative environment record containing no bindings.
Set env’s environment record to be envRec.
Set the outer lexical environment reference of env to E.
Return env.

10.2.2.3

NewObjectEnvironment (O, E)

When the abstract operation NewObjectEnvironmentis called with an Object O and a Lexical Environment E(or null) as arguments, the following steps are performed:
1.
2.
3.
4.
5.

Let env be a new Lexical Environment.
Let envRec be a new object environment record containing O as the binding object.
Set env’s environment record to be envRec.
Set the outer lexical environment reference of env to E.
Return env.

10.2.3 The Global Environment
The global environment is a unique Lexical Environment which is created before any ECMAScript code is
executed. The global environment’s Environment Record is an object environment record whose binding
object is the global object (15.1). The global environment’s outer environment reference is null.
As ECMAScript code is executed, additional properties may be added to the global object and the initialproperties may be modified.

10.3 Execution Contexts
When control is transferred to ECMAScript executable code, control is entering an execution context. Active
execution contexts logically form a stack. The top execution context on this logical stack is the running
execution context. A new execution context is created whenever control is transferred from the executablecode associated with the currently running execution context to executable code that is not associated with56

© Ecma International 2009that execution context. The newly created execution context is pushed onto the stack and becomes therunning execution context.
An execution context contains whatever state is necessary to track the execution progress of its associatedcode. In addition, each execution context has the state components listed in Table 19.
Table 19 —Execution Context State Components
Component

Purpose

LexicalEnvironment

Identifies the Lexical Environment used to resolve identifier references
made by code within this execution context.

VariableEnvironment

Identifies the Lexical Environment whose environment record holds
bindings created by VariableStatements and FunctionDeclarations within
this execution context.

ThisBinding

The value associated with the this keyword within ECMAScript code
associated with this execution context.

The LexicalEnvironment and VariableEnvironment components of an execution context are always Lexical
Environments. When an execution context is created its LexicalEnvironment and VariableEnvironment
components initially have the same value. The value of the VariableEnvironment component never changes
while the value of the LexicalEnvironment component may change during execution of code within an
execution context.
In most situations only the running execution context (the top of the execution context stack) is directly
manipulated by algorithms within this specification.
Hence when the terms “LexicalEnvironment”,“VariableEnvironment” and “ThisBinding” are used without qualification they are in reference to thosecomponents of the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefactof an ECMAScript implementation. It is impossible for an ECMAScript program to access an execution
context.
10.3.1 Identifier Resolution
Identifier resolution is the process of determining the binding of an Identifier using the LexicalEnvironment of
the running execution context. During execution of ECMAScript code, the syntactic production
PrimaryExpression : Identifier is evaluated using the following algorithm:
1.
2.
3.

Let env be the running execution context’s LexicalEnvironment.
If the syntactic production that is being evaluated is contained in a strict mode code, then let strict be true,else let strict be false.
Return the result of calling GetIdentifierReference function passing env, Identifier, and strict as arguments.

The result of evaluating an identifier is always a value of type Reference with its referenced name component
equal to the Identifier String.

10.4 Establishing an Execution Context
Evaluation of global code or code using the eval function (15.1.2.1) establishes and enters a new execution
context. Every invocation of an ECMAScript code function (13.2.1) also establishes and enters a new
execution context, even if a function is calling itself recursively. Every return exits an execution context. A
thrown exception may also exit one or more execution contexts.
When control enters an execution context, the execution context’s ThisBinding is set, its VariableEnvironment
and initial LexicalEnvironment are defined, and declaration binding instantiation (10.5) is performed. The exact
manner in which these actions occur depend on the type of code being entered.

© Ecma International 200957

10.4.1 Entering Global CodeThe following steps are performed when control enters the execution context for global code:
1.
2.

Initialize the execution context using the global code as described in 10.4.1.1.
Perform Declaration Binding Instantiation as described in 10.5 using the global code.

10.4.1.1

Initial Global Execution Context

The following steps are performed to initialize a global execution context for ECMAScript code C:
1.
2.
3.

Set the VariableEnvironment to the Global Environment.
Set the LexicalEnvironment to the Global Environment.
Set the ThisBinding to the global object.

10.4.2 Entering Eval CodeThe following steps are performed when control enters the execution context for eval code:
1.

2.

3.

4.

If there is no calling context or if the eval code is not being evaluated by a direct call (15.1.2.1.1) to the evalfunction then,a. Initialize the execution context as if it was a global execution context using the eval code as C as
described in 10.4.1.1.
Else,
a. Set the ThisBinding to the same value as the ThisBinding of the calling execution context.
b. Set the LexicalEnvironment to the same value as the LexicalEnvironment of the calling execution
context.
c. Set the VariableEnvironment to the same value as the VariableEnvironment of the calling execution
context.
If the eval code is strict code, thena. Let strictVarEnv be the result of calling NewDeclarativeEnvironment passing the
LexicalEnvironment as the argument.
b. Set the LexicalEnvironment to strictVarEnv.
c. Set the VariableEnvironment to strictVarEnv.
Perform Declaration Binding Instantiation as described in 10.5 using the eval code.

10.4.2.1

Strict Mode Restrictions

The eval code cannot instantiate variable or function bindings in the variable environment of the calling
context that invoked the eval if either the code of the calling context or the eval code is strict code. Insteadsuch bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code.
10.4.3 Entering Function CodeThe following steps are performed when control enters the execution context for function code contained infunction object F, a caller provided thisArg, and a caller provided argumentsList:
1.
2.
3.
4.
5.
6.
7.
8.
9.

58

If the function code is strict code, set the ThisBinding to thisArg.
Else if thisArg is null or undefined, set the ThisBinding to the global object.
Else if Type(thisArg) is not Object, set the ThisBinding to ToObject(thisArg).
Else set the ThisBinding to thisArg.
Let localEnv be the result of calling NewDeclarativeEnvironment passing the value of the [[Scope]] internal
property of F as the argument.
Set the LexicalEnvironment to localEnv.
Set the VariableEnvironment to localEnv.
Let code be the value of F’s [[Code]] internal property.
Perform Declaration Binding Instantiation using the function code code and argumentList as described in
10.5.

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10.5 Declaration Binding Instantiation
Every execution context has an associated VariableEnvironment. Variables and functions declared in
ECMAScript code evaluated in an execution context are added as bindings in that VariableEnvironment’s
Environment Record. For function code, parameters are also added as bindings to that Environment Record.
Which Environment Record is used to bind a declaration and its kind depends upon the type of ECMAScript
code executed by the execution context, but the remainder of the behaviour is generic. On entering an
execution context, bindings are created in the VariableEnvironment as follows using the caller provided code
and, if it is function code, argument List args:
1.
2.
3.
4.

5.

6.
7.

8.

Let env be the environment record component of the running execution context’s VariableEnvironment.
If code is eval code, then let configurableBindings be true else let configurableBindings be false.
If code is strict mode code, then let strict be true else let strict be false.
If code is function code, thena. Let func be the function whose [[Call]] internal method initiated execution of code. Let names be
the value of func’s [[FormalParameters]] internal property.
b. Let argCount be the number of elements in args.
c. Let n be the number 0.
d. For each String argName in names, in list order doi.
Let n be the current value of n plus 1.
ii.
If n is greater than argCount, let v be undefined otherwise let v be the value of the n’thelement of args.
iii.
Let argAlreadyDeclared be the result of calling env’s HasBinding concrete method passing
argName as the argument.
iv.
If argAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing
argName as the argument.
v.
Call env’s SetMutableBinding concrete method passing argName, v, and strict as thearguments.
For each FunctionDeclaration f in code, in source text order doa. Let fn be the Identifier in FunctionDeclaration f.
b. Let fo be the result of instantiating FunctionDeclaration f as described in Clause 13.
c. Let funcAlreadyDeclared be the result of calling env’s HasBinding concrete method passing fn as
the argument.
d. If funcAlreadyDeclared is false, call env’s CreateMutableBinding concrete method passing fn andconfigurableBindings as the arguments.
e. Call env’s SetMutableBinding concrete method passing fn, fo, and strict as the arguments.
Let argumentsAlreadyDeclared be the result of calling env’s HasBinding concrete method passing"arguments" as the argument
If code is function code and argumentsAlreadyDeclared is false, thena. Let argsObj be the result of calling the abstract operation CreateArgumentsObject (10.6) passing
func, names, args, env and strict as arguments.
b. If strict is true, theni.
Call env’s CreateImmutableBinding concrete method passing the String "arguments" as
the argument.
ii.
Call env’s InitializeImmutableBinding concrete method passing "arguments" andargsObj as arguments.
c. Else,i.
Call env’s CreateMutableBinding concrete method passing the String "arguments" as theargument.
ii.
Call env’s SetMutableBinding concrete method passing "arguments", argsObj, and false
as arguments.
For each VariableDeclaration and VariableDeclarationNoIn d in code, in source text order doa. Let dn be the Identifier in d.
b. Let varAlreadyDeclared be the result of calling env’s HasBinding concrete method passing dn as theargument.
c. If varAlreadyDeclared is false, theni.
Call env’s CreateMutableBinding concrete method passing dn and configurableBindings as
the arguments.

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ii.

Call env’s SetMutableBinding concrete method passing dn, undefined, and strict as thearguments.

10.6 Arguments Object
When control enters an execution context for function code, an arguments object is created unless (asspecified in 10.5) the identifier arguments occurs as an Identifier in the function’s FormalParameterList or
occurs as the Identifier of a VariableDeclaration or FunctionDeclaration contained in the function code.
The arguments object is created by calling the abstract operation CreateArgumentsObject with arguments func
the function object whose code is to be evaluated, names a List containing the function’s formal parameternames, args the actual arguments passed to the [[Call]] internal method, env the variable environment for thefunction code, and strict a Boolean that indicates whether or not the function code is strict code. When
CreateArgumentsObject is called the following steps are performed:
1.
2.
3.
4.
5.
6.
7.

Let len be the number of elements in args.
Let obj be the result of creating a new ECMAScript object.
Set all the internal methods of obj as specified in 8.12.
Set the [[Class]] internal property of obj to "Arguments".
Let Object be the standard built-in Object constructor (15.2.2).
Set the [[Prototype]] internal property of obj to the standard built-in Object prototype object (15.2.4).
Call the [[DefineOwnProperty]] internal method on obj passing "length", the Property Descriptor{[[Value]]: len, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false as arguments.
8. Let map be the result of creating a new object as if by the expression new Object() where Object isthe standard built-in constructor with that name
9. Let mappedNames be an empty List.
10. Let indx = len - 1.
11. Repeat while indx >= 0,
a. Let val be the element of args at 0-origined list position indx.
b. Call the [[DefineOwnProperty]] internal method on obj passing ToString(indx), the property
descriptor {[[Value]]: val, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, andfalse as arguments.
c. If indx is less than the number of elements in names, theni.
Let name be the element of names at 0-origined list position indx.
ii.
If strict is false and name is not an element of mappedNames, then
1. Add name as an element of the list mappedNames.
2. Let g be the result of calling the MakeArgGetter abstract operation with arguments
name and env.
3. Let p be the result of calling the MakeArgSetter abstract operation with arguments
name and env.
4. Call the [[DefineOwnProperty]] internal method of map passing ToString(indx), theProperty Descriptor {[[Set]]: p, [[Get]]: g, [[Configurable]]: true}, and false asarguments.
d. Let indx = indx - 1
12. If mappedNames is not empty, thena. Set the [[ParameterMap]] internal property of obj to map.
b. Set the [[Get]], [[GetOwnProperty]], [[DefineOwnProperty]], and [[Delete]] internal methods of obj
to the definitions provided below.
13. If strict is false, thena. Call the [[DefineOwnProperty]] internal method on obj passing "callee", the property descriptor{[[Value]]: func, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false asarguments.
14. Else, strict is true soa. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
b. Call the [[DefineOwnProperty]] internal method of obj with arguments "caller",
PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]:false}, and false.

60© Ecma International 2009c.

Call the [[DefineOwnProperty]] internal method of obj with arguments "callee",PropertyDescriptor {[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]:false}, and false.
15. Return obj
The abstract operation MakeArgGetter called with String name and environment record env creates a function
object that when executed returns the value bound for name in env. It performs the following steps:
1.
2.

Let body be the result of concatenating the Strings "return ", name, and ";"Return the result of creating a function object as described in 13.2 using no FormalParameterList, body for
FunctionBody, env as Scope, and true for Strict.

The abstract operation MakeArgSetter called with String name and environment record env creates a function
object that when executed sets the value bound for name in env. It performs the following steps:
1.
2.
3.

Let param be the String name concatenated with the String "_arg"
Let body be the String "<name> = <param>;" with <name> replaced by the value of name and <param>
replaced by the value of param.
Return the result of creating a function object as described in 13.2 using a List containing the single String
param as FormalParameterList, body for FunctionBody, env as Scope, and true for Strict.

The [[Get]] internal method of an arguments object for a non-strict mode function with formal parameters whencalled with a property name P performs the following steps:
1.
2.
3.

4.

Let map be the value of the [[ParameterMap]] internal property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as theargument.
If the value of isMapped is undefined, thena. Let v be the result of calling the default [[Get]] internal method (8.12.3) on the arguments object
passing P as the argument.
b. If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
c. Return v.
Else, map contains a formal parameter mapping for P so,a. Return the result of calling the [[Get]] internal method of map passing P as the argument.

The [[GetOwnProperty]] internal method of an arguments object for a non-strict mode function with formal
parameters when called with a property name P performs the following steps:
1.
2.
3.
4.
5.
6.

Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on the arguments
object passing P as the argument.
If desc is undefined then return desc.
Let map be the value of the [[ParameterMap]] internal property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as theargument.
If the value of isMapped is not undefined, thena. Set desc.[[Value]] to the result of calling the [[Get]] internal method of map passing P as theargument.
Return desc.

The [[DefineOwnProperty]] internal method of an arguments object for a non-strict mode function with formal
parameters when called with a property name P, Property Descriptor Desc, and Boolean flag Throw performs
the following steps:
1.
2.
3.
4.
5.

Let map be the value of the [[ParameterMap]] internal property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as theargument.
Let allowed be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on thearguments object passing P, Desc, and false as the arguments.
If allowed is false, thena. If Throw is true then throw a TypeError exception, otherwise return false.
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a.

6.

If IsAccessorDescriptor(Desc) is true, theni.
Call the [[Delete]] internal method of map passing P, and false as the arguments.
b. Elsei.
If Desc.[[Value]] is present, then
1. Call the [[Put]] internal method of map passing P, Desc.[[Value]], and Throw as thearguments.
ii.
If Desc.[[Writable]] is present and its value is false, then
1. Call the [[Delete]] internal method of map passing P and false as arguments.
Return true.

The [[Delete]] internal method of an arguments object for a non-strict mode function with formal parameterswhen called with a property name P and Boolean flag Throw performs the following steps:
1.
2.
3.
4.
5.

Let map be the value of the [[ParameterMap]] internal property of the arguments object.
Let isMapped be the result of calling the [[GetOwnProperty]] internal method of map passing P as theargument.
Let result be the result of calling the default [[Delete]] internal method (8.12.7) on the arguments object
passing P and Throw as the arguments.
If result is true and the value of isMapped is not undefined, thena. Call the [[Delete]] internal method of map passing P, and false as the arguments.
Return result.

NOTE 1For non-strict mode functions the array index (defined in 15.4) named data properties of an arguments object
whose numeric name values are less than the number of formal parameters of the corresponding function object initiallyshare their values with the corresponding argument bindings in the function’s execution context. This means that changing
the property changes the corresponding value of the argument binding and vice-versa. This correspondence is broken if
such a property is deleted and then redefined or if the property is changed into an accessor property. For strict mode
functions, the values of the arguments object‘s properties are simply a copy of the arguments passed to the function andthere is no dynamic linkage between the property values and the formal parameter values.
NOTE 2The ParameterMap object and its property values are used as a device for specifying the arguments object
correspondence to argument bindings. The ParameterMap object and the objects that are the values of its properties are
not directly accessible from ECMAScript code. An ECMAScript implementation does not need to actually create or use
such objects to implement the specified semantics.
NOTE 3
Arguments objects for strict mode functions define non-configurable accessor properties named "caller" and"callee" which throw a TypeError exception on access. The "callee" property has a more specific meaning for nonstrict mode functions and a "caller" property has historically been provided as an implementation-defined extension bysome ECMAScript implementations. The strict mode definition of these properties exists to ensure that neither of them isdefined in any other manner by conforming ECMAScript implementations.

11 Expressions
11.1 Primary Expressions
SyntaxPrimaryExpression :this
Identifier
LiteralArrayLiteral
ObjectLiteral( Expression )11.1.1 The this Keyword
The this keyword evaluates to the value of the ThisBinding of the current execution context.

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11.1.2 Identifier Reference
An Identifier is evaluated by performing Identifier Resolution as specified in 10.3.1. The result of evaluating an
Identifier is always a value of type Reference.
11.1.3 Literal Reference
A Literal is evaluated as described in 7.8.
11.1.4 Array Initialiser
An array initialiser is an expression describing the initialisation of an Array object, written in a form of a literal.
It is a list of zero or more expressions, each of which represents an array element, enclosed in square
brackets. The elements need not be literals; they are evaluated each time the array initialiser is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in theelement list is not preceded by an AssignmentExpression (i.e., a comma at the beginning or after anothercomma), the missing array element contributes to the length of the Array and increases the index ofsubsequent elements. Elided array elements are not defined. If an element is elided at the end of an array,that element does not contribute to the length of the Array.
SyntaxArrayLiteral :[ Elisionopt ][ ElementList ][ ElementList , Elisionopt ]ElementList :Elisionopt AssignmentExpression
ElementList , Elisionopt AssignmentExpression
Elision :,Elision ,Semantics
The production ArrayLiteral : [ Elisionopt ] is evaluated as follows:
1.
2.
3.
4.

Let array be the result of creating a new object as if by the expression new Array() where Array is
the standard built-in constructor with that name.
Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
Call the [[Put]] internal method of array with arguments "length", pad, and false.
Return array.

The production ArrayLiteral : [ ElementList ] is evaluated as follows:
1.

Return the result of evaluating ElementList.

The production ArrayLiteral : [ ElementList , Elisionopt ] is evaluated as follows:
1.
2.
3.
4.
5.

Let array be the result of evaluating ElementList.
Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
Let len be the result of calling the [[Get]] internal method of array with argument "length".
Call the [[Put]] internal method of array with arguments "length", ToUint32(pad+len), and false.
Return array.

The production ElementList : Elisionopt AssignmentExpression is evaluated as follows:© Ecma International 2009

63

1.
2.
3.
4.
5.
6.

Let array be the result of creating a new object as if by the expression new Array() where Array is
the standard built-in constructor with that name.
Let firstIndex be the result of evaluating Elision; if not present, use the numeric value zero.
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be GetValue(initResult).
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(firstIndex), the Property
Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, andfalse.
Return array.

The production ElementList : ElementList , Elisionopt AssignmentExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.

Let array be the result of evaluating ElementList.
Let pad be the result of evaluating Elision; if not present, use the numeric value zero.
Let initResult be the result of evaluating AssignmentExpression.
Let initValue be GetValue(initResult).
Let len be the result of calling the [[Get]] internal method of array with argument "length".
Call the [[DefineOwnProperty]] internal method of array with arguments ToString(ToUint32((pad+len)) and
the Property Descriptor { [[Value]]: initValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]:true}, and false.
Return array.

The production Elision : , is evaluated as follows:
1.

Return the numeric value 1.

The production Elision : Elision , is evaluated as follows:
1.
2.

Let preceding be the result of evaluating Elision.
Return preceding+1.

NOTE[[DefineOwnProperty]] is used to ensure that own properties are defined for the array even if the standard
built-in Array prototype object has been modified in a manner that would preclude the creation of new own properties
using [[Put]].

11.1.5 Object Initialiser
An object initialiser is an expression describing the initialisation of an Object, written in a form resembling aliteral. It is a list of zero or more pairs of property names and associated values, enclosed in curly braces. The
values need not be literals; they are evaluated each time the object initialiser is evaluated.
SyntaxObjectLiteral :{ }{ PropertyNameAndValueList }{ PropertyNameAndValueList , }PropertyNameAndValueList :PropertyAssignment
PropertyNameAndValueList , PropertyAssignment
PropertyAssignment :PropertyName : AssignmentExpression
get PropertyName ( ) { FunctionBody }set PropertyName ( PropertySetParameterList ) { FunctionBody }64© Ecma International 2009

PropertyName :IdentifierName
StringLiteral
NumericLiteral
PropertySetParameterList :Identifier
Semantics
The production ObjectLiteral : { } is evaluated as follows:
1.

Return a new object created as if by the expression new Object() where Object is the standard builtin constructor with that name.

The productions ObjectLiteral : { PropertyNameAndValueList } andObjectLiteral : { PropertyNameAndValueList ,} are evaluated as follows:
1.

Return the result of evaluating PropertyNameAndValueList.

The production PropertyNameAndValueList : PropertyAssignment is evaluated as follows:
1.
2.
3.
4.

Let obj be the result of creating a new object as if by the expression new Object() where Object is thestandard built-in constructor with that name.
Let propId be the result of evaluating PropertyAssignment.
Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name, propId.descriptor, andfalse.
Return obj.

The production
PropertyNameAndValueList : PropertyNameAndValueList , PropertyAssignmentis evaluated as follows:
1.
2.
3.
4.

5.
6.

Let obj be the result of evaluating PropertyNameAndValueList.
Let propId be the result of evaluating PropertyAssignment.
Let previous be the result of calling the [[GetOwnProperty]] internal method of obj with argument
propId.name.
If previous is not undefined then throw a SyntaxError exception if any of the following conditions are truea. This production is contained in strict code and IsDataDescriptor(previous) is true and
IsDataDescriptor(propId.descriptor) is true.
b. IsDataDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true.
c. IsAccessorDescriptor(previous) is true and IsDataDescriptor(propId.descriptor) is true.
d. IsAccessorDescriptor(previous) is true and IsAccessorDescriptor(propId.descriptor) is true andeither both previous and propId.descriptor have [[Get]] fields or both previous and propId.descriptor
have [[Set]] fields
Call the [[DefineOwnProperty]] internal method of obj with arguments propId.name, propId.descriptor, andfalse.
Return obj.

If the above steps would throw a SyntaxError then an implementation must treat the error as an early error(Clause 16).
The production PropertyAssignment : PropertyName : AssignmentExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let propName be the result of evaluating PropertyName.
Let exprValue be the result of evaluating AssignmentExpression.
Let propValue be GetValue(exprValue).
Let desc be the Property Descriptor{[[Value]]: propValue, [[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}
Return Property Identifier (propName, desc).

© Ecma International 200965

The production PropertyAssignment : get PropertyName ( ) { FunctionBody } is evaluated as follows:
1.
2.

3.
4.

Let propName be the result of evaluating PropertyName.
Let closure be the result of creating a new Function object as specified in 13.2 with an empty parameter list
and body specified by FunctionBody. Pass in the LexicalEnvironment of the running execution context as the
Scope. Pass in true as the Strict flag if the PropertyAssignment is contained in strict code or if its
FunctionBody is strict code.
Let desc be the Property Descriptor{[[Get]]: closure, [[Enumerable]]: true, [[Configurable]]: true}
Return Property Identifier (propName, desc).

The production PropertyAssignment : set PropertyName ( PropertySetParameterList ) { FunctionBody } isevaluated as follows:
1.
2.

3.
4.

Let propName be the result of evaluating PropertyName.
Let closure be the result of creating a new Function object as specified in 13.2 with parameters specified by
PropertySetParameterList and body specified by FunctionBody. Pass in the LexicalEnvironment of therunning execution context as the Scope. Pass in true as the Strict flag if the PropertyAssignment is containedin strict code or if its FunctionBody is strict code.
Let desc be the Property Descriptor{[[Set]]: closure, [[Enumerable]]: true, [[Configurable]]: true}
Return Property Identifier (propName, desc).

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier in a
PropertySetParameterList of a PropertyAssignment that is contained in strict code or if its FunctionBody is strict code.
The production PropertyName : IdentifierName is evaluated as follows:
1.

Return the String value containing the same sequence of characters as the IdentifierName.

The production PropertyName : StringLiteral is evaluated as follows:
1.

Return the SV of the StringLiteral.

The production PropertyName : NumericLiteral is evaluated as follows:1.
2.

Let nbr be the result of forming the value of the NumericLiteral.
Return ToString(nbr).

11.1.6 The Grouping Operator
The production PrimaryExpression : ( Expression ) is evaluated as follows:
1.

Return the result of evaluating Expression. This may be of type Reference.

NOTE
This algorithm does not apply GetValue to the result of evaluating Expression. The principal motivation for thisis so that operators such as delete and typeof may be applied to parenthesised expressions.

11.2 Left-Hand-Side Expressions
SyntaxMemberExpression :PrimaryExpression
FunctionExpression
MemberExpression [ Expression ]MemberExpression . IdentifierName
new MemberExpression Arguments

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NewExpression :MemberExpression
new NewExpression
CallExpression :MemberExpression Arguments
CallExpression Arguments
CallExpression [ Expression ]CallExpression . IdentifierName
Arguments :()( ArgumentList )ArgumentList :AssignmentExpression
ArgumentList , AssignmentExpression
LeftHandSideExpression :NewExpression
CallExpression
11.2.1 Property Accessors
Properties are accessed by name, using either the dot notation:
MemberExpression . IdentifierName
CallExpression . IdentifierNameor the bracket notation:
MemberExpression [ Expression ]CallExpression [ Expression ]The dot notation is explained by the following syntactic conversion:
MemberExpression . IdentifierName
is identical in its behaviour toMemberExpression [ <identifier-name-string> ]and similarly
CallExpression . IdentifierName
is identical in its behaviour to
CallExpression [ <identifier-name-string> ]where <identifier-name-string> is a string literal containing the same sequence of characters after processingof Unicode escape sequences as the IdentifierName.
The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.

Let baseReference be the result of evaluating MemberExpression.
Let baseValue be GetValue(baseReference).
Let propertyNameReference be the result of evaluating Expression.
Let propertyNameValue be GetValue(propertyNameReference).
Call CheckObjectCoercible(baseValue).
Let propertyNameString be ToString(propertyNameValue).
If the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let
strict be false.

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8.

Return a value of type Reference whose base value is baseValue and whose referenced name ispropertyNameString, and whose strict mode flag is strict.

The production CallExpression : CallExpression [ Expression ] is evaluated in exactly the same manner, exceptthat the contained CallExpression is evaluated in step 1.
11.2.2 The new Operator
The production NewExpression : new NewExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let ref be the result of evaluating NewExpression.
Let constructor be GetValue(ref).
If Type(constructor) is not Object, throw a TypeError exception.
If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
Return the result of calling the [[Construct]] internal method on constructor, providing no arguments (thatis, an empty list of arguments).

The production MemberExpression : new MemberExpression Arguments is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let ref be the result of evaluating MemberExpression.
Let constructor be GetValue(ref).
Let argList be the result of evaluating Arguments, producing an internal list of argument values (11.2.4).
If Type(constructor) is not Object, throw a TypeError exception.
If constructor does not implement the [[Construct]] internal method, throw a TypeError exception.
Return the result of calling the [[Construct]] internal method on constructor, providing the list argList as theargument values.

11.2.3 Function Calls
The production CallExpression : MemberExpression Arguments is evaluated as follows:
1.
2.
3.
4.
5.
6.

7.
8.

Let ref be the result of evaluating MemberExpression.
Let func be GetValue(ref).
Let argList be the result of evaluating Arguments, producing an internal list of argument values (see 11.2.4).
If Type(func) is not Object, throw a TypeError exception.
If IsCallable(func) is false, throw a TypeError exception.
If Type(ref) is Reference, thena. If IsPropertyReference(ref) is true, theni.
Let thisValue be GetBase(ref).
b. Else, the base of ref is an Environment Recordi.
Let thisValue be the result of calling the ImplicitThisValue concrete method of
GetBase(ref).
Else, Type(ref) is not Reference.
a. Let thisValue be undefined.
Return the result of calling the [[Call]] internal method on func, providing thisValue as the this value andproviding the list argList as the argument values.

The production CallExpression : CallExpression Arguments is evaluated in exactly the same manner, except thatthe contained CallExpression is evaluated in step 1.
NOTE
The returned result will never be of type Reference if func is a native ECMAScript object. Whether calling ahost object can return a value of type Reference is implementation-dependent. If a value of type Reference is returned, it
must be a non-strict Property Reference.

11.2.4 Argument Lists
The evaluation of an argument list produces a List of values (see 8.8).
The production Arguments : ( ) is evaluated as follows:68© Ecma International 2009

1.

Return an empty List.

The production Arguments : ( ArgumentList ) is evaluated as follows:
1.

Return the result of evaluating ArgumentList.

The production ArgumentList : AssignmentExpression is evaluated as follows:
1.
2.
3.

Let ref be the result of evaluating AssignmentExpression.
Let arg be GetValue(ref).
Return a List whose sole item is arg.

The production ArgumentList : ArgumentList , AssignmentExpression is evaluated as follows:
1.
2.
3.
4.

Let precedingArgs be the result of evaluating ArgumentList.
Let ref be the result of evaluating AssignmentExpression.
Let arg be GetValue(ref).
Return a List whose length is one greater than the length of precedingArgs and whose items are the items ofprecedingArgs, in order, followed at the end by arg which is the last item of the new list.

11.2.5 Function Expressions
The production MemberExpression : FunctionExpression is evaluated as follows:
1.

Return the result of evaluating FunctionExpression.

11.3 Postfix Expressions
SyntaxPostfixExpression :LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++LeftHandSideExpression [no LineTerminator here] -11.3.1 Postfix Increment Operator
The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] ++ is evaluated as follows:
1.
2.

3.
4.
5.
6.

Let lhs be the result of evaluating LeftHandSideExpression.
Throw a SyntaxError exception if the following conditions are all true:•Type(lhs) is Reference is true•
IsStrictReference(lhs) is true•Type(GetBase(lhs)) is Enviroment Record•
GetReferencedName(lhs) is either "eval" or "arguments"
Let oldValue be ToNumber(GetValue(lhs)).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see11.6.3).
Call PutValue(lhs, newValue).
Return oldValue.

11.3.2 Postfix Decrement Operator
The production PostfixExpression : LeftHandSideExpression [no LineTerminator here] -- is evaluated as follows:
1.
2.

Let lhs be the result of evaluating LeftHandSideExpression.
Throw a SyntaxError exception if the following conditions are all true:•Type(lhs) is Reference is true

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3.
4.
5.
6.

•
IsStrictReference(lhs) is true•Type(GetBase(lhs)) is Enviroment Record•
GetReferencedName(lhs) is either "eval" or "arguments"
Let oldValue be ToNumber(GetValue(lhs)).
Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the operator (11.6.3).
Call PutValue(lhs, newValue).
Return oldValue.

11.4 Unary Operators
SyntaxUnaryExpression :PostfixExpression
delete UnaryExpression
void UnaryExpression
typeof UnaryExpression++ UnaryExpression-- UnaryExpression+ UnaryExpression- UnaryExpression~ UnaryExpression! UnaryExpression
11.4.1 The delete Operator
The production UnaryExpression : delete UnaryExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let ref be the result of evaluating UnaryExpression.
If Type(ref) is not Reference, return true.
If IsUnresolvableReference(ref) then,a. If IsStrictReference(ref) is true, throw a SyntaxError exception.
b. Else, return true.
If IsPropertyReference(ref) is true, thena. Return the result of calling the [[Delete]] internal method on ToObject(GetBase(ref)) providing
GetReferencedName(ref) and IsStrictReference(ref) as the arguments.
Else, ref is a Reference to an Environment Record binding, so
a. If IsStrictReference(ref) is true, throw a SyntaxError exception.
b. Let bindings be GetBase(ref).
c. Return the result of calling the DeleteBinding concrete method of bindings, providing
GetReferencedName(ref) as the argument.

NOTE
When a delete operator occurs within strict mode code, a SyntaxError exception is thrown if its
UnaryExpression is a direct reference to a variable, function argument, or function name. In addition, if a delete operator
occurs within strict mode code and the property to be deleted has the attribute { [[Configurable]]: false }, a TypeError
exception is thrown.

11.4.2 The void Operator
The production UnaryExpression : void UnaryExpression is evaluated as follows:
1.
2.
3.

Let expr be the result of evaluating UnaryExpression.
Call GetValue(expr).
Return undefined.

NOTE

70

GetValue must be called even though its value is not used because it may have observable side-effects.

© Ecma International 2009

11.4.3 The typeof Operator
The production UnaryExpression : typeof UnaryExpression is evaluated as follows:
1.
2.
3.

Let val be the result of evaluating UnaryExpression.
If Type(val) is Reference, thena. If IsUnresolvableReference(val) is true, return "undefined".
b. Let val be GetValue(val).
Return a String determined by Type(val) according to Table 20.
Table 20 — typeof Operator Results
Type of val

ResultUndefined"undefined"Null"object"

Boolean"boolean"

Number"number"String"string"

Object (native and does
not implement [[Call]])"object"

Object (native or host anddoes implement [[Call]])"function"

Object (host and does not
implement [[Call]])

Implementation-defined except may
not be "undefined", "boolean","number", or "string".

11.4.4 Prefix Increment Operator
The production UnaryExpression : ++ UnaryExpression is evaluated as follows:
1.
2.

3.
4.
5.
6.

Let expr be the result of evaluating UnaryExpression.
Throw a SyntaxError exception if the following conditions are all true:•Type(expr) is Reference is true•
IsStrictReference(expr) is true•Type(GetBase(expr)) is Enviroment Record•
GetReferencedName(expr) is either "eval" or "arguments"
Let oldValue be ToNumber(GetValue(expr)).
Let newValue be the result of adding the value 1 to oldValue, using the same rules as for the + operator (see11.6.3).
Call PutValue(expr, newValue).
Return newValue.

11.4.5 Prefix Decrement Operator
The production UnaryExpression : -- UnaryExpression is evaluated as follows:
1.
2.

3.

Let expr be the result of evaluating UnaryExpression.
Throw a SyntaxError exception if the following conditions are all true:•Type(expr) is Reference is true•
IsStrictReference(expr) is true•Type(GetBase(expr)) is Enviroment Record•
GetReferencedName(expr) is either "eval" or "arguments"
Let oldValue be ToNumber(GetValue(expr)).

© Ecma International 2009

71

4.
5.
6.

Let newValue be the result of subtracting the value 1 from oldValue, using the same rules as for the operator (see 11.6.3).
Call PutValue(expr, newValue).
Return newValue.

11.4.6 Unary + Operator
The unary + operator converts its operand to Number type.
The production UnaryExpression : + UnaryExpression is evaluated as follows:1.
2.

Let expr be the result of evaluating UnaryExpression.
Return ToNumber(GetValue(expr)).

11.4.7 Unary - Operator
The unary - operator converts its operand to Number type and then negates it. Note that negating +0
produces −0, and negating −0 produces +0.
The production UnaryExpression : - UnaryExpression is evaluated as follows:
1.
2.
3.
4.

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToNumber(GetValue(expr)).
If oldValue is NaN, return NaN.
Return the result of negating oldValue; that is, compute a Number with the same magnitude but opposite
sign.

11.4.8 Bitwise NOT Operator ( ~ )The production UnaryExpression : ~ UnaryExpression is evaluated as follows:
1.
2.
3.

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToInt32(GetValue(expr)).
Return the result of applying bitwise complement to oldValue. The result is a signed 32-bit integer.

11.4.9 Logical NOT Operator ( ! )The production UnaryExpression : ! UnaryExpression is evaluated as follows:
1.
2.
3.
4.

Let expr be the result of evaluating UnaryExpression.
Let oldValue be ToBoolean(GetValue(expr)).
If oldValue is true, return false.
Return true.

11.5 Multiplicative Operators
SyntaxMultiplicativeExpression :UnaryExpression
MultiplicativeExpression * UnaryExpression
MultiplicativeExpression / UnaryExpression
MultiplicativeExpression % UnaryExpression
Semantics
The production MultiplicativeExpression : MultiplicativeExpression @ UnaryExpression, where @ stands for oneof the operators in the above definitions, is evaluated as follows:72© Ecma International 2009

1.
2.
3.
4.
5.
6.
7.

Let left be the result of evaluating MultiplicativeExpression.
Let leftValue be GetValue(left).
Let right be the result of evaluating UnaryExpression.
Let rightValue be GetValue(right).
Let leftNum be ToNumber(leftValue).
Let rightNum be ToNumber(rightValue).
Return the result of applying the specified operation (*, /, or %) to leftNum and rightNum. See the Notes
below 11.5.1, 11.5.2, 11.5.3.

11.5.1 Applying the * Operator
The * operator performs multiplication, producing the product of its operands. Multiplication is commutative.
Multiplication is not always associative in ECMAScript, because of finite precision.
The result of a floating-point multiplication is governed by the rules of IEEE 754 binary double-precision
arithmetic:• If either operand is NaN, the result is NaN.
• The sign of the result is positive if both operands have the same sign, negative if theoperands have different signs.
• Multiplication of an infinity by a zero results in NaN.
• Multiplication of an infinity by an infinity results in an infinity. The sign is determined by therule already stated above.
• Multiplication of an infinity by a finite non-zero value results in a signed infinity. The sign isdetermined by the rule already stated above.
• In the remaining cases, where neither an infinity or NaN is involved, the product is computed
and rounded to the nearest representable value using IEEE 754 round-to-nearest mode. If
the magnitude is too large to represent, the result is then an infinity of appropriate sign. If
the magnitude is too small to represent, the result is then a zero of appropriate sign. The
ECMAScript language requires support of gradual underflow as defined by IEEE 754.
11.5.2 Applying the / Operator
The / operator performs division, producing the quotient of its operands. The left operand is the dividend andthe right operand is the divisor. ECMAScript does not perform integer division. The operands and result of alldivision operations are double-precision floating-point numbers. The result of division is determined by the
specification of IEEE 754 arithmetic:• If either operand is NaN, the result is NaN.
• The sign of the result is positive if both operands have the same sign, negative if theoperands have different signs.
• Division of an infinity by an infinity results in NaN.
• Division of an infinity by a zero results in an infinity. The sign is determined by the rulealready stated above.
• Division of an infinity by a non-zero finite value results in a signed infinity. The sign isdetermined by the rule already stated above.
• Division of a finite value by an infinity results in zero. The sign is determined by the rulealready stated above.
• Division of a zero by a zero results in NaN; division of zero by any other finite value resultsin zero, with the sign determined by the rule already stated above.
• Division of a non-zero finite value by a zero results in a signed infinity. The sign isdetermined by the rule already stated above.
• In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, thequotient is computed and rounded to the nearest representable value using IEEE 754 roundto-nearest mode. If the magnitude is too large to represent, the operation overflows; theresult is then an infinity of appropriate sign. If the magnitude is too small to represent, the© Ecma International 200973

operation underflows and the result is a zero of the appropriate sign. The ECMAScript
language requires support of gradual underflow as defined by IEEE 754.
11.5.3 Applying the % Operator
The % operator yields the remainder of its operands from an implied division; the left operand is the dividend
and the right operand is the divisor.
NOTE
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts floatingpoint operands.

The result of a floating-point remainder operation as computed by the % operator is not the same as the“remainder” operation defined by IEEE 754. The IEEE 754 “remainder” operation computes the remainderfrom a rounding division, not a truncating division, and so its behaviour is not analogous to that of the usual
integer remainder operator. Instead the ECMAScript language defines % on floating-point operations tobehave in a manner analogous to that of the Java integer remainder operator; this may be compared with the
C library function fmod.
The result of an ECMAScript floating-point remainder operation is determined by the rules of IEEE arithmetic:
•
•
•
•
•
•

If either operand is NaN, the result is NaN.
The sign of the result equals the sign of the dividend.
If the dividend is an infinity, or the divisor is a zero, or both, the result is NaN.
If the dividend is finite and the divisor is an infinity, the result equals the dividend.
If the dividend is a zero and the divisor is finite, the result is the same as the dividend.
In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, thefloating-point remainder r from a dividend n and a divisor d is defined by the mathematical
relation r = n − (d * q) where q is an integer that is negative only if n/d is negative andpositive only if n/d is positive, and whose magnitude is as large as possible without
exceeding the magnitude of the true mathematical quotient of n and d.

11.6 Additive Operators
SyntaxAdditiveExpression :MultiplicativeExpression
AdditiveExpression + MultiplicativeExpression
AdditiveExpression - MultiplicativeExpression
11.6.1 The Addition operator ( + )The addition operator either performs string concatenation or numeric addition.
The production AdditiveExpression : AdditiveExpression + MultiplicativeExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.
8.

Let lref be the result of evaluating AdditiveExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be GetValue(rref).
Let lprim be ToPrimitive(lval).
Let rprim be ToPrimitive(rval).
If Type(lprim) is String or Type(rprim) is String, thena. Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)
Return the result of applying the addition operation to ToNumber(lprim) and ToNumber(rprim). See theNote below 11.6.3.

NOTE 1
No hint is provided in the calls to ToPrimitive in steps 5 and 6. All native ECMAScript objects except Dateobjects handle the absence of a hint as if the hint Number were given; Date objects handle the absence of a hint as if thehint String were given. Host objects may handle the absence of a hint in some other manner.

74© Ecma International 2009

NOTE 2
Step 7 differs from step 3 of the comparison algorithm for the relational operators (11.8.5), by using the
logical-or operation instead of the logical-and operation.

11.6.2 The Subtraction Operator ( - )The production AdditiveExpression : AdditiveExpression - MultiplicativeExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.

Let lref be the result of evaluating AdditiveExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating MultiplicativeExpression.
Let rval be GetValue(rref).
Let lnum be ToNumber(lval).
Let rnum be ToNumber(rval).
Return the result of applying the subtraction operation to lnum and rnum. See the note below 11.6.3.

11.6.3 Applying the Additive Operators to Numbers
The + operator performs addition when applied to two operands of numeric type, producing the sum of theoperands. The - operator performs subtraction, producing the difference of two numeric operands.
Addition is a commutative operation, but not always associative.
The result of an addition is determined using the rules of IEEE 754 binary double-precision arithmetic:
•
•
•
•
•

If either operand is NaN, the result is NaN.
The sum of two infinities of opposite sign is NaN.
The sum of two infinities of the same sign is the infinity of that sign.
The sum of an infinity and a finite value is equal to the infinite operand.
The sum of two negative zeros is −0. The sum of two positive zeros, or of two zeros ofopposite sign, is +0.
• The sum of a zero and a nonzero finite value is equal to the nonzero operand.
• The sum of two nonzero finite values of the same magnitude and opposite sign is +0.
• In the remaining cases, where neither an infinity, nor a zero, nor NaN is involved, and theoperands have the same sign or have different magnitudes, the sum is computed androunded to the nearest representable value using IEEE 754 round-to-nearest mode. If themagnitude is too large to represent, the operation overflows and the result is then an infinityof appropriate sign. The ECMAScript language requires support of gradual underflow as
defined by IEEE 754.
The - operator performs subtraction when applied to two operands of numeric type, producing the differenceof its operands; the left operand is the minuend and the right operand is the subtrahend. Given numeric
operands a and b, it is always the case that a–b produces the same result as a +(–b).

11.7 Bitwise Shift Operators
SyntaxShiftExpression :AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression
11.7.1 The Left Shift Operator ( << )Performs a bitwise left shift operation on the left operand by the amount specified by the right operand.

© Ecma International 200975

The production ShiftExpression : ShiftExpression << AdditiveExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.
8.

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
Let lnum be ToInt32(lval).
Let rnum be ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum
& 0x1F.
Return the result of left shifting lnum by shiftCount bits. The result is a signed 32-bit integer.

11.7.2 The Signed Right Shift Operator ( >> )Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right
operand.
The production ShiftExpression : ShiftExpression >> AdditiveExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.
8.

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
Let lnum be ToInt32(lval).
Let rnum be ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum
& 0x1F.
Return the result of performing a sign-extending right shift of lnum by shiftCount bits. The most significant
bit is propagated. The result is a signed 32-bit integer.

11.7.3 The Unsigned Right Shift Operator ( >>> )Performs a zero-filling bitwise right shift operation on the left operand by the amount specified by the right
operand.
The production ShiftExpression : ShiftExpression >>> AdditiveExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.
8.

76

Let lref be the result of evaluating ShiftExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating AdditiveExpression.
Let rval be GetValue(rref).
Let lnum be ToUint32(lval).
Let rnum be ToUint32(rval).
Let shiftCount be the result of masking out all but the least significant 5 bits of rnum, that is, compute rnum
& 0x1F.
Return the result of performing a zero-filling right shift of lnum by shiftCount bits. Vacated bits are filledwith zero. The result is an unsigned 32-bit integer.

© Ecma International 2009

11.8 Relational Operators
SyntaxRelationalExpression :ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression
RelationalExpressionNoIn :ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression
NOTE
The “NoIn” variants are needed to avoid confusing the in operator in a relational expression with the inoperator in a for statement.

Semantics
The result of evaluating a relational operator is always of type Boolean, reflecting whether the relationshipnamed by the operator holds between its two operands.
The RelationalExpressionNoIn productions are evaluated in the same manner as the RelationalExpression
productions except that the contained RelationalExpressionNoIn is evaluated instead of the contained
RelationalExpression.
11.8.1 The Less-than Operator ( < )The production RelationalExpression : RelationalExpression < ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
If r is undefined, return false. Otherwise, return r.

11.8.2 The Greater-than Operator ( > )The production RelationalExpression : RelationalExpression > ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
Let r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see11.8.5).
If r is undefined, return false. Otherwise, return r.

© Ecma International 200977

11.8.3 The Less-than-or-equal Operator ( <= )The production RelationalExpression : RelationalExpression <= ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
Let r be the result of performing abstract relational comparison rval < lval with LeftFirst equal to false. (see11.8.5).
If r is true or undefined, return false. Otherwise, return true.

11.8.4 The Greater-than-or-equal Operator ( >= )The production RelationalExpression : RelationalExpression >= ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
Let r be the result of performing abstract relational comparison lval < rval. (see 11.8.5)
If r is true or undefined, return false. Otherwise, return true.

11.8.5 The Abstract Relational Comparison Algorithm
The comparison x < y, where x and y are values, produces true, false, or undefined (which indicates that atleast one operand is NaN). In addition to x and y the algorithm takes a Boolean flag named LeftFirst as aparameter. The flag is used to control the order in which operations with potentially visible side-effects are
performed upon x and y. It is necessary because ECMAScript specifies left to right evaluation of expressions.
The default value of LeftFirst is true and indicates that the x parameter corresponds to an expression that
occurs to the left of the y parameter’s corresponding expression. If LeftFirst is false, the reverse is the caseand operations must be performed upon y before x. Such a comparison is performed as follows:
1.
2.
3.

4.

78

If the LeftFirst flag is true, thena. Let px be the result of calling ToPrimitive(x, hint Number).
b. Let py be the result of calling ToPrimitive(y, hint Number).
Else the order of evaluation needs to be reversed to preserve left to right evaluationa. Let py be the result of calling ToPrimitive(y, hint Number).
b. Let px be the result of calling ToPrimitive(x, hint Number).
If it is not the case that both Type(px) is String and Type(py) is String, thena. Let nx be the result of calling ToNumber(px). Because px and py are primitive values evaluation
order is not important.
b. Let ny be the result of calling ToNumber(py).
c. If nx is NaN, return undefined.
d. If ny is NaN, return undefined.
e. If nx and ny are the same Number value, return false.
f. If nx is +0 and ny is −0, return false.
g. If nx is −0 and ny is +0, return false.
h. If nx is +∞, return false.
i. If ny is +∞, return true.
j. If ny is −∞, return false.
k. If nx is −∞, return true.
l. If the mathematical value of nx is less than the mathematical value of ny —note that thesemathematical values are both finite and not both zero—return true. Otherwise, return false.
Else, both px and py are Strings
a. If py is a prefix of px, return false. (A String value p is a prefix of String value q if q can be theresult of concatenating p and some other String r. Note that any String is a prefix of itself, because r
may be the empty String.)© Ecma International 2009

b.
c.
d.
e.
f.
NOTE 1

If px is a prefix of py, return true.
Let k be the smallest nonnegative integer such that the character at position k within px is different
from the character at position k within py. (There must be such a k, for neither String is a prefix ofthe other.)
Let m be the integer that is the code unit value for the character at position k within px.
Let n be the integer that is the code unit value for the character at position k within py.
If m < n, return true. Otherwise, return false.
Step 3 differs from step 7 in the algorithm for the addition operator + (11.6.1) in using and instead of or.

NOTE 2The comparison of Strings uses a simple lexicographic ordering on sequences of code unit values. There is noattempt to use the more complex, semantically oriented definitions of character or string equality and collating orderdefined in the Unicode specification. Therefore String values that are canonically equal according to the Unicode standard
could test as unequal. In effect this algorithm assumes that both Strings are already in normalised form. Also, note that for
strings containing supplementary characters, lexicographic ordering on sequences of UTF-16 code unit values differs fromthat on sequences of code point values.

11.8.6 The instanceof operator
The production RelationalExpression: RelationalExpression instanceof ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
If Type(rval) is not Object, throw a TypeError exception.
If rval does not have a [[HasInstance]] internal method, throw a TypeError exception.
Return the result of calling the [[HasInstance]] internal method of rval with argument lval.

11.8.7 The in operator
The production RelationalExpression : RelationalExpression in ShiftExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating RelationalExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating ShiftExpression.
Let rval be GetValue(rref).
If Type(rval) is not Object, throw a TypeError exception.
Return the result of calling the [[HasProperty]] internal method of rval with argument ToString(lval).

11.9 Equality Operators
SyntaxEqualityExpression :RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression
EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression
EqualityExpressionNoIn :RelationalExpressionNoIn
EqualityExpressionNoIn == RelationalExpressionNoIn
EqualityExpressionNoIn != RelationalExpressionNoIn
EqualityExpressionNoIn === RelationalExpressionNoIn
EqualityExpressionNoIn !== RelationalExpressionNoIn© Ecma International 200979

Semantics
The result of evaluating an equality operator is always of type Boolean, reflecting whether the relationshipnamed by the operator holds between its two operands.
The EqualityExpressionNoIn productions are evaluated in the same manner as the EqualityExpression
productions except that the contained EqualityExpressionNoIn and RelationalExpressionNoIn are evaluatedinstead of the contained EqualityExpression and RelationalExpression, respectively.
11.9.1 The Equals Operator ( == )The production EqualityExpression : EqualityExpression == RelationalExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
Return the result of performing abstract equality comparison rval == lval. (see 11.9.3).

11.9.2 The Does-not-equals Operator ( != )The production EqualityExpression : EqualityExpression != RelationalExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
Let r be the result of performing abstract equality comparison rval == lval. (see 11.9.3).
If r is true, return false. Otherwise, return true.

11.9.3 The Abstract Equality Comparison Algorithm
The comparison x == y, where x and y are values, produces true or false. Such a comparison is performed asfollows:
1.

2.
3.
4.
5.
6.
7.
8.

80

If Type(x) is the same as Type(y), thena.
If Type(x) is Undefined, return true.
b. If Type(x) is Null, return true.
c.
If Type(x) is Number, theni.
If x is NaN, return false.
ii.
If y is NaN, return false.
iii.
If x is the same Number value as y, return true.
iv.
If x is +0 and y is −0, return true.
v.
If x is −0 and y is +0, return true.
vi.
Return false.
d. If Type(x) is String, then return true if x and y are exactly the same sequence of characters (samelength and same characters in corresponding positions). Otherwise, return false.
e. If Type(x) is Boolean, return true if x and y are both true or both false. Otherwise, return false.
f. Return true if x and y refer to the same object. Otherwise, return false.
If x is null and y is undefined, return true.
If x is undefined and y is null, return true.
If Type(x) is Number and Type(y) is String,
return the result of the comparison x == ToNumber(y).
If Type(x) is String and Type(y) is Number,
return the result of the comparison ToNumber(x) == y.
If Type(x) is Boolean, return the result of the comparison ToNumber(x) == y.
If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
If Type(x) is either String or Number and Type(y) is Object,return the result of the comparison x == ToPrimitive(y).

© Ecma International 2009

9.

If Type(x) is Object and Type(y) is either String or Number,
return the result of the comparison ToPrimitive(x) == y.
10. Return false.
NOTE 1Given the above definition of equality:•

String comparison can be forced by: "" + a == "" + b.

•

Numeric comparison can be forced by: +a == +b.

•

Boolean comparison can be forced by: !a == !b.

NOTE 2The equality operators maintain the following invariants:••

A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of A and B.

NOTE 3The equality operator is not always transitive. For example, there might be two distinct String objects, each
representing the same String value; each String object would be considered equal to the String value by the == operator,
but the two String objects would not be equal to each other.
NOTE 4Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt touse the more complex, semantically oriented definitions of character or string equality and collating order defined in the
Unicode specification. Therefore Strings values that are canonically equal according to the Unicode standard could test as
unequal. In effect this algorithm assumes that both Strings are already in normalised form.

11.9.4 The Strict Equals Operator ( === )The production EqualityExpression : EqualityExpression === RelationalExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
Return the result of performing the strict equality comparison rval === lval. (See 11.9.6)

11.9.5 The Strict Does-not-equal Operator ( !== )The production EqualityExpression : EqualityExpression !== RelationalExpression is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let lref be the result of evaluating EqualityExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating RelationalExpression.
Let rval be GetValue(rref).
Let r be the result of performing strict equality comparison rval === lval. (See 11.9.6)
If r is true, return false. Otherwise, return true.

11.9.6 The Strict Equality Comparison Algorithm
The comparison x === y, where x and y are values, produces true or false. Such a comparison is performed
as follows:
1.
2.
3.
4.

If Type(x) is different from Type(y), return false.
If Type(x) is Undefined, return true.
If Type(x) is Null, return true.
If Type(x) is Number, thena. If x is NaN, return false.
b. If y is NaN, return false.
c. If x is the same Number value as y, return true.
d. If x is +0 and y is −0, return true.
e. If x is −0 and y is +0, return true.
f. Return false.

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81

5.
6.
7.

If Type(x) is String, then return true if x and y are exactly the same sequence of characters (same length andsame characters in corresponding positions); otherwise, return false.
If Type(x) is Boolean, return true if x and y are both true or both false; otherwise, return false.
Return true if x and y refer to the same object. Otherwise, return false.

NOTE

This algorithm differs from the SameValue Algorithm (9.12) in its treatment of signed zeroes and NaNs.

11.10 Binary Bitwise Operators
SyntaxBitwiseANDExpression :EqualityExpression
BitwiseANDExpression & EqualityExpression
BitwiseANDExpressionNoIn :EqualityExpressionNoIn
BitwiseANDExpressionNoIn & EqualityExpressionNoIn
BitwiseXORExpression :BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression
BitwiseXORExpressionNoIn :BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn
BitwiseORExpression :BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression
BitwiseORExpressionNoIn :BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn
Semantics
The production A : A @ B, where @ is one of the bitwise operators in the productions above, is evaluated asfollows:
1.
2.
3.
4.
5.
6.
7.

Let lref be the result of evaluating A.
Let lval be GetValue(lref).
Let rref be the result of evaluating B.
Let rval be GetValue(rref).
Let lnum be ToInt32(lval).
Let rnum be ToInt32(rval).
Return the result of applying the bitwise operator @ to lnum and rnum. The result is a signed 32 bit integer.

11.11 Binary Logical Operators
Syntax
LogicalANDExpression :BitwiseORExpression
LogicalANDExpression && BitwiseORExpression
LogicalANDExpressionNoIn :BitwiseORExpressionNoIn
LogicalANDExpressionNoIn && BitwiseORExpressionNoIn

82© Ecma International 2009

LogicalORExpression :LogicalANDExpression
LogicalORExpression || LogicalANDExpression
LogicalORExpressionNoIn :LogicalANDExpressionNoIn
LogicalORExpressionNoIn || LogicalANDExpressionNoIn
Semantics
The production LogicalANDExpression : LogicalANDExpression && BitwiseORExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let lref be the result of evaluating LogicalANDExpression.
Let lval be GetValue(lref).
If ToBoolean(lval) is false, return lval.
Let rref be the result of evaluating BitwiseORExpression.
Return GetValue(rref).

The production LogicalORExpression : LogicalORExpression || LogicalANDExpression is evaluated as follows:
1.
2.
3.
4.
5.

Let lref be the result of evaluating LogicalORExpression.
Let lval be GetValue(lref).
If ToBoolean(lval) is true, return lval.
Let rref be the result of evaluating LogicalANDExpression.
Return GetValue(rref).

The LogicalANDExpressionNoIn and LogicalORExpressionNoIn productions are evaluated in the same manneras the LogicalANDExpression and LogicalORExpression productions except that the contained
LogicalANDExpressionNoIn, BitwiseORExpressionNoIn and LogicalORExpressionNoIn are evaluated instead of thecontained LogicalANDExpression, BitwiseORExpression and LogicalORExpression, respectively.
NOTE
The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always
be the value of one of the two operand expressions.

11.12 Conditional Operator ( ? : )Syntax
ConditionalExpression :LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression
ConditionalExpressionNoIn :LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpression : AssignmentExpressionNoIn
Semantics
The production ConditionalExpression : LogicalORExpression ? AssignmentExpression : AssignmentExpression isevaluated as follows:
1.
2.
3.

Let lref be the result of evaluating LogicalORExpression.
If ToBoolean(GetValue(lref)) is true, thena. Let trueRef be the result of evaluating the first AssignmentExpression.
b. Return GetValue(trueRef).
Elsea. Let falseRef be the result of evaluating the second AssignmentExpression.
b. Return GetValue(falseRef).

© Ecma International 200983

The ConditionalExpressionNoIn production is evaluated in the same manner as the ConditionalExpression
production
exceptthatthecontained
LogicalORExpressionNoIn,
AssignmentExpression
andAssignmentExpressionNoIn are evaluated instead of the contained LogicalORExpression, first
AssignmentExpression and second AssignmentExpression, respectively.
NOTE
The grammar for a ConditionalExpression in ECMAScript is a little bit different from that in C and Java, which
each allow the second subexpression to be an Expression but restrict the third expression to be a ConditionalExpression.
The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by either arm of aconditional and to eliminate the confusing and fairly useless case of a comma expression as the centre expression.

11.13 Assignment Operators
SyntaxAssignmentExpression :ConditionalExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression
AssignmentExpressionNoIn :ConditionalExpressionNoIn
LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn
AssignmentOperator : one of
=
*=
/=

%=

+=

-=

<<=

>>=

>>>= &=^=|=

Semantics
The AssignmentExpressionNoIn productions are evaluated in the same manner as the AssignmentExpression
productions except that the contained ConditionalExpressionNoIn and AssignmentExpressionNoIn are evaluatedinstead of the contained ConditionalExpression and AssignmentExpression, respectively.
11.13.1

Simple Assignment ( = )The production AssignmentExpression : LeftHandSideExpression = AssignmentExpression is evaluated as follows:
1.
2.
3.
4.

5.
6.

Let lref be the result of evaluating LeftHandSideExpression.
Let rref be the result of evaluating AssignmentExpression.
Let rval be GetValue(rref).
Throw a SyntaxError exception if the following conditions are all true:•Type(lref) is Reference is true•
IsStrictReference(lref) is true•Type(GetBase(lref)) is Enviroment Record•
GetReferencedName(lref) is either "eval" or "arguments"
Call PutValue(lref, rval).
Return rval.

NOTE
When an assignment occurs within strict mode code, its LeftHandSide must not evaluate to an unresolvablereference. If it does a ReferenceError exception is thrown upon assignment. The LeftHandSide also may not be areference to a data property with the attribute value {[[Writable]]:false}, to an accessor property with the attribute value{[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal property has the value false. In
these cases a TypeError exception is thrown.

11.13.2

Compound Assignment ( op= )The production AssignmentExpression : LeftHandSideExpression @ = AssignmentExpression, where @ represents
one of the operators indicated above, is evaluated as follows:84© Ecma International 2009

1.
2.
3.
4.
5.
6.

7.
8.

Let lref be the result of evaluating LeftHandSideExpression.
Let lval be GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Let rval be GetValue(rref).
Let r be the result of applying operator @ to lval and rval.
Throw a SyntaxError exception if the following conditions are all true:•Type(lref) is Reference is true•
IsStrictReference(lref) is true•Type(GetBase(lref)) is Enviroment Record•
GetReferencedName(lref) is either "eval" or "arguments"
Call PutValue(lref, r).
Return r.

NOTE

See NOTE 11.13.1.

11.14 Comma Operator ( , )SyntaxExpression :AssignmentExpression
Expression , AssignmentExpression
ExpressionNoIn :AssignmentExpressionNoIn
ExpressionNoIn , AssignmentExpressionNoIn
Semantics
The production Expression : Expression , AssignmentExpression is evaluated as follows:
1.
2.
3.
4.

Let lref be the result of evaluating Expression.
Call GetValue(lref).
Let rref be the result of evaluating AssignmentExpression.
Return GetValue(rref).

The ExpressionNoIn production is evaluated in the same manner as the Expression production except that thecontained ExpressionNoIn and AssignmentExpressionNoIn are evaluated instead of the contained Expression and
AssignmentExpression, respectively.
NOTE

GetValue must be called even though its value is not used because it may have observable side-effects.

© Ecma International 200985

12 Statements
SyntaxStatement :Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement
Semantics
A Statement can be part of a LabelledStatement, which itself can be part of a LabelledStatement, and so on. The
labels introduced this way are collectively referred to as the “current label set” when describing the semantics
of individual statements. A LabelledStatement has no semantic meaning other than the introduction of a label toa label set. The label set of an IterationStatement or a SwitchStatement initially contains the single element
empty. The label set of any other statement is initially empty.
NOTE
Several widely used implementations of ECMAScript are known to support the use of FunctionDeclaration as aStatement. However there are significant and irreconcilable variations among the implementations in the semantics applied
to such FunctionDeclarations. Because of these irreconcilable difference, the use of a FunctionDeclaration as a Statement
results in code that is not reliably portable among implementations. It is recommended that ECMAScript implementations
either disallow this usage of FunctionDeclaration or issue a warning when such a usage is encountered. Future editions of
ECMAScript may define alternative portable means for declaring functions in a Statement context.

12.1 Block
SyntaxBlock :
{ StatementListopt }StatementList :Statement
StatementList Statement
Semantics
The production Block : { } is evaluated as follows:
1.

Return (normal, empty, empty).

The production Block : { StatementList }is evaluated as follows:
1.

Return the result of evaluating StatementList.

The production StatementList : Statement is evaluated as follows:86© Ecma International 2009

1.
2.
3.

Let s be the result of evaluating Statement.
If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as
if no exception were thrown.)
Return s.

The production StatementList : StatementList Statement is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let sl be the result of evaluating StatementList.
If sl is an abrupt completion, return sl.
Let s be the result of evaluating Statement.
If an exception was thrown, return (throw, V, empty) where V is the exception. (Execution now proceeds as
if no exception were thrown.)
If s.value is empty, let V = sl.value, otherwise let V = s.value.
Return (s.type, V, s.target).

12.2 Variable Statement
Syntax
VariableStatement :var VariableDeclarationList ;VariableDeclarationList :VariableDeclaration
VariableDeclarationList , VariableDeclaration
VariableDeclarationListNoIn :VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn
VariableDeclaration :Identifier Initialiseropt
VariableDeclarationNoIn :Identifier InitialiserNoInopt
Initialiser := AssignmentExpression
InitialiserNoIn := AssignmentExpressionNoIn
A variable statement declares variables that are created as defined in 10.5. Variables are initialised toundefined when created. A variable with an Initialiser is assigned the value of its AssignmentExpression when
the VariableStatement is executed, not when the variable is created.
Semantics
The production VariableStatement : var VariableDeclarationList ; is evaluated as follows:1.
2.

Evaluate VariableDeclarationList.
Return (normal, empty, empty).

The production VariableDeclarationList :VariableDeclaration is evaluated as follows:1.

Evaluate VariableDeclaration.

The production VariableDeclarationList : VariableDeclarationList , VariableDeclaration is evaluated as follows:© Ecma International 2009

87

1.
2.

Evaluate VariableDeclarationList.
Evaluate VariableDeclaration.

The production VariableDeclaration : Identifier is evaluated as follows:
1.

Return a String value containing the same sequence of characters as in the Identifier.

The production VariableDeclaration : Identifier Initialiser is evaluated as follows:
1.
2.
3.
4.
5.

Let lhs be the result of evaluating Identifier as described in 11.1.2.
Let rhs be the result of evaluating Initialiser.
Let value be GetValue(rhs).
Call PutValue(lhs, value).
Return a String value containing the same sequence of characters as in the Identifier.

NOTE

The String value of a VariableDeclaration is used in the evaluation of for-in statements (12.6.4).

If a VariableDeclaration is nested within a with statement and the Identifier in the VariableDeclaration is thesame as a property name of the binding object of the with statement’s object environment record, then step 4
will assign value to the property instead of to the VariableEnvironment binding of the Identifier.
The production Initialiser : = AssignmentExpression is evaluated as follows:
1. Return the result of evaluating AssignmentExpression.
The VariableDeclarationListNoIn, VariableDeclarationNoIn and InitialiserNoIn productions are evaluated in thesame manner as the VariableDeclarationList, VariableDeclaration and Initialiser productions except that thecontained VariableDeclarationListNoIn, VariableDeclarationNoIn, InitialiserNoIn and AssignmentExpressionNoIn
are evaluated instead of the contained VariableDeclarationList, VariableDeclaration, Initialiser andAssignmentExpression, respectively.
12.2.1 Strict Mode Restrictions
It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict code and its Identifier
is either "eval" or "arguments".

12.3 Empty Statement
SyntaxEmptyStatement :;Semantics
The production EmptyStatement : ; is evaluated as follows:
1. Return (normal, empty, empty).

12.4 Expression Statement
SyntaxExpressionStatement :[lookahead ∉ {{, function}] Expression ;NOTE
An ExpressionStatement cannot start with an opening curly brace because that might make it ambiguous with a
Block. Also, an ExpressionStatement cannot start with the function keyword because that might make it ambiguous with aFunctionDeclaration.

88© Ecma International 2009

Semantics
The production ExpressionStatement : [lookahead ∉ {{, function}] Expression; is evaluated as follows:1.
2.

Let exprRef be the result of evaluating Expression.
Return (normal, GetValue(exprRef), empty).

12.5 The if Statement
SyntaxIfStatement :if ( Expression ) Statement else Statement
if ( Expression ) Statement
Each else for which the choice of associated if is ambiguous shall be associated with the nearest possible
if that would otherwise have no corresponding else.
Semantics
The production IfStatement : if ( Expression ) Statement else Statement is evaluated as follows:
1.
2.
3.

Let exprRef be the result of evaluating Expression.
If ToBoolean(GetValue(exprRef)) is true, thena. Return the result of evaluating the first Statement.
Else,
a. Return the result of evaluating the second Statement.

The production IfStatement : if ( Expression ) Statement is evaluated as follows:
1. Let exprRef be the result of evaluating Expression.
2. If ToBoolean(GetValue(exprRef)) is false, return (normal, empty, empty).
3. Return the result of evaluating Statement.

12.6 Iteration Statements
Syntax
IterationStatement :do Statement while ( Expression );while ( Expression ) Statementfor (ExpressionNoInopt; Expressionopt ; Expressionopt ) Statementfor ( var VariableDeclarationListNoIn; Expressionopt ; Expressionopt ) Statementfor ( LeftHandSideExpression in Expression ) Statementfor ( var VariableDeclarationNoIn in Expression ) Statement
12.6.1 The do-while Statement
The production do Statement while ( Expression ); is evaluated as follows:
1.
2.
3.

Let V = empty.
Let iterating be true.
Repeat, while iterating is true
a. Let stmt be the result of evaluating Statement.
b. If stmt.value is not empty, let V = stmt.value.
c. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
ii.
If stmt is an abrupt completion, return stmt.
d. Let exprRef be the result of evaluating Expression.

© Ecma International 2009

89

4.

e. If ToBoolean(GetValue(exprRef)) is false, set iterating to false.
Return (normal, V, empty);

12.6.2 The while Statement
The production IterationStatement : while ( Expression ) Statement is evaluated as follows:
1.
2.

Let V = empty.
Repeata. Let exprRef be the result of evaluating Expression.
b. If ToBoolean(GetValue(exprRef)) is false, return (normal, V, empty).
c. Let stmt be the result of evaluating Statement.
d. If stmt.value is not empty, let V = stmt.value.
e. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt.type is break and stmt.target is in the current label set, then
1. Return (normal, V, empty).
ii.
If stmt is an abrupt completion, return stmt.

12.6.3 The for Statement
The production
IterationStatement : for (ExpressionNoInopt ; Expressionopt ; Expressionopt) Statement
is evaluated as follows:
1.
2.
3.

If ExpressionNoIn is present, then.
a. Let exprRef be the result of evaluating ExpressionNoIn.
b. Call GetValue(exprRef). (This value is not used.)
Let V = empty.
Repeata. If the first Expression is present, theni.
Let testExprRef be the result of evaluating the first Expression.
ii.
If GetValue(testExprRef) is false, return (normal, V, empty).
b. Let stmt be the result of evaluating Statement.
c. If stmt.value is not empty, let V = stmt.valued. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
e. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt is an abrupt completion, return stmt.
f. If the second Expression is present, theni.
Let incExprRef be the result of evaluating the second Expression.
ii.
Call GetValue(incExprRef). (This value is not used.)

The production
IterationStatement : for ( var VariableDeclarationListNoIn ; Expressionopt ; Expressionopt ) Statement
is evaluated as follows:
1.
2.
3.

90

Evaluate VariableDeclarationListNoIn.
Let V = empty.
Repeata. If the first Expression is present, theni.
Let testExprRef be the result of evaluating the first Expression.
ii.
If GetValue(testExprRef) is false, then return (normal, V, empty).
b. Let stmt be the result of evaluating Statement.
c. If stmt.value is not empty, let V = stmt.value.
d. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
e. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt is an abrupt completion, return stmt.
f. If the second Expression is present, then.
i.
Let incExprRef be the result of evaluating the second Expression.
ii.
Call GetValue(incExprRef). (This value is not used.)© Ecma International 2009

12.6.4 The for-in Statement
The production IterationStatement : for ( LeftHandSideExpression in Expression ) Statement is evaluated asfollows:
1.
2.
3.
4.
5.
6.

Let exprRef be the result of evaluating the Expression.
Let experValue be GetValue(exprRef).
If experValue is null or undefined, return (normal, empty, empty).
Let obj be ToObject(experValue).
Let V = empty.
Repeata. Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is nosuch property, return (normal, V, empty).
b. Let lhsRef be the result of evaluating the LeftHandSideExpression ( it may be evaluated repeatedly).
c. Call PutValue(lhsRef, P).
d. Let stmt be the result of evaluating Statement.
e. If stmt.value is not empty, let V = stmt.value.
f. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
g. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt is an abrupt completion, return stmt.

The production
IterationStatement : for ( var VariableDeclarationNoIn in Expression ) Statement
is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.

Let varName be the result of evaluating VariableDeclarationNoIn.
Let exprRef be the result of evaluating the Expression.
Let experValue be GetValue(exprRef).
If experValue is null or undefined, return (normal, empty, empty).
Let obj be ToObject(experValue).
Let V = empty.
Repeata. Let P be the name of the next property of obj whose [[Enumerable]] attribute is true. If there is nosuch property, return (normal, V, empty).
b. Let varRef be the result of evaluating varName as if it were an Identifier Reference (11.1.2); it may
be evaluated repeatedly.
c. Call PutValue(varRef, P).
d. Let stmt be the result of evaluating Statement.
e. If stmt.value is not empty, let V = stmt.value.
f. If stmt.type is break and stmt.target is in the current label set, return (normal, V, empty).
g. If stmt.type is not continue || stmt.target is not in the current label set, theni.
If stmt is an abrupt completion, return stmt.

The mechanics and order of enumerating the properties (step 6.a in the first algorithm, step 7.a in the second)
is not specified. Properties of the object being enumerated may be deleted during enumeration. If a property
that has not yet been visited during enumeration is deleted, then it will not be visited. If new properties areadded to the object being enumerated during enumeration, the newly added properties are not guaranteed to
be visited in the active enumeration.
Enumerating the properties of an object includes enumerating properties of its prototype, and the prototype of
the prototype, and so on, recursively; but a property of a prototype is not enumerated if it is “shadowed”
because some previous object in the prototype chain has a property with the same name.
NOTE

See NOTE 11.13.1.

© Ecma International 200991

12.7 The continue Statement
SyntaxContinueStatement :continue [no LineTerminator here] Identifieropt ;Semantics
A program is considered syntactically incorrect if either of the following is true:••

The program contains a continue statement without the optional Identifier, which is notnested, directly or indirectly (but not crossing function boundaries), within an
IterationStatement.
The program contains a continue statement with the optional Identifier, where Identifier
does not appear in the label set of an enclosing (but not crossing function boundaries)
IterationStatement.

A ContinueStatement without an Identifier is evaluated as follows:
1.

Return (continue, empty, empty).

A ContinueStatement with the optional Identifier is evaluated as follows:
1. Return (continue, empty, Identifier).

12.8 The break Statement
SyntaxBreakStatement :break [no LineTerminator here] Identifieropt ;Semantics
A program is considered syntactically incorrect if either of the following is true:••

The program contains a break statement without the optional Identifier, which is notnested, directly or indirectly (but not crossing function boundaries), within an
IterationStatement or a SwitchStatement.
The program contains a break statement with the optional Identifier, where Identifier does
not appear in the label set of an enclosing (but not crossing function boundaries) Statement.

A BreakStatement without an Identifier is evaluated as follows:
1.

Return (break, empty, empty).

A BreakStatement with an Identifier is evaluated as follows:
1.

Return (break, empty, Identifier).

12.9 The return Statement
SyntaxReturnStatement :return [no LineTerminator here] Expressionopt ;92© Ecma International 2009

Semantics
An ECMAScript program is considered syntactically incorrect if it contains a return statement that is notwithin a FunctionBody. A return statement causes a function to cease execution and return a value to thecaller. If Expression is omitted, the return value is undefined. Otherwise, the return value is the value ofExpression.
The production ReturnStatement : return [no LineTerminator here] Expressionopt ; is evaluated as:
1.
2.
3.

If the Expression is not present, return (return, undefined, empty).
Let exprRef be the result of evaluating Expression.
Return (return, GetValue(exprRef), empty).

12.10 The with Statement
SyntaxWithStatement :with ( Expression ) Statement
The with statement adds an object environment record for a computed object to the lexical environment of
the current execution context. It then executes a statement using this augmented lexical environment. Finally,
it restores the original lexical environment.
Semantics
The production WithStatement : with ( Expression ) Statement is evaluated as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.

Let val be the result of evaluating Expression.
Let obj be ToObject(GetValue(val)).
Let oldEnv be the running execution context’s LexicalEnvironment.
Let newEnv be the result of calling NewObjectEnvironment passing obj and oldEnv as the arguments
Set the provideThis flag of newEnv to true.
Set the running execution context’s LexicalEnvironment to newEnv.
Let C be the result of evaluating Statement but if an exception is thrown during the evaluation, let C be(throw, V, empty), where V is the exception. (Execution now proceeds as if no exception were thrown.)
Set the running execution context’s Lexical Environment to oldEnv.
Return C.

NOTE
No matter how control leaves the embedded Statement, whether normally or by some form of abrupt
completion or exception, the LexicalEnvironment is always restored to its former state.

12.10.1

Strict Mode Restrictions

Strict mode code may not include a WithStatement. The occurrence of a WithStatement in such a context istreated as a SyntaxError.

12.11 The switch Statement
Syntax
SwitchStatement :switch ( Expression ) CaseBlock
CaseBlock :{ CaseClausesopt }{ CaseClausesopt DefaultClause CaseClausesopt }© Ecma International 200993

CaseClauses :CaseClause
CaseClauses CaseClause
CaseClause :case Expression : StatementListopt
DefaultClause :default : StatementListopt
Semantics
The production SwitchStatement : switch ( Expression ) CaseBlock is evaluated as follows:
1.
2.
3.
4.

Let exprRef be the result of evaluating Expression.
Let R be the result of evaluating CaseBlock, passing it GetValue(exprRef) as a parameter.
If R.type is break and R.target is in the current label set, return (normal, R.value, empty).
Return R.

The production CaseBlock : { CaseClausesopt } is given an input parameter, input, and is evaluated as follows:
1.
2.
3.
4.

5.

Let V = empty.
Let A be the list of CaseClause items in source text order.
Let searching be true.
Repeat, while searching is true
a. Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V, empty).
b. Let clauseSelector be the result of evaluating C.
c. If input is equal to clauseSelector as defined by the === operator, theni.
Set searching to false.
ii.
If C has a StatementList, then
1. Evaluate C’s StatementList and let R be the result.
2. If R is an abrupt completion, then return R.
3. Let V = R.value.
Repeata. Let C be the next CaseClause in A. If there is no such CaseClause, return (normal, V, empty).
b. If C has a StatementList, theni.
Evaluate C’s StatementList and let R be the result.
ii.
If R.value is not empty, then let V = R.value.
iii.
If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseBlock : { CaseClausesopt DefaultClause CaseClausesopt } is given an input parameter, input,
and is evaluated as follows:
1.
2.
3.
4.
5.

6.
7.

94

Let V = empty.
Let A be the list of CaseClause items in the first CaseClauses, in source text order.
Let B be the list of CaseClause items in the second CaseClauses, in source text order.
Let found be false.
Repeat letting C be in order each CaseClause in A
a. If found is false, theni.
Let clauseSelector be the result of evaluating C.
ii.
If input is equal to clauseSelector as defined by the === operator, then set found to true.
b. If found is true, theni.
If C has a StatementList, then
1. Evaluate C’s StatementList and let R be the result.
2. If R.value is not empty, then let V = R.value.
3. R is an abrupt completion, then return (R.type, V, R.target).
Let foundInB be false.
If found is false, thena. Repeat, while foundInB is false and all elements of B have not been processed© Ecma International 2009

i.
ii.
iii.

8.

9.

Let C be the next CaseClause in B.
Let clauseSelector be the result of evaluating C.
If input is equal to clauseSelector as defined by the === operator, then
1. Set foundInB to true.
2. If C has a StatementList, thena Evaluate C’s StatementList and let R be the result.
b If R.value is not empty, then let V = R.value.
c R is an abrupt completion, then return (R.type, V, R.target).
If foundInB is false and the DefaultClause has a StatementList, thena. Evaluate the DefaultClause’s StatementList and let R be the result.
b. If R.value is not empty, then let V = R.value.
c. If R is an abrupt completion, then return (R.type, V, R.target).
Repeat (Note that if step 7.a.i has been performed this loop does not start at the beginning of B)
a. Let C be the next CaseClause in B. If there is no such CaseClause, return (normal, V, empty).
b. If C has a StatementList, theni.
Evaluate C’s StatementList and let R be the result.
ii.
If R.value is not empty, then let V = R.value.
iii.
If R is an abrupt completion, then return (R.type, V, R.target).

The production CaseClause : case Expression : StatementListopt is evaluated as follows:1.
2.

Let exprRef be the result of evaluating Expression.
Return GetValue(exprRef).

NOTE
Evaluating CaseClause does not execute the associated StatementList. It simply evaluates the Expression andreturns the value, which the CaseBlock algorithm uses to determine which StatementList to start executing.

12.12 Labelled Statements
SyntaxLabelledStatement :Identifier : Statement
Semantics
A Statement may be prefixed by a label. Labelled statements are only used in conjunction with labelled breakand continue statements. ECMAScript has no goto statement.
An ECMAScript program is considered syntactically incorrect if it contains a LabelledStatement that is enclosed
by a LabelledStatement with the same Identifier as label. This does not apply to labels appearing within the bodyof a FunctionDeclaration that is nested, directly or indirectly, within a labelled statement.
The production Identifier : Statement is evaluated by adding Identifier to the label set of Statement and thenevaluating Statement. If the LabelledStatement itself has a non-empty label set, these labels are also added to
the label set of Statement before evaluating it. If the result of evaluating Statement is (break, V, L) where L isequal to Identifier, the production results in (normal, V, empty).
Prior to the evaluation of a LabelledStatement, the contained Statement is regarded as possessing an empty
label set, unless it is an IterationStatement or a SwitchStatement, in which case it is regarded as possessing alabel set consisting of the single element, empty.

12.13 The throw Statement
SyntaxThrowStatement :throw [no LineTerminator here] Expression ;© Ecma International 200995

Semantics
The production ThrowStatement : throw [no LineTerminator here] Expression ; is evaluated as:
1.
2.

Let exprRef be the result of evaluating Expression.
Return (throw, GetValue(exprRef), empty).

12.14 The try Statement
SyntaxTryStatement :try Block Catchtry Block Finallytry Block Catch Finally
Catch :
catch ( Identifier ) Block
Finally :finally Block
The try statement encloses a block of code in which an exceptional condition can occur, such as a runtime
error or a throw statement. The catch clause provides the exception-handling code. When a catch clausecatches an exception, its Identifier is bound to that exception.
Semantics
The production TryStatement : try Block Catch is evaluated as follows:
1.
2.
3.

Let B be the result of evaluating Block.
If B.type is not throw, return B.
Return the result of evaluating Catch with parameter B.

The production TryStatement : try Block Finally is evaluated as follows:
1.
2.
3.
4.

Let B be the result of evaluating Block.
Let F be the result of evaluating Finally.
If F.type is normal, return B.
Return F.

The production TryStatement : try Block Catch Finally is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let B be the result of evaluating Block.
If B.type is throw, thena. Let C be the result of evaluating Catch with parameter B.
Else, B.type is not throw,a. Let C be B.
Let F be the result of evaluating Finally.
If F.type is normal, return C.
Return F.

The production Catch : catch ( Identifier ) Block is evaluated as follows:
1.
2.
3.
4.

Let C be the parameter that has been passed to this production.
Let oldEnv be the running execution context’s LexicalEnvironment.
Let catchEnv be the result of calling NewDeclarativeEnvironment passing oldEnv as the argument.
Call the CreateMutableBinding concrete method of catchEnv passing the Identifier String value as theargument.

96© Ecma International 2009

5.
6.
7.
8.
9.

Call the SetMutableBinding concrete method of catchEnv passing the Identifier, C, and false as arguments.
Note that the last argument is immaterial in this situation.
Set the running execution context’s LexicalEnvironment to catchEnv.
Let B be the result of evaluating Block.
Set the running execution context’s LexicalEnvironment to oldEnv.
Return B.

NOTE

No matter how control leaves the Block the LexicalEnvironment is always restored to its former state.

The production Finally : finally Block is evaluated as follows:
1.

Return the result of evaluating Block.

12.14.1

Strict Mode Restrictions

It is an SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier of the Catchproduction is either "eval" or "arguments".

12.15 The debugger statement
SyntaxDebuggerStatement :debugger ;Semantics
Evaluating the DebuggerStatement production may allow an implementation to cause a breakpoint when run
under a debugger. If a debugger is not present or active this statement has no observable effect.
The production DebuggerStatement : debugger ; is evaluated as follows:
1.
2.
3.

If an implementation defined debugging facility is available and enabled, thena. Perform an implementation defined debugging action.
b. Let result be an implementation defined Completion value.
Elsea. Let result be (normal, empty, empty).
Return result.

13 Function Definition
SyntaxFunctionDeclaration :function Identifier ( FormalParameterListopt ) { FunctionBody }FunctionExpression :function Identifieropt ( FormalParameterListopt ) { FunctionBody }FormalParameterList :Identifier
FormalParameterList , Identifier
FunctionBody :SourceElementsopt© Ecma International 200997

Semantics
The production
FunctionDeclaration : function Identifier ( FormalParameterListopt ) { FunctionBody }is instantiated as follows during Declaration Binding instantiation (10.5):
1.

Return the result of creating a new Function object as specified in 13.2 with parameters specified byFormalParameterListopt, and body specified by FunctionBody. Pass in the VariableEnvironment of the running
execution context as the Scope. Pass in true as the Strict flag if the FunctionDeclaration is contained in strict code
or if its FunctionBody is strict code.

The production
FunctionExpression : function ( FormalParameterListopt ) { FunctionBody }is evaluated as follows:
1.

Return the result of creating a new Function object as specified in 13.2 with parameters specified byFormalParameterListopt and body specified by FunctionBody. Pass in the LexicalEnvironment of the running
execution context as the Scope. Pass in true as the Strict flag if the FunctionExpression is contained in strict code or
if its FunctionBody is strict code.

The production
FunctionExpression : function Identifier ( FormalParameterListopt ) { FunctionBody }is evaluated as follows:
1.
2.
3.
4.
5.
6.

Let funcEnv be the result of calling NewDeclarativeEnvironment passing the running execution context’s Lexical
Environment as the argument
Let envRec be funcEnv’s environment record.
Call the CreateImmutableBinding(N) concrete method of envRec passing the String value of Identifier as theargument.
Let closure be the result of creating a new Function object as specified in 13.2 with parameters specified byFormalParameterListopt and body specified by FunctionBody. Pass in funcEnv as the Scope. Pass in true as the
Strict flag if the FunctionExpression is contained in strict code or if its FunctionBody is strict code.
Call the InitializeImmutableBinding(N,V) concrete method of envRec passing the String value of Identifier andclosure as the arguments.
Return closure.

NOTE
The Identifier in a FunctionExpression can be referenced from inside the FunctionExpression's FunctionBody toallow the function to call itself recursively. However, unlike in a FunctionDeclaration, the Identifier in a FunctionExpression
cannot be referenced from and does not affect the scope enclosing the FunctionExpression.

The production FunctionBody : SourceElementsopt is evaluated as follows:
1.

2.
3.

The code of this FunctionBody is strict mode code if it is part of a FunctionDeclaration or FunctionExpression thatis contained in strict mode code or if the Directive Prologue (14.1) of its SourceElements contains a Use Strict
Directive or if any of the conditions in 10.1.1 apply. If the code of this FunctionBody is strict mode code,
SourceElements is evaluated in the following steps as strict mode code. Otherwise, SourceElements is evaluated in
the following steps as non-strict mode code.
If SourceElements is present return the result of evaluating SourceElements.
Else return (normal, undefined, empty).

13.1 Strict Mode Restrictions
It is a SyntaxError if any Identifier value occurs more than once within a FormalParameterList of a strict mode
FunctionDeclaration or FunctionExpression.
It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs within a FormalParameterListof a strict mode FunctionDeclaration or FunctionExpression.

98© Ecma International 2009

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the Identifier of a strictmode FunctionDeclaration or FunctionExpression.

13.2 Creating Function Objects
Given an optional parameter list specified by FormalParameterList, a body specified by FunctionBody, a Lexical
Environment specified by Scope, and a Boolean flag Strict, a Function object is constructed as follows:
1. Create a new native ECMAScript object and let F be that object.
2. Set all the internal methods, except for [[Get]], of F as described in 8.12.
3. Set the [[Class]] internal property of F to "Function".
4. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
5. Set the [[Get]] internal property of F as described in 15.3.5.4.
6. Set the [[Call]] internal property of F as described in 13.2.1.
7. Set the [[Construct]] internal property of F as described in 13.2.2.
8. Set the [[HasInstance]] internal property of F as described in 15.3.5.3.
9. Set the [[Scope]] internal property of F to the value of Scope.
10. Let names be a List containing, in left to right textual order, the Strings corresponding to the identifiers ofFormalParameterList.
11. Set the [[FormalParameters]] internal property of F to names.
12. Set the [[Code]] internal property of F to FunctionBody.
13. Set the [[Extensible]] internal property of F to true.
14. Let len be the number of formal parameters specified in FormalParameterList. If no parameters are specified, let len
be 0.
15. Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor {[[Value]]:len, [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and false.
16. Let proto be the result of creating a new object as would be constructed by the expression new Object()where
Object is the standard built-in constructor with that name.
17. Call the [[DefineOwnProperty]] internal method of proto with arguments "constructor", Property Descriptor{[[Value]]: F, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true}, and false.
18. Call the [[DefineOwnProperty]] internal method of F with arguments "prototype", Property Descriptor{[[Value]]: proto, { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false}, and false.
19. If Strict is true, thena. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
b. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor{[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
c. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments", PropertyDescriptor{[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
20. Return F.
NOTE
A prototype property is automatically created for every function, to allow for the possibility that the function
will be used as a constructor.

13.2.1 [[Call]]When the [[Call]] internal method for a Function object F is called with a this value and a list of arguments, thefollowing steps are taken:
1.
2.
3.
4.
5.
6.

Let funcCtx be the result of establishing a new execution context for function code using the value of F's[[FormalParameters]] internal property, the passed arguments List args, and the this value as described in
10.4.3.
Let result be the result of evaluating the FunctionBody that is the value of F's [[Code]] internal property. If
F does not have a [[Code]] internal property or if its value is an empty FunctionBody, then result is (normal,
undefined, empty).
Exit the execution context funcCtx, restoring the previous execution context.
If result.type is throw then throw result.value.
If result.type is return then return result.value.
Otherwise result.type must be normal. Return undefined.

© Ecma International 200999

13.2.2 [[Construct]]
When the [[Construct]] internal method for a Function object F is called with a possibly empty list of arguments,
the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

Let obj be a newly created native ECMAScript object.
Set all the internal methods of obj as specified in 8.12.
Set the [[Class]] internal property of obj to "Object".
Set the [[Extensible]] internal property of obj to true.
Let proto be the value of calling the [[Get]] internal property of F with argument "prototype".
If Type(proto) is Object, set the [[Prototype]] internal property of obj to proto.
If Type(proto) is not Object, set the [[Prototype]] internal property of obj to the standard built-in Object prototype
object as described in 15.2.4.
8. Let result be the result of calling the [[Call]] internal property of F, providing obj as the this value and providing theargument list passed into [[Construct]] as args.
9. If Type(result) is Object then return result.
10. Return obj.
13.2.3 The [[ThrowTypeError]] Function Object

1.
2.
3.
4.
5.
6.
7.
8.
9.

The [[ThrowTypeError]] object is a unique function object that is defined once as follows:
Create a new native ECMAScript object and let F be that object.
Set all the internal methods of F as described in 8.12.
Set the [[Class]] internal property of F to "Function".
Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in 15.3.3.1.
Set the [[Call]] internal property of F as described in 13.2.1.
Set the [[Scope]] internal property of F to the Global Environment.
Set the [[FormalParameters]] internal property of F to an empty List.
Set the [[Code]] internal property of F to be a FunctionBody that unconditionally throws a TypeError exception andperforms no other action.
10. Call the [[DefineOwnProperty]] internal method of F with arguments "length", Property Descriptor {[[Value]]: 0,[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false}, and false.
11. Set the [[Extensible]] internal property of F to false.
12. Let [[ThrowTypeError]] be F.

14 Program
Syntax
Program :SourceElementsopt
SourceElements :SourceElement
SourceElements SourceElement
SourceElement :Statement
FunctionDeclaration
Semantics
The production Program : SourceElementsopt is evaluated as follows:1.

The code of this Program is strict mode code if the Directive Prologue (14.1) of its SourceElements contains
a Use Strict Directive or if any of the conditions of 10.1.1 apply. If the code of this Program is strict mode
code, SourceElements is evaluated in the following steps as strict mode code. Otherwise SourceElements isevaluated in the following steps as non-strict mode code.

100© Ecma International 2009

2.
3.
4.
5.
6.

If SourceElements is not present, return (normal, empty, empty).
Let progCxt be a new execution context for global code as described in 10.4.1.
Let result be the result of evaluating SourceElements.
Exit the execution context progCxt.
Return result.

NOTE
The processes for initiating the evaluation of a Program and for dealing with the result of such an evaluation
are defined by an ECMAScript implementation and not by this specification.

The production SourceElements : SourceElements SourceElement is evaluated as follows:
1.
2.
3.
4.
5.

Let headResult be the result of evaluating SourceElements.
If headResult is an abrupt completion, return headResult
Let tailResult be result of evaluating SourceElement.
If tailResult.value is empty, let V = headResult.value, otherwise let V = tailResult.value.
Return (tailResult.type, V, tailResult.target)

The production SourceElement : Statement is evaluated as follows:
1.

Return the result of evaluating Statement.

The production SourceElement : FunctionDeclaration is evaluated as follows:
1.

Return (normal, empty, empty).

14.1 Directive Prologues and the Use Strict Directive
A Directive Prologue is the longest sequence of ExpressionStatement productions occurring as the initial
SourceElement productions of a Program or FunctionBody and where each ExpressionStatement in the sequence
consists entirely of a StringLiteral token followed a semicolon. The semicolon may appear explicitly or may beinserted by automatic semicolon insertion. A Directive Prologue may be an empty sequence.
A Use Strict Directive is an ExpressionStatement in a Directive Prologue whose StringLiteral is either the exact
character sequences "use strict" or 'use strict'. A Use Strict Directive may not contain an
EscapeSequence or LineContinuation.
A Directive Prologue may contain more than one Use Strict Directive. However, an implementation may issue
a warning if this occurs.
NOTE
The ExpressionStatement productions of a Directive Prologue are evaluated normally during evaluation of thecontaining SourceElements production. Implementations may define implementation specific meanings for
ExpressionStatement productions which are not a Use Strict Directive and which occur in a Directive Prologue. If an
appropriate notification mechanism exists, an implementation should issue a warning if it encounters in a Directive
Prologue an ExpressionStatement that is not a Use Strict Directive or which does not have a meaning defined by the
implementation.

15 Standard Built-in ECMAScript Objects
There are certain built-in objects available whenever an ECMAScript program begins execution. One, theglobal object, is part of the lexical environment of the executing program. Others are accessible as initialproperties of the global object.
Unless specified otherwise, the [[Class]] internal property of a built-in object is "Function" if that built-in
object has a [[Call]] internal property, or "Object" if that built-in object does not have a [[Call]] internal
property. Unless specified otherwise, the [[Extensible]] internal property of a built-in object initially has thevalue true.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore areconstructors: they are functions intended for use with the new operator. For each built-in function, this© Ecma International 2009101

specification describes the arguments required by that function and properties of the Function object. For eachbuilt-in constructor, this specification furthermore describes properties of the prototype object of that
constructor and properties of specific object instances returned by a new expression that invokes that
constructor.
Unless otherwise specified in the description of a particular function, if a function or constructor described inthis clause is given fewer arguments than the function is specified to require, the function or constructor shallbehave exactly as if it had been given sufficient additional arguments, each such argument being the
undefined value.
Unless otherwise specified in the description of a particular function, if a function or constructor described inthis clause is given more arguments than the function is specified to allow, the extra arguments are evaluatedby the call and then ignored by the function. However, an implementation may define implementation specificbehaviour relating to such arguments as long as the behaviour is not the throwing of a TypeError exceptionthat is predicated simply on the presence of an extra argument.
NOTE
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so byadding new functions rather than adding new parameters to existing functions.

Every built-in function and every built-in constructor has the Function prototype object, which is the initial valueof the expression Function.prototype (15.3.4), as the value of its [[Prototype]] internal property.
Unless otherwise specified every built-in prototype object has the Object prototype object, which is the initialvalue of the expression Object.prototype (15.2.4), as the value of its [[Prototype]] internal property,
except the Object prototype object itself.
None of the built-in functions described in this clause that are not constructors shall implement the[[Construct]] internal method unless otherwise specified in the description of a particular function. None of thebuilt-in functions described in this clause shall have a prototype property unless otherwise specified in thedescription of a particular function.
This clause generally describes distinct behaviours for when a constructor is “called as a function” and forwhen it is “called as part of a new expression”. The “called as a function” behaviour corresponds to the
invocation of the constructor’s [[Call]] internal method and the “called as part of a new expression” behaviour
corresponds to the invocation of the constructor’s [[Construct]] internal method.
Every built-in Function object described in this clause—whether as a constructor, an ordinary function, orboth—has a length property whose value is an integer. Unless otherwise specified, this value is equal to thelargest number of named arguments shown in the subclause headings for the function description, including
optional parameters.
NOTE
For example, the Function object that is the initial value of the slice property of the String prototype object is
described under the subclause heading “String.prototype.slice (start, end)” which shows the two named arguments startand end; therefore the value of the length property of that Function object is 2.

In every case, the length property of a built-in Function object described in this clause has the attributes{ [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }. Every other property described in thisclause has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true } unless otherwise
specified.

15.1 The Global Object
The unique global object is created before control enters any execution context.
Unless otherwise specified, the standard built-in properties of the global object have attributes {[[Writable]]:true, [[Enumerable]]: false, [[Configurable]]: true}.
The global object does not have a [[Construct]] internal property; it is not possible to use the global object as aconstructor with the new operator.

102© Ecma International 2009

The global object does not have a [[Call]] internal property; it is not possible to invoke the global object as afunction.
The values of the [[Prototype]] and [[Class]] internal properties of the global object are implementationdependent.
In addition to the properties defined in this specification the global object may have additional host definedproperties. This may include a property whose value is the global object itself; for example, in the HTML
document object model the window property of the global object is the global object itself.
15.1.1 Value Properties of the Global Object
15.1.1.1

NaN

The value of NaN is NaN (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false,[[Configurable]]: false }.
15.1.1.2

Infinity

The value of Infinity is +∞ (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]:false, [[Configurable]]: false }.
15.1.1.3

undefined

The value of undefined is undefined (see 8.1). This property has the attributes { [[Writable]]: false,[[Enumerable]]: false, [[Configurable]]: false }.
15.1.2 Function Properties of the Global Object
15.1.2.1

eval (x)When the eval function is called with one argument x, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

If Type(x) is not String, return x.
Let prog be the ECMAScript code that is the result of parsing x as a Program. If the parse fails, throw a
SyntaxError exception (but see also clause 16).
Let evalCtx be the result of establishing a new execution context (10.4.2) for the eval code prog.
Let result be the result of evaluating the program prog.
Exit the running execution context evalCtx, restoring the previous execution context.
If result.type is normal and its completion value is a value V, then return the value V.
If result.type is normal and its completion value is empty, then return the value undefined.
Otherwise, result.type must be throw. Throw result.value as an exception.

15.1.2.1.1

Direct Call to Eval

A direct call to the eval function is one that is expressed as a CallExpression that meets the following two
conditions:
The Reference that is the result of evaluating the MemberExpression in the CallExpression has an environment
record as its base value and its reference name is "eval".
The result of calling the abstract operation GetValue with that Reference as the argument is the standard builtin function defined in 15.1.2.1.

© Ecma International 2009103

15.1.2.2

parseInt (string , radix)

The parseInt function produces an integer value dictated by interpretation of the contents of the stringargument according to the specified radix. Leading white space in string is ignored. If radix is undefined or 0,
it is assumed to be 10 except when the number begins with the character pairs 0x or 0X, in which case a radixof 16 is assumed. If radix is 16, number may also optionally begin with the character pairs 0x or 0X.
When the parseInt function is called, the following steps are taken:
1.
2.

Let inputString be ToString(string).
Let S be a newly created substring of inputString consisting of the first character that is not a
StrWhiteSpaceChar and all characters following that character. (In other words, remove leading whitespace.)
3. Let sign be 1.
4. If S is not empty and the first character of S is a minus sign -, let sign be −1.
5. If S is not empty and the first character of S is a plus sign + or a minus sign -, then remove the firstcharacter from S.
6. Let R = ToInt32(radix).
7. Let stripPrefix be true.
8. If R ≠ 0, thena. If R < 2 or R > 36, then return NaN.
b. If R ≠ 16, let stripPrefix be false.
9. Else, R = 0
a. Let R = 10.
10. If stripPrefix is true, thena. If the length of S is at least 2 and the first two characters of S are either “0x” or “0X”, then remove
the first two characters from S and let R = 16.
11. If S contains any character that is not a radix-R digit, then let Z be the substring of S consisting of all
characters before the first such character; otherwise, let Z be S.
12. If Z is empty, return NaN.
13. Let mathInt be the mathematical integer value that is represented by Z in radix-R notation, using the letters
A-Z and a-z for digits with values 10 through 35. (However, if R is 10 and Z contains more than 20
significant digits, every significant digit after the 20th may be replaced by a 0 digit, at the option of theimplementation; and if R is not 2, 4, 8, 10, 16, or 32, then mathInt may be an implementation-dependent
approximation to the mathematical integer value that is represented by Z in radix-R notation.)
14. Let number be the Number value for mathInt.
15. Return sign × number.
NOTE
parseInt may interpret only a leading portion of string as an integer value; it ignores any characters thatcannot be interpreted as part of the notation of an integer, and no indication is given that any such characters wereignored.

15.1.2.3

parseFloat (string)

The parseFloat function produces a Number value dictated by interpretation of the contents of the stringargument as a decimal literal.
When the parseFloat function is called, the following steps are taken:
1.
2.
3.
4.
5.

Let inputString be ToString(string).
Let trimmedString be a substring of inputString consisting of the leftmost character that is not a
StrWhiteSpaceChar and all characters to the right of that character.(In other words, remove leading whitespace.)
If neither trimmedString nor any prefix of trimmedString satisfies the syntax of a StrDecimalLiteral (see9.3.1), return NaN.
Let numberString be the longest prefix of trimmedString, which might be trimmedString itself, that satisfies
the syntax of a StrDecimalLiteral.
Return the Number value for the MV of numberString.

104© Ecma International 2009

NOTE
parseFloat may interpret only a leading portion of string as a Number value; it ignores any characters thatcannot be interpreted as part of the notation of an decimal literal, and no indication is given that any such characters wereignored.

15.1.2.4

isNaN (number)

Returns true if the argument coerces to NaN, and otherwise returns false.
1.
2.

If ToNumber(number) is NaN, return true.
Otherwise, return false.

NOTE
A reliable way for ECMAScript code to test if a value X is a NaN is an expression of the form X !== X. The
result will be true if and only if X is a NaN.

15.1.2.5

isFinite (number)

Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.
1.
2.

If ToNumber(number) is NaN, +∞, or −∞, return false.
Otherwise, return true.

15.1.3 URI Handling Function Properties
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) andtransport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language
itself does not provide any support for using URIs except for functions that encode and decode URIs as
described in 15.1.3.1, 15.1.3.2, 15.1.3.3 and 15.1.3.4.
NOTE
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages;
these functions are beyond the scope of this standard.

A URI is composed of a sequence of components separated by component separators. The general form is:
Scheme : First / Second ; Third ? Fourth
where the italicised names represent components and the “:”, “/”, “;” and “?” are reserved characters usedas separators. The encodeURI and decodeURI functions are intended to work with complete URIs; they
assume that any reserved characters in the URI are intended to have special meaning and so are notencoded. The encodeURIComponent and decodeURIComponent functions are intended to work with theindividual component parts of a URI; they assume that any reserved characters represent text and so must be
encoded so that they are not interpreted as reserved characters when the component is part of a complete
URI.
The following lexical grammar specifies the form of encoded URIs.
uri :::
uriCharactersopt
uriCharacters :::uriCharacter uriCharactersopt
uriCharacter :::uriReserved
uriUnescaped
uriEscaped
uriReserved ::: one of; / ? : @© Ecma International 2009

&

=

+

$

,

105

uriUnescaped :::uriAlpha
DecimalDigit
uriMark
uriEscaped :::% HexDigit HexDigit
uriAlpha ::: one ofa b c dA B C D

e
E

f
F

g
G

h
H

i
I

uriMark ::: one of- _ . !

~

*

'

(

)

j
J

k
K

l
L

m
M

n
N

o
O

p
P

q
Q

r
R

s
S

t
T

u
U

v
V

w
W

x
X

y
Y

z
Z

When a character to be included in a URI is not listed above or is not intended to have the special meaning
sometimes given to the reserved characters, that character must be encoded. The character is transformed
into its UTF-8 encoding, with surrogate pairs first converted from UTF-16 to the corresponding code pointvalue. (Note that for code units in the range [0,127] this results in a single octet with the same value.) The
resulting sequence of octets is then transformed into a String with each octet represented by an escapesequence of the form “%xx”.
The encoding and escaping process is described by the abstract operation Encode taking two Stringarguments string and unescapedSet.
1.
2.
3.
4.

Let strLen be the number of characters in string.
Let R be the empty String.
Let k be 0.
Repeata. If k equals strLen, return R.
b. Let C be the character at position k within string.
c. If C is in unescapedSet, theni.
Let S be a String containing only the character C.
ii.
Let R be a new String value computed by concatenating the previous value of R and S.
d. Else, C is not in unescapedSeti.
If the code unit value of C is not less than 0xDC00 and not greater than 0xDFFF, throw a
URIError exception.
ii.
If the code unit value of C is less than 0xD800 or greater than 0xDBFF, then
1. Let V be the code unit value of C.
iii.
Else,
1. Increase k by 1.
2. If k equals strLen, throw a URIError exception.
3. Let kChar be the code unit value of the character at position k within string.
4. If kChar is less than 0xDC00 or greater than 0xDFFF, throw a URIError
exception.
5. Let V be (((the code unit value of C) – 0xD800) * 0x400 + (kChar – 0xDC00) +0x10000).
iv.
Let Octets be the array of octets resulting by applying the UTF-8 transformation to V, andlet L be the array size.
v.
Let j be 0.
vi.
Repeat, while j < L
1. Let jOctet be the value at position j within Octets.
2. Let S be a String containing three characters “%XY” where XY are two uppercase
hexadecimal digits encoding the value of jOctet.
3. Let R be a new String value computed by concatenating the previous value of R andS.
4. Increase j by 1.
e. Increase k by 1.

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The unescaping and decoding process is described by the abstract operation Decode taking two Stringarguments string and reservedSet.
1.
2.
3.
4.

Let strLen be the number of characters in string.
Let R be the empty String.
Let k be 0.
Repeata. If k equals strLen, return R.
b. Let C be the character at position k within string.
c. If C is not ‘%’, theni.
Let S be the String containing only the character C.
d. Else, C is ‘%’i.
Let start be k.
ii.
If k + 2 is greater than or equal to strLen, throw a URIError exception.
iii.
If the characters at position (k+1) and (k + 2) within string do not represent hexadecimal
digits, throw a URIError exception.
iv.
Let B be the 8-bit value represented by the two hexadecimal digits at position (k + 1) and (k+ 2).
v.
Increment k by 2.
vi.
If the most significant bit in B is 0, then
1. Let C be the character with code unit value B.
2. If C is not in reservedSet, thena Let S be the String containing only the character C.
3. Else, C is in reservedSeta Let S be the substring of string from position start to position k included.
vii.
Else, the most significant bit in B is 1
1. Let n be the smallest non-negative number such that (B << n) & 0x80 is equal to 0.
2. If n equals 1 or n is greater than 4, throw a URIError exception.
3. Let Octets be an array of 8-bit integers of size n.
4. Put B into Octets at position 0.
5. If k + (3 * (n – 1)) is greater than or equal to strLen, throw a URIError exception.
6. Let j be 1.
7. Repeat, while j < na Increment k by 1.
b If the character at position k is not ‘%’, throw a URIError exception.
c If the characters at position (k +1) and (k + 2) within string do not
represent hexadecimal digits, throw a URIError exception.
d Let B be the 8-bit value represented by the two hexadecimal digits at
position (k + 1) and (k + 2).
e If the two most significant bits in B are not 10, throw a URIError
exception.
f Increment k by 2.
g Put B into Octets at position j.
h Increment j by 1.
8. Let V be the value obtained by applying the UTF-8 transformation to Octets, that is,from an array of octets into a 32-bit value. If Octets does not contain a valid UTF-8
encoding of a Unicode code point throw a URIError exception.
9. If V is less than 0x10000, thena Let C be the character with code unit value V.
b If C is not in reservedSet, theni.
Let S be the String containing only the character C.
c Else, C is in reservedSeti.
Let S be the substring of string from position start to position k
included.
10. Else, V is ≥ 0x10000a If V is greater than 0x10FFFF, throw a URIError exception.
b Let L be (((V – 0x10000) & 0x3FF) + 0xDC00).
c Let H be ((((V – 0x10000) >> 10) & 0x3FF) + 0xD800).
d Let S be the String containing the two characters with code unit values H
and L.

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107

e.
f.

Let R be a new String value computed by concatenating the previous value of R and S.
Increase k by 1.

NOTE
The syntax of Uniform Resource Identifiers is given in RFC 2396. A formal description and implementation of
UTF-8 is given in RFC 3629.
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the higherorder bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initialoctet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bits of that octet contain bits from thevalue of the character to be encoded. The following octets all have the higher-order bit set to 1 and the following bit set to0, leaving 6 bits in each to contain bits from the character to be encoded. The possible UTF-8 encodings of ECMAScript
characters are specified in Table 21.

Table 21 — UTF-8 Encodings
Code Unit Value
0x0000 - 0x007F
0x0080 - 0x07FF
0x0800 - 0xD7FF
0xD800 - 0xDBFF
followed by
0xDC00 – 0xDFFF
0xD800 - 0xDBFF
not followed by
0xDC00 – 0xDFFF
0xDC00 – 0xDFFF
0xE000 - 0xFFFF

Representation
00000000 0zzzzzzz
00000yyy yyzzzzzz
xxxxyyyy yyzzzzzz
110110vv vvwwwwxx
followed by110111yy yyzzzzzz

1st Octet

2nd Octet

3rd Octet

0zzzzzzz
110yyyyy
1110xxxx

10zzzzzz
10yyyyyy

10zzzzzz

11110uuu

10uuwwww

10xxyyyy

1110xxxx

10yyyyyy

10zzzzzz

4th Octet

10zzzzzz

causes URIError
causes URIError
xxxxyyyy yyzzzzzz

Where

uuuuu = vvvv + 1
to account for the addition of 0x10000 as in Surrogates, section 3.7, of the Unicode Standard.
The range of code unit values 0xD800-0xDFFF is used to encode surrogate pairs; the above transformation combines a
UTF-16 surrogate pair into a UTF-32 representation and encodes the resulting 21-bit value in UTF-8. Decoding
reconstructs the surrogate pair.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence C0 80 must notdecode into the character U+0000. Implementations of the Decode algorithm are required to throw a URIError when
encountering such invalid sequences.

15.1.3.1

decodeURI (encodedURI)

The decodeURI function computes a new version of a URI in which each escape sequence and UTF-8
encoding of the sort that might be introduced by the encodeURI function is replaced with the character that it
represents. Escape sequences that could not have been introduced by encodeURI are not replaced.
When the decodeURI function is called with one argument encodedURI, the following steps are taken:
1.
2.
3.

Let uriString be ToString(encodedURI).
Let reservedURISet be a String containing one instance of each character valid in uriReserved plus “#”.
Return the result of calling Decode(uriString, reservedURISet)

NOTE

108

The character “#” is not decoded from escape sequences even though it is not a reserved URI character.

© Ecma International 2009

15.1.3.2

decodeURIComponent (encodedURIComponent)

The decodeURIComponent function computes a new version of a URI in which each escape sequence and
UTF-8 encoding of the sort that might be introduced by the encodeURIComponent function is replaced withthe character that it represents.
When the decodeURIComponent function is called with one argument encodedURIComponent, the following
steps are taken:
1.
2.
3.

Let componentString be ToString(encodedURIComponent).
Let reservedURIComponentSet be the empty String.
Return the result of calling Decode(componentString, reservedURIComponentSet)

15.1.3.3

encodeURI (uri)The encodeURI function computes a new version of a URI in which each instance of certain characters is
replaced by one, two or three escape sequences representing the UTF-8 encoding of the character.
When the encodeURI function is called with one argument uri, the following steps are taken:
1.
2.
3.

Let uriString be ToString(uri).
Let unescapedURISet be a String containing one instance of each character valid in uriReserved anduriUnescaped plus “#”.
Return the result of calling Encode(uriString, unescapedURISet)NOTE
The character “#” is not encoded to an escape sequence even though it is not a reserved or unescaped URI
character.

15.1.3.4

encodeURIComponent (uriComponent)

The encodeURIComponent function computes a new version of a URI in which each instance of certain
characters is replaced by one, two or three escape sequences representing the UTF-8 encoding of thecharacter.
When the encodeURIComponent function is called with one argument uriComponent, the following steps aretaken:
1.
2.
3.

Let componentString be ToString(uriComponent).
Let unescapedURIComponentSet be a String containing one instance of each character valid in
uriUnescaped.
Return the result of calling Encode(componentString, unescapedURIComponentSet)

15.1.4 Constructor Properties of the Global Object
15.1.4.1

Object ( . . . )See 15.2.1 and 15.2.2.
15.1.4.2

Function ( . . . )See 15.3.1 and 15.3.2.
15.1.4.3

Array ( . . . )See 15.4.1 and 15.4.2.

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15.1.4.4

String ( . . . )See 15.5.1 and 15.5.2.
15.1.4.5

Boolean ( . . . )See 15.6.1 and 15.6.2.
15.1.4.6

Number ( . . . )See 15.7.1 and 15.7.2.
15.1.4.7

Date ( . . . )See 15.9.2.
15.1.4.8

RegExp ( . . . )See 15.10.3 and 15.10.4.
15.1.4.9

Error ( . . . )See 15.11.1 and 15.11.2.
15.1.4.10 EvalError ( . . . )See 15.11.6.1.
15.1.4.11 RangeError ( . . . )See 15.11.6.2.
15.1.4.12 ReferenceError ( . . . )See 15.11.6.3.
15.1.4.13 SyntaxError ( . . . )See 15.11.6.4.
15.1.4.14 TypeError ( . . . )See 15.11.6.5.
15.1.4.15 URIError ( . . . )See 15.11.6.6.
15.1.5 Other Properties of the Global Object
15.1.5.1

Math

See 15.8.

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15.1.5.2

JSON

See 15.12.

15.2 Object Objects
15.2.1 The Object Constructor Called as a Function
When Object is called as a function rather than as a constructor, it performs a type conversion.
15.2.1.1

Object ( [ value ] )When the Object function is called with no arguments or with one argument value, the following steps aretaken:
1.
2.

If value is null, undefined or not supplied, create and return a new Object object exactly as if the standard
built-in Object constructor had been called with the same arguments (15.2.2.1).
Return ToObject(value).

15.2.2 The Object Constructor
When Object is called as part of a new expression, it is a constructor that may create an object.
15.2.2.1

new Object ( [ value ] )When the Object constructor is called with no arguments or with one argument value, the following steps aretaken:
1.

2.
3.
4.
5.
6.
7.
8.

If value is supplied, thena. If Type(value) is Object, theni.
If the value is a native ECMAScript object, do not create a new object but simply return
value.
ii.
If the value is a host object, then actions are taken and a result is returned in an
implementation-dependent manner that may depend on the host object.
b. If Type(value) is String, return ToObject(value).
c. If Type(value) is Boolean, return ToObject(value).
d. If Type(value) is Number, return ToObject(value).
Asset: The argument value was not supplied or its type was Null or Undefined.
Let obj be a newly created native ECMAScript object.
Set the [[Prototype]] internal property of obj t to the standard built-in Object prototype object (15.2.4).
Set the [[Class]] internal property of obj to "Object".
Set the [[Extensible]] internal property of obj to true.
Set the all the internal methods of obj as specified in 8.12
Return obj.

15.2.3 Properties of the Object Constructor
The value of the [[Prototype]] internal property of the Object constructor is the standard built-in Function
prototype object.
Besides the internal properties and the length property (whose value is 1), the Object constructor has thefollowing properties:
15.2.3.1

Object.prototype

The initial value of Object.prototype is the standard built-in Object prototype object (15.2.4).

© Ecma International 2009111

This property has the attributes {[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.2.3.2

Object.getPrototypeOf ( O )When the getPrototypeOf function is called with argument O, the following steps are taken:
1.
2.

If Type(O) is not Object throw a TypeError exception.
Return the value of the [[Prototype]] internal property of O.

15.2.3.3

Object.getOwnPropertyDescriptor ( O, P )When the getOwnPropertyDescriptor function is called, the following steps are taken:
1.
2.
3.
4.

If Type(O) is not Object throw a TypeError exception.
Let name be ToString(P).
Let desc be the result of calling the [[GetOwnProperty]] internal method of O with argument name.
Return the result of calling FromPropertyDescriptor(desc) (8.10.4).

15.2.3.4

Object.getOwnPropertyNames ( O )When the getOwnPropertyNames function is called, the following steps are taken:
1.
2.
3.
4.

5.

If Type(O) is not Object throw a TypeError exception.
Let array be the result of creating a new object as if by the expression new Array () where Array is thestandard built-in constructor with that name.
Let n be 0.
For each named own property P of O
a. Let name be the String value that is the name of P.
b. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(n), thePropertyDescriptor {[[Value]]: name, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]:true}, and false.
c. Increment n by 1.
Return array.

NOTE
If O is a String instance, the set of own properties processed in step 4 includes the implicit properties definedin 15.5.5.2 that correspond to character positions within the object’s [[PrimitiveValue]] String.

15.2.3.5

Object.create ( O [, Properties] )The create function creates a new object with a specified prototype. When the create function is called, thefollowing steps are taken:
1.
2.
3.
4.
5.

If Type(O) is not Object or Null throw a TypeError exception.
Let obj be the result of creating a new object as if by the expression new Object() where Object is thestandard built-in constructor with that nameSet the [[Prototype]] internal property of obj to O.
If the argument Properties is present and not undefined, add own properties to obj as if by calling thestandard built-in function Object.defineProperties with arguments obj and Properties.
Return obj.

15.2.3.6

Object.defineProperty ( O, P, Attributes )The defineProperty function is used to add an own property and/or update the attributes of an existing own
property of an object. When the defineProperty function is called, the following steps are taken:
1.
2.
3.
4.
5.

If Type(O) is not Object throw a TypeError exception.
Let name be ToString(P).
Let desc be the result of calling ToPropertyDescriptor with Attributes as the argument.
Call the [[DefineOwnProperty]] internal method of O with arguments name, desc, and true.
Return O.

112© Ecma International 2009

15.2.3.7

Object.defineProperties ( O, Properties )The defineProperties function is used to add own properties and/or update the attributes of existing own
properties of an object. When the defineProperties function is called, the following steps are taken:
1.
2.
3.
4.
5.

6.
7.

If Type(O) is not Object throw a TypeError exception.
Let props be ToObject(Properties).
Let names be an internal list containing the names of each enumerable own property of props.
Let descriptors be an empty internal List.
For each element P of names in list order,
a. Let descObj be the result of calling the [[Get]] internal method of props with P as the argument.
b. Let desc be the result of calling ToPropertyDescriptor with descObj as the argument.
c. Append desc to the end of descriptors.
For each element desc of descriptors in list order,
a. Call the [[DefineOwnProperty]] internal method of O with arguments P, desc, and true.
Return O.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration
order must be used to order the list elements in step 3 of this algorithm.
15.2.3.8

Object.seal ( O )When the seal function is called, the following steps are taken:
1.
2.

3.
4.

If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
a. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
b. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
c. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
Set the [[Extensible]] internal property of O to false.
Return O.

15.2.3.9

Object.freeze ( O )When the freeze function is called, the following steps are taken:
1.
2.

3.
4.

If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
a. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
b. If IsDataDescriptor(desc) is true, theni.
If desc.[[Writable]] is true, set desc.[[Writable]] to false.
c. If desc.[[Configurable]] is true, set desc.[[Configurable]] to false.
d. Call the [[DefineOwnProperty]] internal method of O with P, desc, and true as arguments.
Set the [[Extensible]] internal property of O to false.
Return O.

15.2.3.10 Object.preventExtensions ( O )When the preventExtensions function is called, the following steps are taken:
1.
2.
3.

If Type(O) is not Object throw a TypeError exception.
Set the [[Extensible]] internal property of O to false.
Return O.

15.2.3.11 Object.isSealed ( O )When the isSealed function is called with argument O, the following steps are taken:
1.
2.

If Type(O) is not Object throw a TypeError exception.
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113

3.
4.

a. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
b. If desc.[[Configurable]] is true, then return false.
If the [[Extensible]] internal property of O is false, then return true.
Otherwise, return false.

15.2.3.12 Object.isFrozen ( O )When the isFrozen function is called with argument O, the following steps are taken:
1.
2.

3.
4.

If Type(O) is not Object throw a TypeError exception.
For each named own property name P of O,
a. Let desc be the result of calling the [[GetOwnProperty]] internal method of O with P.
b. If IsDataDescriptor(desc) is true theni.
If desc.[[Writable]] is true, return false.
c. If desc.[[Configurable]] is true, then return false.
If the [[Extensible]] internal property of O is false, then return true.
Otherwise, return false.

15.2.3.13 Object.isExtensible ( O )When the isExtensible function is called with argument O, the following steps are taken:
1.
2.

If Type(O) is not Object throw a TypeError exception.
Return the Boolean value of the [[Extensible]] internal property of O.

15.2.3.14 Object.keys ( O )When the keys function is called with argument O, the following steps are taken:
1.
2.
3.
4.
5.

6.

If the Type(O) is not Object, throw a TypeError exception.
Let n be the number of own enumerable properties of O
Let array be the result of creating a new Object as if by the expression new Array(n) where Array is
the standard built-in constructor with that name.
Let index be 0.
For each own enumerable property of O whose name String is P
a. Call the [[DefineOwnProperty]] internal method of array with arguments ToString(index), thePropertyDescriptor {[[Value]]: P, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true},and false.
b. Increment index by 1.
Return array.

If an implementation defines a specific order of enumeration for the for-in statement, that same enumeration
order must be used in step 5 of this algorithm.
15.2.4 Properties of the Object Prototype Object
The value of the [[Prototype]] internal property of the Object prototype object is null, the value of the [[Class]]
internal property is "Object", and the initial value of the [[Extensible]] internal property is true.
15.2.4.1

Object.prototype.constructor

The initial value of Object.prototype.constructor is the standard built-in Object constructor.
15.2.4.2

Object.prototype.toString ( )When the toString method is called, the following steps are taken:
1.
2.

Let O be the result of calling ToObject passing the this value as the argument.
Let class be the value of the [[Class]] internal property of O.

114© Ecma International 2009

3.

Return the String value that is the result of concatenating the three Strings "[object ", class, and "]".

15.2.4.3

Object.prototype.toLocaleString ( )When the toLocaleString method is called, the following steps are taken:
1.
2.
3.
4.

Let O be the result of calling ToObject passing the this value as the argument.
Let toString be the result of calling the [[Get]] internal method of O passing "toString" as the argument.
If IsCallable(toString) is false, throw a TypeError exception.
Return the result of calling the [[Call]] internal method of toString passing O as the this value and noarguments.

NOTE 1This function is provided to give all Objects a generic toLocaleString interface, even though not all may
use it. Currently, Array, Number, and Date provide their own locale-sensitive toLocaleString methods.
NOTE 2
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.

15.2.4.4

Object.prototype.valueOf ( )When the valueOf method is called, the following steps are taken:
1.
2.
3.

Let O be the result of calling ToObject passing the this value as the argument.
If O is the result of calling the Object constructor with a host object (15.2.2.1), thena. Return either O or another value such as the host object originally passed to the constructor. The
specific result that is returned is implementation-defined.
Return O.

15.2.4.5

Object.prototype.hasOwnProperty (V)When the hasOwnProperty method is called with argument V, the following steps are taken:
1.
2.
3.
4.
5.

Let P be ToString(V).
Let O be the result of calling ToObject passing the this value as the argument.
Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
If desc is undefined, return false.
Return true.

NOTE 1

Unlike [[HasProperty]] (8.12.6), this method does not consider objects in the prototype chain.

NOTE 2The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1
in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

15.2.4.6

Object.prototype.isPrototypeOf (V)When the isPrototypeOf method is called with argument V, the following steps are taken:
1.
2.
3.

If V is not an object, return false.
Let O be the result of calling ToObject passing the this value as the argument.
Repeata. Let V be the value of the [[Prototype]] internal property of V.
b. if V is null, return falsec. If O and V refer to the same object, return true.

NOTE
The ordering of steps 1 and 2 is chosen to preserve the behaviour specified by previous editions of thisspecification for the case where V is not an object and the this value is undefined or null.

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15.2.4.7

Object.prototype.propertyIsEnumerable (V)When the propertyIsEnumerable method is called with argument V, the following steps are taken:
1.
2.
3.
4.
5.

Let P be ToString(V).
Let O be the result of calling ToObject passing the this value as the argument.
Let desc be the result of calling the [[GetOwnProperty]] internal method of O passing P as the argument.
If desc is undefined, return false.
Return the value of desc.[[Enumerable]].

NOTE 1This method does not consider objects in the prototype chain.

NOTE 2The ordering of steps 1 and 2 is chosen to ensure that any exception that would have been thrown by step 1
in previous editions of this specification will continue to be thrown even if the this value is undefined or null.

15.2.5 Properties of Object Instances
Object instances have no special properties beyond those inherited from the Object prototype object.

15.3 Function Objects
15.3.1 The Function Constructor Called as a Function
When Function is called as a function rather than as a constructor, it creates and initialises a new Function
object. Thus the function call Function(…) is equivalent to the object creation expression new
Function(…) with the same arguments.
15.3.1.1

Function (p1, p2, … , pn, body)

When the Function function is called with some arguments p1, p2, … , pn, body (where n might be 0, that is,there are no “p” arguments, and where body might also not be provided), the following steps are taken:
1.

Create and return a new Function object as if the standard built-in constructor Function was used in a new
expression with the same arguments (15.3.2.1).

15.3.2 The Function Constructor
When Function is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.3.2.1

new Function (p1, p2, … , pn, body)

The last argument specifies the body (executable code) of a function; any preceding arguments specify formal
parameters.
When the Function constructor is called with some arguments p1, p2, … , pn, body (where n might be 0, thatis, there are no “p” arguments, and where body might also not be provided), the following steps are taken:
1.
2.
3.
4.
5.

Let argCount be the total number of arguments passed to this function invocation.
Let P be the empty String.
If argCount = 0, let body be the empty String.
Else if argCount = 1, let body be that argument.
Else, argCount > 1
a. Let firstArg be the first argument.
b. Let P be ToString(firstArg).
c. Let k be 2.
d. Repeat, while k < argCounti.
Let nextArg be the k’ th argument.

116© Ecma International 2009

ii.

Let P be the result of concatenating the previous value of P, the String "," (a comma), andToString(nextArg).
iii.
Increase k by 1.
e. Let body be the k’ th argument.
6. Let body be ToString(body).
7. If P is not parsable as a FormalParameterListopt then throw a SyntaxError exception.
8. If body is not parsable as FunctionBody then throw a SyntaxError exception.
9. If body is strict mode code (see 10.1.1) then let strict be true, else let strict be false.
10. If strict is true, throw any exceptions specified in 13.1 that apply.
11. Return a new Function object created as specified in 13.2 passing P as the FormalParameterList and body as
the FunctionBody. Pass in the Global Environment as the Scope parameter and strict as the Strict flag.
A prototype property is automatically created for every function, to provide for the possibility that thefunction will be used as a constructor.
NOTE
It is permissible but not necessary to have one argument for each formal parameter to be specified. For
example, all three of the following expressions produce the same result:
new Function("a", "b", "c", "return a+b+c")
new Function("a, b, c", "return a+b+c")
new Function("a,b", "c", "return a+b+c")

15.3.3 Properties of the Function Constructor
The Function constructor is itself a Function object and its [[Class]] is "Function". The value of the[[Prototype]] internal property of the Function constructor is the standard built-in Function prototype object(15.3.4).
The value of the [[Extensible]] internal property of the Function constructor is true.
The Function constructor has the following properties:
15.3.3.1

Function.prototype

The initial value of Function.prototype is the standard built-in Function prototype object (15.3.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.3.3.2

Function.length

This is a data property with a value of 1. This property has the attributes { [[Writable]]: false, [[Enumerable]]:false, [[Configurable]]: false }.
15.3.4 Properties of the Function Prototype Object
The Function prototype object is itself a Function object (its [[Class]] is "Function") that, when invoked,
accepts any arguments and returns undefined.
The value of the [[Prototype]] internal property of the Function prototype object is the standard built-in Object
prototype object (15.2.4). The initial value of the [[Extensible]] internal property of the Function prototype
object is true.
The Function prototype object does not have a valueOf property of its own; however, it inherits the valueOfproperty from the Object prototype Object.
The length property of the Function prototype object is 0.

© Ecma International 2009117

15.3.4.1

Function.prototype.constructor

The initial value of Function.prototype.constructor is the built-in Function constructor.
15.3.4.2

Function.prototype.toString ( )An implementation-dependent representation of the function is returned. This representation has the syntax of
a FunctionDeclaration. Note in particular that the use and placement of white space, line terminators, andsemicolons within the representation String is implementation-dependent.
The toString function is not generic; it throws a TypeError exception if its this value is not a Function
object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.3.4.3

Function.prototype.apply (thisArg, argArray)

When the apply method is called on an object func with arguments thisArg and argArray, the following steps
are taken:
1.
2.

If IsCallable(func) is false, then throw a TypeError exception.
If argArray is null or undefined, thena. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value
and an empty list of arguments.
3. If Type(argArray) is not Object, then throw a TypeError exception.
4. Let len be the result of calling the [[Get]] internal method of argArray with argument "length".
5. If len is null or undefined, then throw a TypeError exception.
6. Let n be ToUint32(len).
7. If n is not equal to ToNumber(len), then throw a TypeError exception.
8. Let argList be an empty List.
9. Let index be 0.
10. Repeat while index < na. Let indexName be ToString(index).
b. Let nextArg be the result of calling the [[Get]] internal method of argArray with indexName as theargument.
c. Append nextArg as the last element of argList.
d. Set index to index + 1.
11. Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value andargList as the list of arguments.
The length property of the apply method is 2.
NOTE
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where aundefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result ispassed as the this value.

15.3.4.4

Function.prototype.call (thisArg [ , arg1 [ , arg2, … ] ] )When the call method is called on an object func with argument thisArg and optional arguments arg1, arg2
etc, the following steps are taken:
1.
2.
3.
4.

If IsCallable(func) is false, then throw a TypeError exception.
Let argList be an empty List.
If this method was called with more than one argument then in left to right order starting with arg1 append
each argument as the last element of argList
Return the result of calling the [[Call]] internal method of func, providing thisArg as the this value andargList as the list of arguments.

The length property of the call method is 1.

118© Ecma International 2009

NOTE
The thisArg value is passed without modification as the this value. This is a change from Edition 3, where aundefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result ispassed as the this value.

15.3.4.5

Function.prototype.bind (thisArg [, arg1 [, arg2, …]])

The bind method takes one or more arguments, thisArg and (optionally) arg1, arg2, etc, and returns a new
function object by performing the following steps:
1.
2.
3.

Let Target be the this value.
If IsCallable(Target) is false, throw a TypeError exception.
Let A be a new (possibly empty) internal list of all of the argument values provided after thisArg (arg1, arg2etc), in order.
4. Let F be a new native ECMAScript object .
5. Set all the internal methods, except for [[Get]], of F as specified in 8.12.
6. Set the [[Get]] internal property of F as specified in 15.3.5.4.
7. Set the [[TargetFunction]] internal property of F to Target.
8. Set the [[BoundThis]] internal property of F to the value of thisArg.
9. Set the [[BoundArgs]] internal property of F to A.
10. Set the [[Class]] internal property of F to "Function".
11. Set the [[Prototype]] internal property of F to the standard built-in Function prototype object as specified in
15.3.3.1.
12. Set the [[Call]] internal property of F as described in 15.3.4.5.1.
13. Set the [[Construct]] internal property of F as described in 15.3.4.5.2.
14. Set the [[HasInstance]] internal property of F as described in 15.3.4.5.3.
15. If the [[Class]] internal property of Target is "Function", thena. Let L be the length property of Target minus the length of A.
b. Set the length own property of F to either 0 or L, whichever is larger.
16. Else set the length own property of F to 0.
17. Set the attributes of the length own property of F to the values specified in 15.3.5.1.
18. Set the [[Extensible]] internal property of F to true.
19. Let thrower be the [[ThrowTypeError]] function Object (13.2.3).
20. Call the [[DefineOwnProperty]] internal method of F with arguments "caller", PropertyDescriptor{[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
21. Call the [[DefineOwnProperty]] internal method of F with arguments "arguments", PropertyDescriptor{[[Get]]: thrower, [[Set]]: thrower, [[Enumerable]]: false, [[Configurable]]: false}, and false.
22. Return F.
The length property of the bind method is 1.
NOTE
Function objects created using Function.prototype.bind do not have a prototype property or the[[Code]], [[FormalParameters]], and [[Scope]] internal properties.

15.3.4.5.1[[Call]]When the [[Call]] internal method of a function object, F, which was created using the bind function is calledwith a this value and a list of arguments ExtraArgs, the following steps are taken:
1.
2.
3.
4.
5.

Let boundArgs be the value of F’s [[BoundArgs]] internal property.
Let boundThis be the value of F’s [[BoundThis]] internal property.
Let target be the value of F’s [[TargetFunction]] internal property.
Let args be a new list containing the same values as the list boundArgs in the same order followed by thesame values as the list ExtraArgs in the same order.
Return the result of calling the [[Call]] internal method of target providing boundThis as the this value andproviding args as the arguments.

15.3.4.5.2[[Construct]]

When the [[Construct]] internal method of a function object, F that was created using the bind function is calledwith a list of arguments ExtraArgs, the following steps are taken:© Ecma International 2009

119

1.
2.
3.
4.
5.

Let target be the value of F’s [[TargetFunction]] internal property.
If target has no [[Construct]] internal method, a TypeError exception is thrown.
Let boundArgs be the value of F’s [[BoundArgs]] internal property.
Let args be a new list containing the same values as the list boundArgs in the same order followed by thesame values as the list ExtraArgs in the same order.
Return the result of calling the [[Construct]] internal method of target providing args as the arguments.

15.3.4.5.3[[HasInstance]] (V)When the [[HasInstance]] internal method of a function object F, that was created using the bind function iscalled with argument V, the following steps are taken:
1.
2.
3.

Let target be the value of F’s [[TargetFunction]] internal property.
If target has no [[HasInstance]] internal method, a TypeError exception is thrown.
Return the result of calling the [[HasInstance]] internal method of target providing V as the argument.

15.3.5 Properties of Function Instances
In addition to the required internal properties, every function instance has a [[Call]] internal property and in
most cases use a different version of the [[Get]] internal property. Depending on how they are created (see8.6.2 ,13.2, 15, and 15.3.4.5), function instances may have a [[HasInstance]] internal property, a [[Scope]]
internal property, a [[Construct]] internal property, a [[FormalParameters]] internal property, a [[Code]] internal
property, a [[TargetFunction]] internal property, a [[BoundThis]] internal property, and a [[BoundArgs]] internal
property.
The value of the [[Class]] internal property is "Function".
Function instances that correspond to strict mode functions (13.2) and function instances created using the
Function.prototype.bind method (15.3.4.5) have properties named “caller” and “arguments” that throw a
TypeError exception. An ECMAScript implementation must not associate any implementation specificbehaviour with accesses of these properties from strict mode function code.
15.3.5.1

lengthThe value of the length property is an integer that indicates the “typical” number of arguments expected by
the function. However, the language permits the function to be invoked with some other number of arguments.
The behaviour of a function when invoked on a number of arguments other than the number specified by its
length property depends on the function. This property has the attributes { [[Writable]]: false, [[Enumerable]]:false, [[Configurable]]: false }.
15.3.5.2

prototypeThe value of the prototype property is used to initialise the [[Prototype]] internal property of a newly createdobject before the Function object is invoked as a constructor for that newly created object. This property has
the attribute { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
NOTEFunction objects created using Function.prototype.bind do not have a prototype property.

15.3.5.3[[HasInstance]] (V)Assume F is a Function object.
When the [[HasInstance]] internal method of F is called with value V, the following steps are taken:
1.
2.
3.
4.

If V is not an object, return false.
Let O be the result of calling the [[Get]] internal method of F with property name "prototype".
If Type(O) is not Object, throw a TypeError exception.
Repeata. Let V be the value of the [[Prototype]] internal property of V.

120© Ecma International 2009b.
c.

If V is null, return false.
If O and V refer to the same object, return true.

NOTE
Function objects created using Function.prototype.bind have a different implementation of[[HasInstance]] defined in 15.3.4.5.3.

15.3.5.4[[Get]] (P)Function objects use a variation of the [[Get]] internal method used for other native ECMAScript objects(8.12.3).
Assume F is a Function object. When the [[Get]] internal method of F is called with property name P, thefollowing steps are taken:
1.
2.
3.

Let v be the result of calling the default [[Get]] internal method (8.12.3) on F passing P as the property nameargument.
If P is "caller" and v is a strict mode Function object, throw a TypeError exception.
Return v.

NOTE

Function objects created using Function.prototype.bind use the default [[Get]] internal method.

15.4 Array Objects
Array objects give special treatment to a certain class of property names. A property name P (in the form of aString value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to232−1. A property whose property name is an array index is also called an element. Every Array object has alength property whose value is always a nonnegative integer less than 232. The value of the length
property is numerically greater than the name of every property whose name is an array index; whenever aproperty of an Array object is created or changed, other properties are adjusted as necessary to maintain thisinvariant. Specifically, whenever a property is added whose name is an array index, the length property ischanged, if necessary, to be one more than the numeric value of that array index; and whenever the length
property is changed, every property whose name is an array index whose value is not smaller than the newlength is automatically deleted. This constraint applies only to own properties of an Array object and isunaffected by length or array index properties that may be inherited from its prototypes.
An object, O, is said to be sparse if the following algorithm returns true:
1.
2.

3.

Let len be the result of calling the [[Get]] internal method of O with argument "length".
For each integer i in the range 0≤i<ToUint32(len)a. Let elem be the result of calling the [[GetOwnProperty]] internal method of O with argument
ToString(i).
b. If elem is undefined, return true.
Return false.

15.4.1 The Array Constructor Called as a Function
When Array is called as a function rather than as a constructor, it creates and initialises a new Array object.
Thus the function call Array(…) is equivalent to the object creation expression new Array(…) with thesame arguments.
15.4.1.1

Array ( [ item1 [ , item2 [ , … ] ] ] )When the Array function is called the following steps are taken:
1.

Create and return a new Array object exactly as if the standard built-in constructor Array was used in a
new expression with the same arguments (15.4.2).

© Ecma International 2009121

15.4.2 The Array Constructor
When Array is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.4.2.1

new Array ( [ item0 [ , item1 [ , … ] ] ] )This description applies if and only if the Array constructor is given no arguments or at least two arguments.
The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object,
the one that is the initial value of Array.prototype (15.4.3.1).
The [[Class]] internal property of the newly constructed object is set to "Array".
The [[Extensible]] internal property of the newly constructed object is set to true.
The length property of the newly constructed object is set to the number of arguments.
The 0 property of the newly constructed object is set to item0 (if supplied); the 1 property of the newly
constructed object is set to item1 (if supplied); and, in general, for as many arguments as there are, the kproperty of the newly constructed object is set to argument k, where the first argument is considered to beargument number 0. These properties all have the attributes {[[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}.
15.4.2.2

new Array (len)The [[Prototype]] internal property of the newly constructed object is set to the original Array prototype object,
the one that is the initial value of Array.prototype (15.4.3.1). The [[Class]] internal property of the newly
constructed object is set to "Array". The [[Extensible]] internal property of the newly constructed object is set
to true.
If the argument len is a Number and ToUint32(len) is equal to len, then the length property of the newly
constructed object is set to ToUint32(len). If the argument len is a Number and ToUint32(len) is not equal to len,a RangeError exception is thrown.
If the argument len is not a Number, then the length property of the newly constructed object is set to 1 andthe 0 property of the newly constructed object is set to len with attributes {[[Writable]]: true, [[Enumerable]]:true, [[Configurable]]: true}..
15.4.3 Properties of the Array Constructor
The value of the [[Prototype]] internal property of the Array constructor is the Function prototype object(15.3.4).
Besides the internal properties and the length property (whose value is 1), the Array constructor has thefollowing properties:
15.4.3.1

Array.prototype

The initial value of Array.prototype is the Array prototype object (15.4.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.4.3.2

Array.isArray ( arg )The isArray function takes one argument arg, and returns the Boolean value true if the argument is an object
whose class internal property is "Array"; otherwise it returns false. The following steps are taken:

122© Ecma International 2009

1.
2.
3.

If Type(arg) is not Object, return false.
If the value of the [[Class]] internal property of arg is "Array", then return true.
Return false.

15.4.4 Properties of the Array Prototype Object
The value of the [[Prototype]] internal property of the Array prototype object is the standard built-in Object
prototype object (15.2.4).
The Array prototype object is itself an array; its [[Class]] is "Array", and it has a length property (whose
initial value is +0) and the special [[DefineOwnProperty]] internal method described in 15.4.5.1.
In following descriptions of functions that are properties of the Array prototype object, the phrase “this object”
refers to the object that is the this value for the invocation of the function. It is permitted for the this to be an
object for which the value of the [[Class]] internal property is not "Array".
NOTE
The Array prototype object does not have a valueOf property of its own; however, it inherits the valueOfproperty from the standard built-in Object prototype Object.

15.4.4.1

Array.prototype.constructor

The initial value of Array.prototype.constructor is the standard built-in Array constructor.
15.4.4.2

Array.prototype.toString ( )When the toString method is called, the following steps are taken:
1.
2.
3.
4.

Let array be the result of calling ToObject on the this value.
Let func be the result of calling the [[Get]] internal method of array with argument "join".
If IsCallable(func) is false, then let func be the standard built-in method Object.prototype.toString (15.2.4.2).
Return the result of calling the [[Call]] internal method of func providing array as the this value and an
empty arguments list.

NOTE
The toString function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the toString function can beapplied successfully to a host object is implementation-dependent.

15.4.4.3

Array.prototype.toLocaleString ( )The elements of the array are converted to Strings using their toLocaleString methods, and these Strings
are then concatenated, separated by occurrences of a separator String that has been derived in an
implementation-defined locale-specific way. The result of calling this function is intended to be analogous to
the result of toString, except that the result of this function is intended to be locale-specific.
The result is calculated as follows:
1.
2.
3.
4.
5.
6.
7.
8.

Let O be the result of calling ToObject passing the this value as the argument.
Let arrayLen be the result of calling the [[Get]] internal method of array with argument "length".
Let len be ToUint32(arrayLen).
Let separator be the String value for the list-separator String appropriate for the host environment’s current
locale (this is derived in an implementation-defined way).
If len is zero, return the empty String.
Let firstElement be the result of calling the [[Get]] internal method of array with argument "0".
If firstElement is undefined or null, thena. Let R be the empty String.
Elsea. Let elementObj be ToObject(firstElement).

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123

b.

Let func be the result of calling the [[Get]] internal method of elementObj with argument"toLocaleString".
c. If IsCallable(func) is false, throw a TypeError exception.
d. Let R be the result of calling the [[Call]] internal method of func providing elementObj as the thisvalue and an empty arguments list.
9. Let k be 1.
10. Repeat, while k < lena. Let S be a String value produced by concatenating R and separator.
b. Let nextElement be the result of calling the [[Get]] internal method of array with argument
ToString(k).
c. If nextElement is undefined or null, theni.
Let R be the empty String.
d. Elsei.
Let elementObj be ToObject(nextElement).
ii.
Let func be the result of calling the [[Get]] internal method of elementObj with argument"toLocaleString".
iii.
If IsCallable(func) is false, throw a TypeError exception.
iv.
Let R be the result of calling the [[Call]] internal method of func providing elementObj as
the this value and an empty arguments list.
e. Let R be a String value produced by concatenating S and R.
f. Increase k by 1.
11. Return R.
NOTE 1
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.
NOTE 2 The toLocaleString function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the toLocaleString function canbe applied successfully to a host object is implementation-dependent.

15.4.4.4

Array.prototype.concat ( [ item1 [ , item2 [ , … ] ] ] )When the concat method is called with zero or more arguments item1, item2, etc., it returns an array
containing the array elements of the object followed by the array elements of each argument in order.
The following steps are taken:
1.
2.
3.
4.
5.

Let O be the result of calling ToObject passing the this value as the argument.
Let A be a new array created as if by the expression new Array() where Array is the standard built-in
constructor with that name.
Let n be 0.
Let items be an internal List whose first element is O and whose subsequent elements are, in left to rightorder, the arguments that were passed to this function invocation.
Repeat, while items is not empty
a. Remove the first element from items and let E be the value of the element.
b. If the value of the [[Class]] internal property of E is "Array", theni.
Let k be 0.
ii.
Let len be the result of calling the [[Get]] internal method of E with argument "length".
iii.
Repeat, while k < len
1. Let P be ToString(k).
2. Let exists be the result of calling the [[HasProperty]] internal method of E with P.
3. If exists is true, thena Let subElement be the result of calling the [[Get]] internal method of Ewith argument P.
b Call the [[DefineOwnProperty]] internal method of A with arguments
ToString(n), Property Descriptor {[[Value]]: subElement, [[Writable]]:true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
4. Increase n by 1.
5. Increase k by 1.

124© Ecma International 2009

c.

6.

Else, E is not an Arrayi.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property
Descriptor {[[Value]]: E, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true},and false.
ii.
Increase n by 1.
Return A.

The length property of the concat method is 1.
NOTE
The concat function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the concat function can be applied
successfully to a host object is implementation-dependent.

15.4.4.5

Array.prototype.join (separator)

The elements of the array are converted to Strings, and these Strings are then concatenated, separated byoccurrences of the separator. If no separator is provided, a single comma is used as the separator.
The join method takes one argument, separator, and performs the following steps:
1. Let O be the result of calling ToObject passing the this value as the argument.
2. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
3. Let len be ToUint32(lenVal).
4. If separator is undefined, let separator be the single-character String ",".
5. Let sep be ToString(separator).
6. If len is zero, return the empty String.
7. Let element0 be the result of calling the [[Get]] internal method of O with argument "0".
8. If element0 is undefined or null, let R be the empty String; otherwise, Let R be ToString(element0).
9. Let k be 1.
10. Repeat, while k < lena. Let S be the String value produced by concatenating R and sep.
b. Let element be the result of calling the [[Get]] internal method of O with argument ToString(k).
c. If element is undefined or null, Let next be the empty String; otherwise, let next be
ToString(element).
d. Let R be a String value produced by concatenating S and next.
e. Increase k by 1.
11. Return R.
The length property of the join method is 1.
NOTE
The join function is intentionally generic; it does not require that its this value be an Array object. Therefore,
it can be transferred to other kinds of objects for use as a method. Whether the join function can be applied successfully
to a host object is implementation-dependent.

15.4.4.6

Array.prototype.pop ( )The last element of the array is removed from the array and returned.
1.
2.
3.
4.
5.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
Let len be ToUint32(lenVal).
If len is zero,a. Call the [[Put]] internal method of O with arguments "length", 0, and true.
b. Return undefined.
Else, len > 0
a. Let indx be ToString(len–1).
b. Let element be the result of calling the [[Get]] internal method of O with argument indx.
c. Call the [[Delete]] internal method of O with arguments indx and true.
d. Call the [[Put]] internal method of O with arguments "length", indx, and true.

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e.

Return element.

NOTE
The pop function is intentionally generic; it does not require that its this value be an Array object. Therefore it
can be transferred to other kinds of objects for use as a method. Whether the pop function can be applied successfully toa host object is implementation-dependent.

15.4.4.7

Array.prototype.push ( [ item1 [ , item2 [ , … ] ] ] )The arguments are appended to the end of the array, in the order in which they appear. The new length of thearray is returned as the result of the call.
When the push method is called with zero or more arguments item1, item2, etc., the following steps are taken:
1.
2.
3.
4.
5.

6.
7.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
Let n be ToUint32(lenVal).
Let items be an internal List whose elements are, in left to right order, the arguments that were passed to thisfunction invocation.
Repeat, while items is not empty
a. Remove the first element from items and let E be the value of the element.
b. Call the [[Put]] internal method of O with arguments ToString(n), E, and true.
c. Increase n by 1.
Call the [[Put]] internal method of O with arguments "length", n, and true.
Return n.

The length property of the push method is 1.
NOTE
The push function is intentionally generic; it does not require that its this value be an Array object. Therefore
it can be transferred to other kinds of objects for use as a method. Whether the push function can be applied successfully
to a host object is implementation-dependent.

15.4.4.8

Array.prototype.reverse ( )The elements of the array are rearranged so as to reverse their order. The object is returned as the result of
the call.
1.
2.
3.
4.
5.
6.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
Let len be ToUint32(lenVal).
Let middle be floor(len/2).
Let lower be 0.
Repeat, while lower ≠ middle
a. Let upper be len− lower −1.
b. Let upperP be ToString(upper).
c. Let lowerP be ToString(lower).
d. Let lowerValue be the result of calling the [[Get]] internal method of O with argument lowerP.
e. Let upperValue be the result of calling the [[Get]] internal method of O with argument upperP .
f. Let lowerExists be the result of calling the [[HasProperty]] internal method of O with argument
lowerP.
g. Let upperExists be the result of calling the [[HasProperty]] internal method of O with argument
upperP.
h. If lowerExists is true and upperExists is true, theni.
Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
ii.
Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
i. Else if lowerExists is false and upperExists is true, theni.
Call the [[Put]] internal method of O with arguments lowerP, upperValue, and true .
ii.
Call the [[Delete]] internal method of O, with arguments upperP and true.
j. Else if lowerExists is true and upperExists is false, theni.
Call the [[Delete]] internal method of O, with arguments lowerP and true .

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ii.
Call the [[Put]] internal method of O with arguments upperP, lowerValue, and true .
Else, both lowerExists and upperExists are falsei.
No action is required.
l. Increase lower by 1.
Return O .
k.

7.

NOTE
The reverse function is intentionally generic; it does not require that its this value be an Array object.
Therefore, it can be transferred to other kinds of objects for use as a method. Whether the reverse function can beapplied successfully to a host object is implementation-dependent.

15.4.4.9

Array.prototype.shift ( )The first element of the array is removed from the array and returned.
1.
2.
3.
4.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
Let len be ToUint32(lenVal).
If len is zero, thena. Call the [[Put]] internal method of O with arguments "length", 0, and true.
b. Return undefined.
5. Let first be the result of calling the [[Get]] internal method of O with argument "0".
6. Let k be 1.
7. Repeat, while k < lena. Let from be ToString(k).
b. Let to be ToString(k–1).
c. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument
from.
d. If fromPresent is true, theni.
Let fromVal be the result of calling the [[Get]] internal method of O with argument from.
ii.
Call the [[Put]] internal method of O with arguments to, fromVal, and true.
e. Else, fromPresent is falsei.
Call the [[Delete]] internal method of O with arguments to and true.
f. Increase k by 1.
8. Call the [[Delete]] internal method of O with arguments ToString(len–1) and true.
9. Call the [[Put]] internal method of O with arguments "length", (len–1) , and true.
10. Return first.
NOTE
The shift function is intentionally generic; it does not require that its this value be an Array object. Therefore
it can be transferred to other kinds of objects for use as a method. Whether the shift function can be applied
successfully to a host object is implementation-dependent.

15.4.4.10 Array.prototype.slice (start, end)
The slice method takes two arguments, start and end, and returns an array containing the elements of thearray from element start up to, but not including, element end (or through the end of the array if end isundefined). If start is negative, it is treated as length+start where length is the length of the array. If end isnegative, it is treated as length+end where length is the length of the array. The following steps are taken:
1.
2.

Let O be the result of calling ToObject passing the this value as the argument.
Let A be a new array created as if by the expression new Array() where Array is the standard built-in
constructor with that name.
3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4. Let len be ToUint32(lenVal).
5. Let relativeStart be ToInteger(start).
6. If relativeStart is negative, let k be max((len + relativeStart),0); else let k be min(relativeStart, len).
7. If end is undefined, let relativeEnd be len; else let relativeEnd be ToInteger(end).
8. If relativeEnd is negative, let final be max((len + relativeEnd),0); else let final be min(relativeEnd, len).
9. Let n be 0.
10. Repeat, while k < final© Ecma International 2009

127

a.
b.
c.

Let Pk be ToString(k).
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n), Property
Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]:true}, and false.
d. Increase k by 1.
e. Increase n by 1.
11. Return A.
The length property of the slice method is 2.
NOTE
The slice function is intentionally generic; it does not require that its this value be an Array object. Therefore
it can be transferred to other kinds of objects for use as a method. Whether the slice function can be applied
successfully to a host object is implementation-dependent.

15.4.4.11 Array.prototype.sort (comparefn)
The elements of this array are sorted. The sort is not necessarily stable (that is, elements that compare equal
do not necessarily remain in their original order). If comparefn is not undefined, it should be a function that
accepts two arguments x and y and returns a negative value if x < y, zero if x = y, or a positive value if x > y.
Let obj be the result of calling ToObject passing the this value as the argument.
Let len be the result of applying Uint32 to the result of calling the [[Get]] internal method of obj with argument"length".
If comparefn is not undefined and is not a consistent comparison function for the elements of this array (seebelow), the behaviour of sort is implementation-defined.
Let proto be the value of the [[Prototype]] internal property of obj. If proto is not null and there exists an integer
j such that all of the conditions below are satisfied then the behaviour of sort is implementation-defined:•••

obj is sparse (15.4)
0 ≤ j < len
The result of calling the [[HasProperty]] internal method of proto with argument ToString(j) is true.

The behaviour of sort is also implementation defined if obj is sparse and any of the following conditions are
true:
•
•

The [[Extensible]] internal property of obj is false.
Any array index property of obj whose name is a nonnegative integer less than len is a data property
whose [[Configurable]] attribute is false.

The behaviour of sort is also implementation defined if any array index property of obj whose name is anonnegative integer less than len is an accessor property or is a data property whose [[Writable]] attribute isfalse.
Otherwise, the following steps are taken.
1.

2.

Perform an implementation-dependent sequence of calls to the [[Get]] , [[Put]], and [[Delete]] internal
methods of obj and to SortCompare (described below), where the first argument for each call to [[Get]],[[Put]], or [[Delete]] is a nonnegative integer less than len and where the arguments for calls to SortCompare
are results of previous calls to the [[Get]] internal method. The throw argument to the [[Put]] and [[Delete]]internal methods will be the value true. If obj is not sparse then [[Delete]] must not be called.
Return obj.

128

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The returned object must have the following two properties.
••

There must be some mathematical permutation π of the nonnegative integers less than len, such thatfor every nonnegative integer j less than len, if property old[j] existed, then new[π(j)] is exactly thesame value as old[j],. But if property old[j] did not exist, then new[π(j)] does not exist.
Then for all nonnegative integers j and k, each less than len, if SortCompare(j,k) < 0 (see SortCompare
below), then π(j) < π(k).

Here the notation old[j] is used to refer to the hypothetical result of calling the [[Get]] internal method of obj
with argument j before this function is executed, and the notation new[j] to refer to the hypothetical result of
calling the [[Get]] internal method of obj with argument j after this function has been executed.
A function comparefn is a consistent comparison function for a set of values S if all of the requirements below
are met for all values a, b, and c (possibly the same value) in the set S: The notation a <CF b means
comparefn(a,b) < 0; a =CF b means comparefn(a,b) = 0 (of either sign); and a >CF b means comparefn(a,b) > 0.
•
•
•
•
•
•
•

Calling comparefn(a,b) always returns the same value v when given a specific pair of values a and b as its twoarguments. Furthermore, Type(v) is Number, and v is not NaN. Note that this implies that exactly one of a <CF b,
a =CF b, and a >CF b will be true for a given pair of a and b.
Calling comparefn(a,b) does not modify the this object.
a =CF a (reflexivity)
If a =CF b, then b =CF a (symmetry)
If a =CF b and b =CF c, then a =CF c (transitivity of =CF)If a <CF b and b <CF c, then a <CF c (transitivity of <CF)
If a >CF b and b >CF c, then a >CF c (transitivity of >CF)NOTE
The above conditions are necessary and sufficient to ensure that comparefn divides the set S into equivalenceclasses and that these equivalence classes are totally ordered.

When the SortCompare abstract operation is called with two arguments j and k, the following steps are taken:
1. Let jString be ToString(j).
2. Let kString be ToString(k).
3. Let hasj be the result of calling the [[HasProperty]] internal method of obj with argument jString.
4. Let hask be the result of calling the [[HasProperty]] internal method of obj with argument kString.
5. If hasj and hask are both false, then return +0.
6. If hasj is false, then return 1.
7. If hask is false, then return –1.
8. Let x be the result of calling the [[Get]] internal method of obj with argument jString.
9. Let y be the result of calling the [[Get]] internal method of obj with argument kString.
10. If x and y are both undefined, return +0.
11. If x is undefined, return 1.
12. If y is undefined, return −1.
13. If the argument comparefn is not undefined, thena. If IsCallable(comparefn) is false, throw a TypeError exception.
b. Return the result of calling the [[Call]] internal method of comparefn passing undefined as the thisvalue and with arguments x and y.
14. Let xString be ToString(x).
15. Let yString be ToString(y).
16. If xString < yString, return −1.
17. If xString > yString, return 1.
18. Return +0.
NOTE 1
Because non-existent property values always compare greater than undefined property values, andundefined always compares greater than any other value, undefined property values always sort to the end of the result,followed by non-existent property values.
NOTE 2The sort function is intentionally generic; it does not require that its this value be an Array object. Therefore,
it can be transferred to other kinds of objects for use as a method. Whether the sort function can be applied successfully
to a host object is implementation-dependent.

© Ecma International 2009129

15.4.4.12 Array.prototype.splice (start, deleteCount [ , item1 [ , item2 [ , … ] ] ] )When the splice method is called with two or more arguments start, deleteCount and (optionally) item1, item2,
etc., the deleteCount elements of the array starting at array index start are replaced by the arguments item1,
item2, etc. An Array object containing the deleted elements (if any) is returned. The following steps are taken:
1.
2.

Let O be the result of calling ToObject passing the this value as the argument.
Let A be a new array created as if by the expression new Array()where Array is the standard built-in
constructor with that name.
3. Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
4. Let len be ToUint32(lenVal).
5. Let relativeStart be ToInteger(start).
6. If relativeStart is negative, let actualStart be max((len + relativeStart),0); else let actualStart bemin(relativeStart, len).
7. Let actualDeleteCount be min(max(ToInteger(deleteCount),0), len – actualStart).
8. Let k be 0.
9. Repeat, while k < actualDeleteCount
a. Let from be ToString(relativeStart+k).
b. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument
from.
c. If fromPresent is true, theni.
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
ii.
Call the [[DefineOwnProperty]] internal method of A with arguments ToString(k), Property
Descriptor {[[Value]]: fromValue, [[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}, and false.
d. Increment k by 1.
10. Let items be an internal List whose elements are, in left to right order, the portion of the actual argument list
starting with item1. The list will be empty if no such items are present.
11. Let itemCount be the number of elements in items.
12. If itemCount < actualDeleteCount, thena. Let k be actualStart.
b. Repeat, while k < (len – actualDeleteCount)i.
Let from be ToString(k+actualDeleteCount).
ii.
Let to be ToString(k+itemCount).
iii.
Let fromPresent be the result of calling the [[HasProperty]] internal method of O withargument from.
iv.
If fromPresent is true, then
1. Let fromValue be the result of calling the [[Get]] internal method of O withargument from.
2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
v.
Else, fromPresent is false
1. Call the [[Delete]] internal method of O with arguments to and true.
vi.
Increase k by 1.
c. Let k be len.
d. Repeat, while k > (len – actualDeleteCount + itemCount)i.
Call the [[Delete]] internal method of O with arguments ToString(k–1) and true.
ii.
Decrease k by 1.
13. Else if itemCount > actualDeleteCount, thena. Let k be (len – actualDeleteCount).
b. Repeat, while k > actualStarti.
Let from be ToString(k + actualDeleteCount – 1).
ii.
Let to be ToString(k + itemCount – 1)iii.
Let fromPresent be the result of calling the [[HasProperty]] internal method of O withargument from.
iv.
If fromPresent is true, then
1. Let fromValue be the result of calling the [[Get]] internal method of O withargument from.
2. Call the [[Put]] internal method of O with arguments to, fromValue, and true.
v.
Else, fromPresent is false
1. Call the [[Delete]] internal method of O with argument to and true.

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vi.
Decrease k by 1.
14. Let k be actualStart.
15. Repeat, while items is not empty
a. Remove the first element from items and let E be the value of that element.
b. Call the [[Put]] internal method of O with arguments ToString(k), E, and true.
c. Increase k by 1.
16. Call the [[Put]] internal method of O with arguments "length", (len – actualDeleteCount + itemCount),
and true.
17. Return A.
The length property of the splice method is 2.
NOTE
The splice function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the splice function can be applied
successfully to a host object is implementation-dependent.

15.4.4.13 Array.prototype.unshift ( [ item1 [ , item2 [ , … ] ] ] )The arguments are prepended to the start of the array, such that their order within the array is the same as theorder in which they appear in the argument list.
When the unshift method is called with zero or more arguments item1, item2, etc., the following steps aretaken:
1.
2.
3.
4.
5.
6.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenVal be the result of calling the [[Get]] internal method of O with argument "length".
Let len be ToUint32(lenVal).
Let argCount be the number of actual arguments.
Let k be len.
Repeat, while k > 0,a. Let from be ToString(k–1).
b. Let to be ToString(k+argCount –1).
c. Let fromPresent be the result of calling the [[HasProperty]] internal method of O with argument
from.
d. If fromPresent is true, theni.
Let fromValue be the result of calling the [[Get]] internal method of O with argument from.
ii.
Call the [[Put]] internal method of O with arguments to, fromValue, and true.
e. Else, fromPresent is falsei.
Call the [[Delete]] internal method of O with arguments to, and true.
f. Decrease k by 1.
7. Let j be 0.
8. Let items be an internal List whose elements are, in left to right order, the arguments that were passed to thisfunction invocation.
9. Repeat, while items is not empty
a. Remove the first element from items and let E be the value of that element.
b. Call the [[Put]] internal method of O with arguments ToString(j), E, and true.
c. Increase j by 1.
10. Call the [[Put]] internal method of O with arguments "length", len+argCount, and true.
11. Return len+argCount.
The length property of the unshift method is 1.
NOTE
The unshift function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the unshift function can beapplied successfully to a host object is implementation-dependent.

© Ecma International 2009131

15.4.4.14 Array.prototype.indexOf ( searchElement [ , fromIndex ] )indexOf compares searchElement to the elements of the array, in ascending order, using the internal Strict
Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of the firstsuch position; otherwise, -1 is returned.
The optional second argument fromIndex defaults to 0 (i.e. the whole array is searched). If it is greater than orequal to the length of the array, -1 is returned, i.e. the array will not be searched. If it is negative, it is used as
the offset from the end of the array to compute fromIndex. If the computed index is less than 0, the whole array
will be searched.
When the indexOf method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If len is 0, return -1.
If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be 0.
If n ≥ len, return -1.
If n ≥ 0, thena. Let k be n.
8. Else, n<0
a. Let k be len - abs(n).
b. If k is less than 0, then let k be 0.
9. Repeat, while k<lena. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
ToString(k).
b. If kPresent is true, theni.
Let elementK be the result of calling the [[Get]] internal method of O with the argument
ToString(k).
ii.
Let same be the result of applying the Strict Equality Comparison Algorithm tosearchElement and elementK.
iii.
If same is true, return k.
c. Increase k by 1.
10. Return -1.
The length property of the indexOf method is 1.
NOTE
The indexOf function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the indexOf function can beapplied successfully to a host object is implementation-dependent.

15.4.4.15 Array.prototype.lastIndexOf ( searchElement [ , fromIndex ] )lastIndexOf compares searchElement to the elements of the array in descending order using the internal
Strict Equality Comparison Algorithm (11.9.6), and if found at one or more positions, returns the index of thelast such position; otherwise, -1 is returned.
The optional second argument fromIndex defaults to the array's length (i.e. the whole array is searched). If it isgreater than or equal to the length of the array, the whole array will be searched. If it is negative, it is used as
the offset from the end of the array to compute fromIndex. If the computed index is less than 0, -1 is returned.
When the lastIndexOf method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If len is 0, return -1.
If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len.

132

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6.
7.
8.

9.

If n ≥ 0, then let k be min(n, len – 1).
Else, n < 0
a. Let k be len - abs(n).
Repeat, while k≥ 0
a. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
ToString(k).
b. If kPresent is true, theni.
Let elementK be the result of calling the [[Get]] internal method of O with the argument
ToString(k).
ii.
Let same be the result of applying the Strict Equality Comparision Algorithm tosearchElement and elementK.
iii.
If same is true, return k.
c. Decrease k by 1.
Return -1.

The length property of the lastIndexOf method is 1.
NOTE
The lastIndexOf function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the lastIndexOf function can beapplied successfully to a host object is implementation-dependent.

15.4.4.16 Array.prototype.every ( callbackfn [ , thisArg ] )callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. every calls callbackfn once for each element present in the array, in ascendingorder, until it finds one where callbackfn returns false. If such an element is found, every immediately returns
false. Otherwise, if callbackfn returned true for all elements, every will return true. callbackfn is called only forelements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is notprovided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
every does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by every is set before the first call to callbackfn. Elements which areappended to the array after the call to every begins will not be visited by callbackfn. If existing elements of thearray are changed, their value as passed to callbackfn will be the value at the time every visits them;elements that are deleted after the call to every begins and before being visited are not visited. every acts
like the "for all" quantifier in mathematics. In particular, for an empty array, it returns true.
When the every method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as thethis value and argument list containing kValue, k, and O.

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133

8.

iii.
If ToBoolean(testResult) is false, return false.
d. Increase k by 1.
Return true.

The length property of the every method is 1.
NOTE
The every function is intentionally generic; it does not require that its this value be an Array object. Therefore
it can be transferred to other kinds of objects for use as a method. Whether the every function can be applied
successfully to a host object is implementation-dependent.

15.4.4.17 Array.prototype.some ( callbackfn [ , thisArg ] )callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. some calls callbackfn once for each element present in the array, in ascendingorder, until it finds one where callbackfn returns true. If such an element is found, some immediately returns
true. Otherwise, some returns false. callbackfn is called only for elements of the array which actually exist; it is
not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is notprovided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
some does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by some is set before the first call to callbackfn. Elements that are appended
to the array after the call to some begins will not be visited by callbackfn. If existing elements of the array arechanged, their value as passed to callbackfn will be the value at the time that some visits them; elements thatare deleted after the call to some begins and before being visited are not visited. some acts like the "exists"quantifier in mathematics. In particular, for an empty array, it returns false.
When the some method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

8.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let testResult be the result of calling the [[Call]] internal method of callbackfn with T as thethis value and argument list containing kValue, k, and O.
iii.
If ToBoolean(testResult) is true, return true.
d. Increase k by 1.
Return false.

The length property of the some method is 1.
NOTE
The some function is intentionally generic; it does not require that its this value be an Array object. Therefore
it can be transferred to other kinds of objects for use as a method. Whether the some function can be applied successfully
to a host object is implementation-dependent.

134© Ecma International 2009

15.4.4.18 Array.prototype.forEach ( callbackfn [ , thisArg ] )callbackfn should be a function that accepts three arguments. forEach calls callbackfn once for each element
present in the array, in ascending order. callbackfn is called only for elements of the array which actually exist;
it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is notprovided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
forEach does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by forEach is set before the first call to callbackfn. Elements which areappended to the array after the call to forEach begins will not be visited by callbackfn. If existing elements of
the array are changed, their value as passed to callback will be the value at the time forEach visits them;elements that are deleted after the call to forEach begins and before being visited are not visited.
When the forEach method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

8.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let k be 0.
Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Call the [[Call]] internal method of callbackfn with T as the this value and argument list
containing kValue, k, and O.
d. Increase k by 1.
Return.

The length property of the forEach method is 1.
NOTE
The forEach function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the forEach function can beapplied successfully to a host object is implementation-dependent.

15.4.4.19 Array.prototype.map ( callbackfn [ , thisArg ] )callbackfn should be a function that accepts three arguments. map calls callbackfn once for each element in thearray, in ascending order, and constructs a new Array from the results. callbackfn is called only for elements of
the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is notprovided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
map does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.

© Ecma International 2009135

The range of elements processed by map is set before the first call to callbackfn. Elements which areappended to the array after the call to map begins will not be visited by callbackfn. If existing elements of thearray are changed, their value as passed to callbackfn will be the value at the time map visits them; elementsthat are deleted after the call to map begins and before being visited are not visited.
When the map method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

9.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let A be a new array created as if by the expression new Array(len) where Array is the standard builtin constructor with that name and len is the value of len.
Let k be 0.
Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let mappedValue be the result of calling the [[Call]] internal method of callbackfn with T as
the this value and argument list containing kValue, k, and O.
iii.
Call the [[DefineOwnProperty]] internal method of A with arguments Pk, Property
Descriptor {[[Value]]: mappedValue, [[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}, and false.
d. Increase k by 1.
Return A.

The length property of the map method is 1.
NOTE
The map function is intentionally generic; it does not require that its this value be an Array object. Therefore it
can be transferred to other kinds of objects for use as a method. Whether the map function can be applied successfully toa host object is implementation-dependent.

15.4.4.20 Array.prototype.filter ( callbackfn [ , thisArg ] )callbackfn should be a function that accepts three arguments and returns a value that is coercible to the
Boolean value true or false. filter calls callbackfn once for each element in the array, in ascending order,and constructs a new array of all the values for which callbackfn returns true. callbackfn is called only forelements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value for each invocation of callbackfn. If it is notprovided, undefined is used instead.
callbackfn is called with three arguments: the value of the element, the index of the element, and the object
being traversed.
filter does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by filter is set before the first call to callbackfn. Elements which areappended to the array after the call to filter begins will not be visited by callbackfn. If existing elements of
the array are changed their value as passed to callbackfn will be the value at the time filter visits them;elements that are deleted after the call to filter begins and before being visited are not visited.
When the filter method is called with one or two arguments, the following steps are taken:
1.
2.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".

136© Ecma International 2009

3.
4.
5.
6.

Let len be ToUint32(lenValue).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If thisArg was supplied, let T be thisArg; else let T be undefined.
Let A be a new array created as if by the expression new Array() where Array is the standard built-in
constructor with that name.
7. Let k be 0.
8. Let to be 0.
9. Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let selected be the result of calling the [[Call]] internal method of callbackfn with T as thethis value and argument list containing kValue, k, and O.
iii.
If ToBoolean(selected) is true, then
1. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(to),Property Descriptor {[[Value]]: kValue, [[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}, and false.
2. Increase to by 1.
d. Increase k by 1.
10. Return A.
The length property of the filter method is 1.
NOTE
The filter function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the filter function can be applied
successfully to a host object is implementation-dependent.

15.4.4.21 Array.prototype.reduce ( callbackfn [ , initialValue ] )callbackfn should be a function that takes four arguments. reduce calls the callback, as a function, once foreach element present in the array, in ascending order.
callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), thecurrentValue (value of the current element), the currentIndex, and the object being traversed. The first timethat callback is called, the previousValue and currentValue can be one of two values. If an initialValue wasprovided in the call to reduce, then previousValue will be equal to initialValue and currentValue will be equal
to the first value in the array. If no initialValue was provided, then previousValue will be equal to the first value
in the array and currentValue will be equal to the second. It is a TypeError if the array contains no elements
and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be mutated by the calls to
callbackfn.
The range of elements processed by reduce is set before the first call to callbackfn. Elements that areappended to the array after the call to reduce begins will not be visited by callbackfn. If existing elements of
the array are changed, their value as passed to callbackfn will be the value at the time reduce visits them;elements that are deleted after the call to filter begins and before being visited are not visited.
When the reduce method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue ).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If len is 0 and initialValue is not present, throw a TypeError exception.
Let k be 0.
If initialValue is present, thena. Set accumulator to initialValue.
Else, initialValue is not present© Ecma International 2009

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a.
b.

Let kPresent be false.
Repeat, while kPresent is false and k < leni.
Let Pk be ToString(k).
ii.
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
Pk.
iii.
If kPresent is true, then
1. Let accumulator be the result of calling the [[Get]] internal method of O withargument Pk.
iv.
Increase k by 1.
c. If kPresent is false, throw a TypeError exception.
9. Repeat, while k < lena. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let accumulator be the result of calling the [[Call]] internal method of callbackfn withundefined as the this value and argument list containing accumulator, kValue, k, and O.
d. Increase k by 1.
10. Return accumulator.
The length property of the reduce method is 1.
NOTE
The reduce function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the reduce function can be applied
successfully to a host object is implementation-dependent.

15.4.4.22 Array.prototype.reduceRight ( callbackfn [ , initialValue ] )callbackfn should be a function that takes four arguments. reduceRight calls the callback, as a function,
once for each element present in the array, in descending order.
callbackfn is called with four arguments: the previousValue (or value from the previous call to callbackfn), thecurrentValue (value of the current element), the currentIndex, and the object being traversed. The first time
the function is called, the previousValue and currentValue can be one of two values. If an initialValue wasprovided in the call to reduceRight, then previousValue will be equal to initialValue and currentValue will beequal to the last value in the array. If no initialValue was provided, then previousValue will be equal to the lastvalue in the array and currentValue will be equal to the second-to-last value. It is a TypeError if the array
contains no elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called but the object may be mutated by thecalls to callbackfn.
The range of elements processed by reduceRight is set before the first call to callbackfn. Elements that areappended to the array after the call to reduceRight begins will not be visited by callbackfn. If existingelements of the array are changed by callbackfn, their value as passed to callbackfn will be the value at thetime reduceRight visits them; elements that are deleted after the call to filter begins and before being
visited are not visited.
When the reduceRight method is called with one or two arguments, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

Let O be the result of calling ToObject passing the this value as the argument.
Let lenValue be the result of calling the [[Get]] internal method of O with the argument "length".
Let len be ToUint32(lenValue ).
If IsCallable(callbackfn) is false, throw a TypeError exception.
If len is 0 and initialValue is not present, throw a TypeError exception.
Let k be len-1.
If initialValue is present, thena. Set accumulator to initialValue.
Else, initialValue is not present
a. Let kPresent be false.

138© Ecma International 2009b.

Repeat, while kPresent is false and k ≥ 0i.
Let Pk be ToString(k).
ii.
Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument
Pk.
iii.
If kPresent is true, then
1. Let accumulator be the result of calling the [[Get]] internal method of O withargument Pk.
iv.
Decrease k by 1.
c. If kPresent is false, throw a TypeError exception.
9. Repeat, while k ≥ 0
a. Let Pk be ToString(k).
b. Let kPresent be the result of calling the [[HasProperty]] internal method of O with argument Pk.
c. If kPresent is true, theni.
Let kValue be the result of calling the [[Get]] internal method of O with argument Pk.
ii.
Let accumulator be the result of calling the [[Call]] internal method of callbackfn with nullas the this value and argument list containing accumulator, kValue, k, and O.
d. Decrease k by 1.
10. Return accumulator.
The length property of the reduceRight method is 1.
NOTE
The reduceRight function is intentionally generic; it does not require that its this value be an Array object.
Therefore it can be transferred to other kinds of objects for use as a method. Whether the reduceRight function can beapplied successfully to a host object is implementation-dependent.

15.4.5 Properties of Array Instances
Array instances inherit properties from the Array prototype object and their [[Class]] internal property value is"Array". Array instances also have the following properties.
15.4.5.1[[DefineOwnProperty]] ( P, Desc, Throw )Array objects use a variation of the [[DefineOwnProperty]] internal method used for other native ECMAScript
objects (8.12.9).
Assume A is an Array object, Desc is a Property Descriptor, and Throw is a Boolean flag.
In the following algorithm, the term “Reject” means “If Throw is true, then throw a TypeError exception, otherwise
return false.”
When the [[DefineOwnProperty]] internal method of A is called with property P, Property Descriptor Desc, andBoolean flag Throw, the following steps are taken:
1.
2.
3.

Let oldLenDesc be the result of calling the [[GetOwnProperty]] internal method of A passing "length" as
the argument. The result will never be undefined or an accessor descriptor because Array objects are
created with a length data property that cannot be deleted or reconfigured.
Let oldLen be oldLenDesc.[[Value]].
If P is "length", thena. If the [[Value]] field of Desc is absent, theni.
Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A
passing "length", Desc, and Throw as arguments.
b. Let newLenDesc be a copy of Desc.
c. Let newLen be ToUint32(Desc.[[Value]]).
d. If newLen is not equal to ToNumber( Desc.[[Value]]), throw a RangeError exception.
e. Set newLenDesc.[[Value] to newLen.
f. If newLen ≥oldLen, theni.
Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A
passing "length", newLenDesc, and Throw as arguments.
g. Reject if oldLenDesc.[[Writable]] is false.
h. If newLenDesc.[[Writable]] is absent or has the value true, let newWritable be true.

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139

i.

4.

5.

Else,
i.

Need to defer setting the [[Writable]] attribute to false in case any elements cannot bedeleted.
ii.
Let newWritable be false.
iii.
Set newLenDesc.[[Writable] to true.
j. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on
A passing "length", newLenDesc, and Throw as arguments.
k. If succeeded is false, return false..
l. While newLen < oldLen repeat,i.
Set oldLen to oldLen – 1.
ii.
Let cannotDelete be the result of calling the [[Delete]] internal method of A passing
ToString(oldLen) and false as arguments.
iii.
If cannotDelete is true, then
1. Set newLenDesc.[[Value] to oldLen+1.
2. If newWritable is false, set newLenDesc.[[Writable] to false.
3. Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing"length", newLenDesc, and false as arguments.
4. Reject.
m. If newWritable is false, theni.
Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length",
Property Descriptor{[[Writable]]: false}, and false as arguments. This call will always
return true.
n. Return true.
Else if P is an array index (15.4), thena. Let index be ToUint32(P).
b. Reject if index ≥ oldLen and oldLenDesc.[[Writable]] is false.
c. Let succeeded be the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on
A passing P, Desc, and false as arguments.
d. Reject if succeeded is false.
e. If index ≥ oldLeni.
Set oldLenDesc.[[Value]] to index + 1.
ii.
Call the default [[DefineOwnProperty]] internal method (8.12.9) on A passing "length",oldLenDesc, and false as arguments. This call will always return true.
f. Return true.
Return the result of calling the default [[DefineOwnProperty]] internal method (8.12.9) on A passing P,
Desc, and Throw as arguments.

15.4.5.2

lengthThe length property of this Array object is a data property whose value is always numerically greater than
the name of every deletable property whose name is an array index.
The length property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE
Attempting to set the length property of an Array object to a value that is numerically less than or equal to thelargest numeric property name of an existing array indexed non-deletable property of the array will result in the length
being set to a numeric value that is one greater than that largest numeric property name. See 15.4.5.1.

15.5 String Objects
15.5.1 The String Constructor Called as a Function
When String is called as a function rather than as a constructor, it performs a type conversion.
15.5.1.1

String ( [ value ] )Returns a String value (not a String object) computed by ToString(value). If value is not supplied, the empty
String "" is returned.

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15.5.2 The String Constructor
When String is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.5.2.1

new String ( [ value ] )The [[Prototype]] internal property of the newly constructed object is set to the standard built-in String
prototype object that is the initial value of String.prototype (15.5.3.1).
The [[Class]] internal property of the newly constructed object is set to "String".
The [[Extensible]] internal property of the newly constructed object is set to true.
The [[PrimitiveValue]] internal property of the newly constructed object is set to ToString(value), or to the empty
String if value is not supplied.
15.5.3 Properties of the String Constructor
The value of the [[Prototype]] internal property of the String constructor is the standard built-in Function
prototype object (15.3.4).
Besides the internal properties and the length property (whose value is 1), the String constructor has thefollowing properties:
15.5.3.1

String.prototype

The initial value of String.prototype is the standard built-in String prototype object (15.5.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.5.3.2

String.fromCharCode ( [ char0 [ , char1 [ , … ] ] ] )Returns a String value containing as many characters as the number of arguments. Each argument specifies
one character of the resulting String, with the first argument specifying the first character, and so on, from left
to right. An argument is converted to a character by applying the operation ToUint16 (9.7) and regarding theresulting 16-bit integer as the code unit value of a character. If no arguments are supplied, the result is theempty String.
The length property of the fromCharCode function is 1.
15.5.4 Properties of the String Prototype Object
The String prototype object is itself a String object (its [[Class]] is "String") whose value is an empty String.
The value of the [[Prototype]] internal property of the String prototype object is the standard built-in Object
prototype object (15.2.4).
15.5.4.1

String.prototype.constructor

The initial value of String.prototype.constructor is the built-in String constructor.
15.5.4.2

String.prototype.toString ( )Returns this String value. (Note that, for a String object, the toString method happens to return the samething as the valueOf method.)© Ecma International 2009141

The toString function is not generic; it throws a TypeError exception if its this value is not a String or a
String object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.5.4.3

String.prototype.valueOf ( )Returns this String value.
The valueOf function is not generic; it throws a TypeError exception if its this value is not a String or String
object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.5.4.4

String.prototype.charAt (pos)Returns a String containing the character at position pos in the String resulting from converting this object to aString. If there is no character at that position, the result is the empty String. The result is a String value, not a
String object.
If pos is a value of Number type that is an integer, then the result of x.charAt(pos) is equal to the result ofx.substring(pos, pos+1).
When the charAt method is called with one argument pos, the following steps are taken:
1.
2.
3.
4.
5.
6.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let position be ToInteger(pos).
Let size be the number of characters in S.
If position < 0 or position ≥ size, return the empty String.
Return a String of length 1, containing one character from S, namely the character at position position, where
the first (leftmost) character in S is considered to be at position 0, the next one at position 1, and so on.

NOTE
The charAt function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.5

String.prototype.charCodeAt (pos)Returns a Number (a nonnegative integer less than 216) representing the code unit value of the character at
position pos in the String resulting from converting this object to a String. If there is no character at that
position, the result is NaN.
When the charCodeAt method is called with one argument pos, the following steps are taken:
1.
2.
3.
4.
5.
6.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let position be ToInteger(pos).
Let size be the number of characters in S.
If position < 0 or position ≥ size, return NaN.
Return a value of Number type, whose value is the code unit value of the character at position position in theString S, where the first (leftmost) character in S is considered to be at position 0, the next one at position 1,
and so on.

NOTE
The charCodeAt function is intentionally generic; it does not require that its this value be a String object.
Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.6

String.prototype.concat ( [ string1 [ , string2 [ , … ] ] ] )When the concat method is called with zero or more arguments string1, string2, etc., it returns a String
consisting of the characters of this object (converted to a String) followed by the characters of each of string1,
string2, etc. (where each argument is converted to a String). The result is a String value, not a String object.
The following steps are taken:

142© Ecma International 2009

1.
2.
3.
4.
5.

6.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let args be an internal list that is a copy of the argument list passed to this function.
Let R be S.
Repeat, while args is not empty
a. Remove the first element from args and let next be the value of that element.
b. Let R be the String value consisting of the characters in the previous value of R followed by thecharacters of ToString(next).
Return R.

The length property of the concat method is 1.
NOTE
The concat function is intentionally generic; it does not require that its this value be a String object.
Therefore it can be transferred to other kinds of objects for use as a method.

15.5.4.7

String.prototype.indexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String, at one or more positions
that are greater than or equal to position, then the index of the smallest such position is returned; otherwise, 1 is returned. If position is undefined, 0 is assumed, so as to search all of the String.
The indexOf method takes two arguments, searchString and position, and performs the following steps:
1.
2.
3.
4.
5.
6.
7.
8.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let searchStr be ToString(searchString).
Let pos be ToInteger(position). (If position is undefined, this step produces the value 0).
Let len be the number of characters in S.
Let start be min(max(pos, 0), len).
Let searchLen be the number of characters in searchStr.
Return the smallest possible integer k not smaller than start such that k+ searchLen is not greater than len,and for all nonnegative integers j less than searchLen, the character at position k+j of S is the same as thecharacter at position j of searchStr); but if there is no such integer k, then return the value -1.

The length property of the indexOf method is 1.
NOTE
The indexOf function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.8

String.prototype.lastIndexOf (searchString, position)

If searchString appears as a substring of the result of converting this object to a String at one or more positions
that are smaller than or equal to position, then the index of the greatest such position is returned; otherwise, 1 is returned. If position is undefined, the length of the String value is assumed, so as to search all of theString.
The lastIndexOf method takes two arguments, searchString and position, and performs the following steps:
1.
2.
3.
4.
5.
6.
7.
8.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let searchStr be ToString(searchString).
Let numPos be ToNumber(position). (If position is undefined, this step produces the value NaN).
If numPos is NaN, let pos be +∞; otherwise, let pos be ToInteger(numPos).
Let len be the number of characters in S.
Let start min(max(pos, 0), len).
Let searchLen be the number of characters in searchStr.

© Ecma International 2009

143

9.

Return the largest possible nonnegative integer k not larger than start such that k+ searchLen is not greaterthan len, and for all nonnegative integers j less than searchLen, the character at position k+j of S is the sameas the character at position j of searchStr; but if there is no such integer k, then return the value -1.

The length property of the lastIndexOf method is 1.
NOTE
The lastIndexOf function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.9

String.prototype.localeCompare (that)

When the localeCompare method is called with one argument that, it returns a Number other than NaN that
represents the result of a locale-sensitive String comparison of the this value (converted to a String) with that(converted to a String). The two Strings are S and That. The two Strings are compared in an implementationdefined fashion. The result is intended to order String values in the sort order specified by the system default
locale, and will be negative, zero, or positive, depending on whether S comes before That in the sort order, theStrings are equal, or S comes after That in the sort order, respectively.
Before perform the comparisons the following steps are performed to prepare the Strings:
1.
2.
3.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let That be ToString(that).

The localeCompare method, if considered as a function of two arguments this and that, is a consistentcomparison function (as defined in 15.4.4.11) on the set of all Strings. Furthermore, localeCompare returns
0 or –0 when comparing two Strings that are considered canonically equivalent by the Unicode standard.
The actual return values are implementation-defined to permit implementers to encode additional informationin the value, but the function is required to define a total ordering on all Strings and to return 0 whencomparing Strings that are considered canonically equivalent by the Unicode standard.
If no language-sensitive comparison at all is available from the host environment, this function may perform a
bitwise comparison.
NOTE 1The localeCompare method itself is not directly suitable as an argument to Array.prototype.sortbecause the latter requires a function of two arguments.
NOTE 2This function is intended to rely on whatever language-sensitive comparison functionality is available to the
ECMAScript environment from the host environment, and to compare according to the rules of the host environment’s
current locale. It is strongly recommended that this function treat Strings that are canonically equivalent according to theUnicode standard as identical (in other words, compare the Strings as if they had both been converted to Normalised
Form C or D first). It is also recommended that this function not honour Unicode compatibility equivalences ordecompositions.
NOTE 3The second parameter to this function is likely to be used in a future version of this standard; it isrecommended that implementations do not use this parameter position for anything else.
NOTE 4The localeCompare function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.10 String.prototype.match (regexp)
When the match method is called with argument regexp, the following steps are taken:
1.
2.
3.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx
be regexp;

144© Ecma International 2009

4.
5.
6.
7.
8.

Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp
is the standard built-in constructor with that name.
Let global be the result of calling the [[Get]] internal method of rx with argument "global".
Let exec be the standard built-in function RegExp.prototype.exec (see 15.10.6.2)
If global is not true, thena. Return the result of calling the [[Call]] internal method of exec with rx as the this value andargument list containing S.
Else, global is true
a. Call the [[Put]] internal method of rx with arguments "lastIndex" and 0.
b. Let A be a new array created as if by the expression new Array() where Array is the standard
built-in constructor with that name.
c. Let previousLastIndex be 0.
d. Let n be 0.
e. Let lastMatch be true.
f. Repeat, while lastMatch is truei.
Let result be the result of calling the [[Call]] internal method of exec with rx as the thisvalue and argument list containing S.
ii.
If result is null, then set lastMatch to false.
iii.
Else, result is not null
1. Let thisIndex be the result of calling the [[Get]] internal method of rx withargument "lastIndex".
2. If thisIndex = previousLastIndex thena Call the [[Put]] internal method of rx with arguments "lastIndex" andthisIndex+1.
b Set previousLastIndex to thisIndex+1.
3. Else, set previousLastIndex to thisIndex.
4. Let matchStr be the result of calling the [[Get]] internal method of result withargument "0".
5. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(n),the Property Descriptor {[[Value]]: matchStr, [[Writable]]: true, [[Enumerable]]:true, [[configurable]]: true}, and false.
6. Increment n.
g. If n = 0, then return null.
h. Return A.

NOTE
The match function is intentionally generic; it does not require that its this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a method.

15.5.4.11 String.prototype.replace (searchValue, replaceValue)First set string according to the following steps:
1.
2.

Call CheckObjectCoercible passing the this value as its argument.
Let string be the result of calling ToString, giving it the this value as its argument.

If searchValue is a regular expression (an object whose [[Class]] internal property is "RegExp"), do thefollowing: If searchValue.global is false, then search string for the first match of the regular expression
searchValue. If searchValue.global is true, then search string for all matches of the regular expression
searchValue. Do the search in the same manner as in String.prototype.match, including the update ofsearchValue.lastIndex. Let m be the number of left capturing parentheses in searchValue (using
NcapturingParens as specified in 15.10.2.1).
If searchValue is not a regular expression, let searchString be ToString(searchValue) and search string for the firstoccurrence of searchString. Let m be 0.
If replaceValue is a function, then for each matched substring, call the function with the following m + 3
arguments. Argument 1 is the substring that matched. If searchValue is a regular expression, the next marguments are all of the captures in the MatchResult (see 15.10.2.1). Argument m + 2 is the offset within string
where the match occurred, and argument m + 3 is string. The result is a String value derived from the original© Ecma International 2009145

input by replacing each matched substring with the corresponding return value of the function call, converted
to a String if need be.
Otherwise, let newstring denote the result of converting replaceValue to a String. The result is a String value
derived from the original input String by replacing each matched substring with a String derived from newstring
by replacing characters in newstring by replacement text as specified in Table 22. These $ replacements aredone left-to-right, and, once such a replacement is performed, the new replacement text is not subject tofurther replacements. For example, "$1,$2".replace(/(\$(\d))/g, "$$1-$1$2") returns "$1$11,$1-$22". A $ in newstring that does not match any of the forms below is left as is.
Table 22 — Replacement Text Symbol Substitutions
Characters

Replacement text

$$
$&
$‘
$’
$n

$
The matched substring.
The portion of string that precedes the matched substring.
The portion of string that follows the matched substring.
The nth capture, where n is a single digit in the range 1 to 9 and $n is not followed
by a decimal digit. If n≤m and the nth capture is undefined, use the empty String
instead. If n>m, the result is implementation-defined.
The nnth capture, where nn is a two-digit decimal number in the range 01 to 99. Ifnn≤m and the nnth capture is undefined, use the empty String instead. If nn>m, theresult is implementation-defined.

$nn

NOTE
The replace function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.12 String.prototype.search (regexp)
When the search method is called with argument regexp, the following steps are taken:
1.
2.
3.
4.
5.

6.

Call CheckObjectCoercible passing the this value as its argument.
Let string be the result of calling ToString, giving it the this value as its argument.
If Type(regexp) is Object and the value of the [[Class]] internal property of regexp is "RegExp", then let rx
be regexp;
Else, let rx be a new RegExp object created as if by the expression new RegExp(regexp) where RegExp
is the standard built-in constructor with that name.
Search the value string from its beginning for an occurrence of the regular expression pattern rx. Let result
be a Number indicating the offset within string where the pattern matched, or –1 if there was no match. The
lastIndex and global properties of regexp are ignored when performing the search. The lastIndex
property of regexp is left unchanged.
Return result.

NOTE
The search function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.13 String.prototype.slice (start, end)
The slice method takes two arguments, start and end, and returns a substring of the result of converting thisobject to a String, starting from character position start and running to, but not including, character position end(or through the end of the String if end is undefined). If start is negative, it is treated as sourceLength+start
where sourceLength is the length of the String. If end is negative, it is treated as sourceLength+end where
sourceLength is the length of the String. The result is a String value, not a String object. The following steps aretaken:
1.
2.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.

146© Ecma International 2009

3.
4.
5.
6.
7.
8.
9.

Let len be the number of characters in S.
Let intStart be ToInteger(start).
If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
If intStart is negative, let from be max(len + intStart,0); else let from be min(intStart, len).
If intEnd is negative, let to be max(len + intEnd,0); else let to be min(intEnd, len).
Let span be max(to – from,0).
Return a String containing span consecutive characters from S beginning with the character at position from.

The length property of the slice method is 2.
NOTE
The slice function is intentionally generic; it does not require that its this value be a String object. Therefore
it can be transferred to other kinds of objects for use as a method.

15.5.4.14 String.prototype.split (separator, limit)
Returns an Array object into which substrings of the result of converting this object to a String have been
stored. The substrings are determined by searching from left to right for occurrences of separator; theseoccurrences are not part of any substring in the returned array, but serve to divide up the String value. The
value of separator may be a String of any length or it may be a RegExp object (i.e., an object whose [[Class]]
internal property is "RegExp"; see 15.10).
The value of separator may be an empty String, an empty regular expression, or a regular expression that canmatch an empty String. In this case, separator does not match the empty substring at the beginning or end of
the input String, nor does it match the empty substring at the end of the previous separator match. (Forexample, if separator is the empty String, the String is split up into individual characters; the length of the result
array equals the length of the String, and each substring contains one character.) If separator is a regularexpression, only the first match at a given position of the this String is considered, even if backtracking could
yield a non-empty-substring match at that position. (For example, "ab".split(/a*?/) evaluates to thearray ["a","b"], while "ab".split(/a*/) evaluates to the array["","b"].)If the this object is (or converts to) the empty String, the result depends on whether separator can match theempty String. If it can, the result array contains no elements. Otherwise, the result array contains one element,
which is the empty String.
If separator is a regular expression that contains capturing parentheses, then each time separator is matched
the results (including any undefined results) of the capturing parentheses are spliced into the output array.
For example,"A<B>bold</B>and<CODE>coded</CODE>".split(/<(\/)?([^<>]+)>/)
evaluates to the array["A", undefined, "B", "bold", "/", "B", "and", undefined,"CODE", "coded", "/", "CODE", ""]If separator is undefined, then the result array contains just one String, which is the this value (converted to aString). If limit is not undefined, then the output array is truncated so that it contains no more than limit
elements.
When the split method is called, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let A be a new array created as if by the expression new Array()where Array is the standard built-in
constructor with that name.
Let lengthA be 0.
If limit is undefined, let lim = 2 32–1; else let lim = ToUint32(limit).
Let s be the number of characters in S.
Let p = 0.
If separator is a RegExp object (its [[Class]] is "RegExp"), let R = separator; otherwise let R =ToString(separator).

© Ecma International 2009147

9. If lim = 0, return A.
10. If separator is undefined, thena. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor{[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
b. Return A.
11. If s = 0, thena. Call SplitMatch(R, S, 0) and let z be its MatchResult result.
b. If z is not failure, return A.
c. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor{[[Value]]: S, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
d. Return A.
12. Let q = p.
13. Repeat, while q ≠ sa. Call SplitMatch(R, S, q) and let z be its MatchResult result.
b. If z is failure, then let q = q+1.
c. Else, z is not failurei.
z must be a State. Let e be z's endIndex and let cap be z's captures array.
ii.
If e = p, then let q = q+1.
iii.
Else, e ≠ p
1. Let T be a String value equal to the substring of S consisting of the characters at
positions p (inclusive) through q (exclusive).
2. Call the [[DefineOwnProperty]] internal method of A with arguments
ToString(lengthA), Property Descriptor {[[Value]]: T, [[Writable]]: true,[[Enumerable]]: true, [[Configurable]]: true}, and false.
3. Increment lengthA by 1.
4. If lengthA = lim, return A.
5. Let p = e.
6. Let i = 0.
7. Repeat, while i is not equal to the number of elements in cap.
a Let i = i+1.
b Call the [[DefineOwnProperty]] internal method of A with arguments
ToString(lengthA), Property Descriptor {[[Value]]: cap[i], [[Writable]]:true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
c Increment lengthA by 1.
d If A.length = lim, return A.
8. Let q = p.
14. Let T be a String value equal to the substring of S consisting of the characters at positions p (inclusive)
through s (exclusive).
15. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(lengthA), Property Descriptor{[[Value]]: T, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
16. Return A.
The abstract operation SplitMatch takes three parameters, a String S, an integer q, and a String or RegExp R,
and performs the following in order to return a MatchResult (see 15.10.2.1):
1.
2.
3.
4.
5.
6.
7.

If R is a RegExp object (its [[Class]] is "RegExp"), thena. Call the [[Match]] internal method of R giving it the arguments S and q, and return the MatchResult
result.
Type(R) must be String. Let r be the number of characters in R.
Let s be the number of characters in S.
If q+r > s then return the MatchResult failure.
If there exists an integer i between 0 (inclusive) and r (exclusive) such that the character at position q+i of S
is different from the character at position i of R, then return failure.
Let cap be an empty array of captures (see 15.10.2.1).
Return the State (q+r, cap). (see 15.10.2.1)

The length property of the split method is 2.
NOTE 1148

The split method ignores the value of separator.global for separators that are RegExp objects.

© Ecma International 2009

NOTE 2The split function is intentionally generic; it does not require that its this value be a String object. Therefore,
it can be transferred to other kinds of objects for use as a method.

15.5.4.15 String.prototype.substring (start, end)
The substring method takes two arguments, start and end, and returns a substring of the result of converting
this object to a String, starting from character position start and running to, but not including, character positionend of the String (or through the end of the String is end is undefined). The result is a String value, not a String
object.
If either argument is NaN or negative, it is replaced with zero; if either argument is larger than the length of theString, it is replaced with the length of the String.
If start is larger than end, they are swapped.
The following steps are taken:
1. Call CheckObjectCoercible passing the this value as its argument.
2. Let S be the result of calling ToString, giving it the this value as its argument.
3. Let len be the number of characters in S.
4. Let intStart be ToInteger(start).
5. If end is undefined, let intEnd be len; else let intEnd be ToInteger(end).
6. Let finalStart be min(max(intStart, 0), len).
7. Let finalEnd be min(max(intEnd, 0), len).
8. Let from be min(finalStart, finalEnd).
9. Let to be max(finalStart, finalEnd).
10. Return a String whose length is to - from, containing characters from S, namely the characters with indicesfrom through to −1, in ascending order.
The length property of the substring method is 2.
NOTE
The substring function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.16 String.prototype.toLowerCase ( )The following steps are taken:
1.
2.
3.
4.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let L be a String where each character of L is either the Unicode lowercase equivalent of the corresponding
character of S or the actual corresponding character of S if no Unicode lowercase equivalent exists.
Return L.

For the purposes of this operation, the 16-bit code units of the Strings are treated as code points in the
Unicode Basic Multilingual Plane. Surrogate code points are directly transferred from S to L without any
mapping.
The result must be derived according to the case mappings in the Unicode character database (this explicitly
includes not only the UnicodeData.txt file, but also the SpecialCasings.txt file that accompanies it in Unicode
2.1.8 and later).
NOTE 1
The case mapping of some characters may produce multiple characters. In this case the result String may not
be the same length as the source String. Because both toUpperCase and toLowerCase have context-sensitive
behaviour, the functions are not symmetrical. In other words, s.toUpperCase().toLowerCase() is not necessarilyequal to s.toLowerCase().
NOTE 2The toLowerCase function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

© Ecma International 2009149

15.5.4.17 String.prototype.toLocaleLowerCase ( )This function works exactly the same as toLowerCase except that its result is intended to yield the correct
result for the host environment’s current locale, rather than a locale-independent result. There will only be adifference in the few cases (such as Turkish) where the rules for that language conflict with the regularUnicode case mappings.
NOTE 1
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.
NOTE 2The toLocaleLowerCase function is intentionally generic; it does not require that its this value be a String
object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.18 String.prototype.toUpperCase ( )This function behaves in exactly the same way as String.prototype.toLowerCase, except thatcharacters are mapped to their uppercase equivalents as specified in the Unicode Character Database.
NOTE
The toUpperCase function is intentionally generic; it does not require that its this value be a String object.
Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.19 String.prototype.toLocaleUpperCase ( )This function works exactly the same as toUpperCase except that its result is intended to yield the correct
result for the host environment’s current locale, rather than a locale-independent result. There will only be adifference in the few cases (such as Turkish) where the rules for that language conflict with the regularUnicode case mappings.
NOTE 1
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.
NOTE 2The toLocaleUpperCase function is intentionally generic; it does not require that its this value be a String
object. Therefore, it can be transferred to other kinds of objects for use as a method.

15.5.4.20 String.prototype.trim ( )The following steps are taken:
1.
2.
3.
4.

Call CheckObjectCoercible passing the this value as its argument.
Let S be the result of calling ToString, giving it the this value as its argument.
Let T be a String value that is a copy of S with both leading and trailing white space removed. The definitionof white space is the union of WhiteSpace and LineTerminator.
Return T.

NOTE
The trim function is intentionally generic; it does not require that its this value be a String object. Therefore, it
can be transferred to other kinds of objects for use as a method.

15.5.5 Properties of String Instances
String instances inherit properties from the String prototype object and their [[Class]] internal property value is"String". String instances also have a [[PrimitiveValue]] internal property, a length property, and a set of
enumerable properties with array index names.
The [[PrimitiveValue]] internal property is the String value represented by this String object. The array index
named properties correspond to the individual characters of the String value. A special [[GetOwnProperty]]internal method is used to specify the number, values, and attributes of the array index named properties.

150© Ecma International 2009

15.5.5.1

lengthThe number of characters in the String value represented by this String object.
Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]: false,[[Enumerable]]: false, [[Configurable]]: false }.
15.5.5.2[[GetOwnProperty]] ( P )String objects use a variation of the [[GetOwnProperty]] internal method used for other native ECMAScript
objects (8.12.1). This special internal method is used to specify the array index named properties of String
objects.
Assume S is a String object and P is a String.
When the [[GetOwnProperty]] internal method of S is called with property name P, the following steps aretaken:
1.
2.
3.
4.
5.
6.
7.
8.
9.

Let desc be the result of calling the default [[GetOwnProperty]] internal method (8.12.1) on S with argumentP.
If desc is not undefined return desc.
If P is not an array index (15.4), return undefined.
Let str be the String value of the [[PrimitiveValue]] internal property of S.
Let index be ToUint32(P).
Let len be the number of characters in str.
If len ≤ index, return undefined.
Let resultStr be a String of length 1, containing one character from str, specifically the character at positionindex, where the first (leftmost) character in str is considered to be at position 0, the next one at position 1,
and so on.
Return a Property Descriptor { [[Value]]: resultStr, [[Enumerable]]: true, [[Writable]]: false,[[Configurable]]: false }

15.6 Boolean Objects
15.6.1 The Boolean Constructor Called as a Function
When Boolean is called as a function rather than as a constructor, it performs a type conversion.
15.6.1.1

Boolean (value)

Returns a Boolean value (not a Boolean object) computed by ToBoolean(value).
15.6.2 The Boolean Constructor
When Boolean is called as part of a new expression it is a constructor: it initialises the newly created object.
15.6.2.1

new Boolean (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Boolean prototype
object, the one that is the initial value of Boolean.prototype (15.6.3.1).
The [[Class]] internal property of the newly constructed Boolean object is set to "Boolean".
The [[PrimitiveValue]] internal property of the newly constructed Boolean object is set to ToBoolean(value).
The [[Extensible]] internal property of the newly constructed object is set to true.

© Ecma International 2009151

15.6.3 Properties of the Boolean Constructor
The value of the [[Prototype]] internal property of the Boolean constructor is the Function prototype object(15.3.4).
Besides the internal properties and the length property (whose value is 1), the Boolean constructor has thefollowing property:
15.6.3.1

Boolean.prototype

The initial value of Boolean.prototype is the Boolean prototype object (15.6.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.6.4 Properties of the Boolean Prototype Object
The Boolean prototype object is itself a Boolean object (its [[Class]] is "Boolean") whose value is false.
The value of the [[Prototype]] internal property of the Boolean prototype object is the standard built-in Object
prototype object (15.2.4).
15.6.4.1

Boolean.prototype.constructor

The initial value of Boolean.prototype.constructor is the built-in Boolean constructor.
15.6.4.2

Boolean.prototype.toString ( )The following steps are taken:
1.
2.
3.
4.
5.

Let B be the this value.
If Type(B) is Boolean, then let b be B.
Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be
the value of the [[PrimitiveValue]] internal property of B.
Else throw a TypeError exception.
If b is true, then return "true"; else return "false".

15.6.4.3

Boolean.prototype.valueOf ( )The following steps are taken:
1.
2.
3.
4.
5.

Let B be the this value.
If Type(B) is Boolean, then let b be B.
Else if Type(B) is Object and the value of the [[Class]] internal property of B is "Boolean", then let b be
the value of the [[PrimitiveValue]] internal property of B.
Else throw a TypeError exception.
Return b.

15.6.5 Properties of Boolean Instances
Boolean instances inherit properties from the Boolean prototype object and their [[Class]] internal property
value is "Boolean". Boolean instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is the Boolean value represented by this Boolean object.

152© Ecma International 2009

15.7 Number Objects
15.7.1 The Number Constructor Called as a Function
When Number is called as a function rather than as a constructor, it performs a type conversion.
15.7.1.1

Number ( [ value ] )Returns a Number value (not a Number object) computed by ToNumber(value) if value was supplied, else
returns +0.
15.7.2 The Number Constructor
When Number is called as part of a new expression it is a constructor: it initialises the newly created object.
15.7.2.1

new Number ( [ value ] )The [[Prototype]] internal property of the newly constructed object is set to the original Number prototype
object, the one that is the initial value of Number.prototype (15.7.3.1).
The [[Class]] internal property of the newly constructed object is set to "Number".
The [[PrimitiveValue]] internal property of the newly constructed object is set to ToNumber(value) if value was
supplied, else to +0.
The [[Extensible]] internal property of the newly constructed object is set to true.
15.7.3 Properties of the Number Constructor
The value of the [[Prototype]] internal property of the Number constructor is the Function prototype object(15.3.4).
Besides the internal properties and the length property (whose value is 1), the Number constructor has thefollowing property:
15.7.3.1

Number.prototype

The initial value of Number.prototype is the Number prototype object (15.7.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.7.3.2

Number.MAX_VALUE

The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which isapproximately 1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.7.3.3

Number.MIN_VALUE

The value of Number.MIN_VALUE is the smallest positive value of the Number type, which is approximately
5 × 10-324.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

© Ecma International 2009153

15.7.3.4

Number.NaN

The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.7.3.5

Number.NEGATIVE_INFINITY

The value of Number.NEGATIVE_INFINITY is −∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.7.3.6

Number.POSITIVE_INFINITY

The value of Number.POSITIVE_INFINITY is +∞.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.7.4 Properties of the Number Prototype Object
The Number prototype object is itself a Number object (its [[Class]] is "Number") whose value is +0.
The value of the [[Prototype]] internal property of the Number prototype object is the standard built-in Object
prototype object (15.2.4).
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not genericand the this value passed to them must be either a Number value or an Object for which the value of the[[Class]] internal property is "Number".
In the following descriptions of functions that are properties of the Number prototype object, the phrase “thisNumber object” refers to either the object that is the this value for the invocation of the function or, if
Type(this value) is Number, an object that is created as if by the expression new Number(this value)
where Number is the standard built-in constructor with that name. Also, the phrase “this Number value” refers
to either the Number value represented by this Number object, that is, the value of the [[PrimitiveValue]]internal property of this Number object or the this value if its type is Number. A TypeError exception isthrown if the this value is neither an object for which the value of the [[Class]] internal property is "Number"or a value whose type is Number.
15.7.4.1

Number.prototype.constructor

The initial value of Number.prototype.constructor is the built-in Number constructor.
15.7.4.2

Number.prototype.toString ( [ radix ] )The optional radix should be an integer value in the inclusive range 2 to 36. If radix not present or is undefined
the Number 10 is used as the value of radix. If ToInteger(radix) is the Number 10 then this Number value isgiven as an argument to the ToString abstract operation; the resulting String value is returned.
If ToInteger(radix) is not an integer between 2 and 36 inclusive throw a RangeError exception. If
ToInteger(radix) is an integer from 2 to 36, but not 10, the result is a String representation of this Number value
using the specified radix. Letters a-z are used for digits with values 10 through 35. The precise algorithm isimplementation-dependent if the radix is not 10, however the algorithm should be a generalization of thatspecified in 9.8.1.
The toString function is not generic; it throws a TypeError exception if its this value is not a Number or a
Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.

154© Ecma International 2009

15.7.4.3

Number.prototype.toLocaleString()

Produces a String value that represents this Number value formatted according to the conventions of the hostenvironment’s current locale. This function is implementation-dependent, and it is permissible, but notencouraged, for it to return the same thing as toString.
NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.

15.7.4.4

Number.prototype.valueOf ( )Returns this Number value.
The valueOf function is not generic; it throws a TypeError exception if its this value is not a Number or a
Number object. Therefore, it cannot be transferred to other kinds of objects for use as a method.
15.7.4.5

Number.prototype.toFixed (fractionDigits)

Return a String containing this Number value represented in decimal fixed-point notation with fractionDigits
digits after the decimal point. If fractionDigits is undefined, 0 is assumed. Specifically, perform the following
steps:
1.
2.
3.
4.
5.
6.
7.
8.

9.

Let f be ToInteger(fractionDigits). (If fractionDigits is undefined, this step produces the value 0).
If f < 0 or f > 20, throw a RangeError exception.
Let x be this Number value.
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, thena. Let s be "-".
b. Let x = –x.
If x ≥ 10 21, thena. Let m = ToString(x).
Else, x < 10 21
a. Let n be an integer for which the exact mathematical value of n ÷ 10f – x is as close to zero as
possible. If there are two such n, pick the larger n.
b. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of thedecimal representation of n (in order, with no leading zeroes).
c. If f ≠ 0, theni.
Let k be the number of characters in m.
ii.
If k ≤ f, then
1. Let z be the String consisting of f+1–k occurrences of the character ‘0’.
2. Let m be the concatenation of Strings z and m.
3. Let k = f + 1.
iii.
Let a be the first k–f characters of m, and let b be the remaining f characters of m.
iv.
Let m be the concatenation of the three Strings a, ".", and b.
Return the concatenation of the Strings s and m.

The length property of the toFixed method is 1.
If the toFixed method is called with more than one argument, then the behaviour is undefined (seeclause 15).
An implementation is permitted to extend the behaviour of toFixed for values of fractionDigits less than 0 or
greater than 20. In this case toFixed would not necessarily throw RangeError for such values.
NOTE
The output of toFixed may be more precise than toString for some values because toString only prints
enough significant digits to distinguish the number from adjacent number values. For example,© Ecma International 2009155

(1000000000000000128).toString() returns "1000000000000000100",
while (1000000000000000128).toFixed(0) returns "1000000000000000128".

15.7.4.6

Number.prototype.toExponential (fractionDigits)

Return a String containing this Number value represented in decmal exponential notation with one digit before
the significand's decimal point and fractionDigits digits after the significand's decimal point. If fractionDigits isundefined, include as many significand digits as necessary to uniquely specify the Number (just like in
ToString except that in this case the Number is always output in exponential notation). Specifically, performthe following steps:
1.
2.
3.
4.
5.

Let x be this Number value.
Let f be ToInteger(fractionDigits).
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, thena. Let s be "-".
b. Let x = –x.
6. If x = +∞, thena. Return the concatenation of the Strings s and "Infinity".
7. If fractionDigits is not undefined and (f < 0 or f > 20), throw a RangeError exception.
8. If x = 0, thena. Let f = 0.
b. Let m be the String consisting of f+1 occurrences of the character ‘0’.
c. Let e = 0.
9. Else, x ≠ 0
a. If fractionDigits is not undefined, theni.
Let e and n be integers such that 10 f ≤ n < 10 f+1 and for which the exact mathematical valueof n × 10 e–f – x is as close to zero as possible. If there are two such sets of e and n, pick thee and n for which n × 10 e–f is larger.
b. Else, fractionDigits is undefinedi.
Let e, n, and f be integers such that f ≥ 0, 10 f ≤ n < 10 f+1, the number value for n × 10 e–f is x,
and f is as small as possible. Note that the decimal representation of n has f+1 digits, n is
not divisible by 10, and the least significant digit of n is not necessarily uniquelydetermined by these criteria.
c. Let m be the String consisting of the digits of the decimal representation of n (in order, with noleading zeroes).
10. If f ≠ 0, thena. Let a be the first character of m, and let b be the remaining f characters of m.
b. Let m be the concatenation of the three Strings a, ".", and b.
11. If e = 0, thena. Let c = "+".
b. Let d = "0".
12. Else
a. If e > 0, then let c = "+".
b. Else, e ≤ 0i.
Let c = "-".
ii.
Let e = –e.
c. Let d be the String consisting of the digits of the decimal representation of e (in order, with noleading zeroes).
13. Let m be the concatenation of the four Strings m, "e", c, and d.
14. Return the concatenation of the Strings s and m.
The length property of the toExponential method is 1.
If the toExponential method is called with more than one argument, then the behaviour is undefined (seeclause 15).

156

© Ecma International 2009

An implementation is permitted to extend the behaviour of toExponential for values of fractionDigits lessthan 0 or greater than 20. In this case toExponential would not necessarily throw RangeError for suchvalues.
NOTE
For implementations that provide more accurate conversions than required by the rules above, it isrecommended that the following alternative version of step 9.b.i be used as a guideline:i.

15.7.4.7

Let e, n, and f be integers such that f ≥ 0, 10f ≤ n < 10f+1, the number value for n × 10e–f is x, and f is as small
as possible. If there are multiple possibilities for n, choose the value of n for which n × 10e–f is closest in value
to x. If there are two such possible values of n, choose the one that is even.

Number.prototype.toPrecision (precision)

Return a String containing this Number value represented either in decimal exponential notation with one digit
before the significand's decimal point and precision–1 digits after the significand's decimal point or in decimal
fixed notation with precision significant digits. If precision is undefined, call ToString (9.8.1) instead.
Specifically, perform the following steps:
1.
2.
3.
4.
5.
6.

Let x be this Number value.
If precision is undefined, return ToString(x).
Let p be ToInteger(precision).
If x is NaN, return the String "NaN".
Let s be the empty String.
If x < 0, thena. Let s be "-".
b. Let x = –x.
7. If x = +∞, thena. Return the concatenation of the Strings s and "Infinity".
8. If p < 1 or p > 21, throw a RangeError exception.
9. If x = 0, thena. Let m be the String consisting of p occurrences of the character ‘0’.
b. Let e = 0.
10. Else x ≠ 0,
a. Let e and n be integers such that 10 p–1 ≤ n < 10 p and for which the exact mathematical value of n ×
10 e–p+1 – x is as close to zero as possible. If there are two such sets of e and n, pick the e and n forwhich n × 10 e–p+1 is larger.
b. Let m be the String consisting of the digits of the decimal representation of n (in order, with noleading zeroes).
c. If e < –6 or e ≥ p, theni.
Let a be the first character of m, and let b be the remaining p–1 characters of m.
ii.
Let m be the concatenation of the three Strings a, ".", and b.
iii.
If e = 0, then
1. Let c = "+" and d = "0".
iv.
Else e ≠ 0,
1. If e > 0, thena Let c = "+".
2. Else e < 0,
a Let c = "-".
b Let e = –e.
3. Let d be the String consisting of the digits of the decimal representation of e (inorder, with no leading zeroes).
v.
Let m be the concatenation of the five Strings s, m, "e", c, and d.
11. If e = p–1, then return the concatenation of the Strings s and m.
12. If e ≥ 0, thena. Let m be the concatenation of the first e+1 characters of m, the character ‘.’, and the remaining p–(e+1) characters of m.
13. Else e < 0,a. Let m be the concatenation of the String "0.", –(e+1) occurrences of the character ‘0’, and theString m.

© Ecma International 2009157

14. Return the concatenation of the Strings s and m.
The length property of the toPrecision method is 1.
If the toPrecision method is called with more than one argument, then the behaviour is undefined (seeclause 15).
An implementation is permitted to extend the behaviour of toPrecision for values of precision less than 1 orgreater than 21. In this case toPrecision would not necessarily throw RangeError for such values.
15.7.5 Properties of Number Instances
Number instances inherit properties from the Number prototype object and their [[Class]] internal property
value is "Number". Number instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is the Number value represented by this Number object.

15.8 The Math Object
The Math object is a single object that has some named properties, some of which are functions.
The value of the [[Prototype]] internal property of the Math object is the standard built-in Object prototype
object (15.2.4). The value of the [[Class]] internal property of the Math object is "Math".
The Math object does not have a [[Construct]] internal property; it is not possible to use the Math object as aconstructor with the new operator.
The Math object does not have a [[Call]] internal property; it is not possible to invoke the Math object as afunction.
NOTE

In this specification, the phrase “the Number value for x” has a technical meaning defined in 8.5.

15.8.1 Value Properties of the Math Object
15.8.1.1

E

The Number value for e, the base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.8.1.2

LN10

The Number value for the natural logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.8.1.3

LN2The Number value for the natural logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.8.1.4

LOG2E

The Number value for the base-2 logarithm of e, the base of the natural logarithms; this value is approximately
1.4426950408889634.

158© Ecma International 2009

This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE

The value of Math.LOG2E is approximately the reciprocal of the value of Math.LN2.

15.8.1.5

LOG10EThe Number value for the base-10 logarithm of e, the base of the natural logarithms; this value isapproximately 0.4342944819032518.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE

The value of Math.LOG10E is approximately the reciprocal of the value of Math.LN10.

15.8.1.6

PI

The Number value for π, the ratio of the circumference of a circle to its diameter, which is approximately
3.1415926535897932.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.8.1.7

SQRT1_2

The Number value for the square root of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
NOTE

The value of Math.SQRT1_2 is approximately the reciprocal of the value of Math.SQRT2.

15.8.1.8

SQRT2The Number value for the square root of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.8.2 Function Properties of the Math Object
Each of the following Math object functions applies the ToNumber abstract operator to each of its arguments(in left-to-right order if there is more than one) and then performs a computation on the resulting Number
value(s).
In the function descriptions below, the symbols NaN, −0, +0, −∞ and +∞ refer to the Number values describedin 8.5.
NOTE
The behaviour of the functions acos, asin, atan, atan2, cos, exp, log, pow, sin, and sqrt is notprecisely specified here except to require specific results for certain argument values that represent boundary cases ofinterest. For other argument values, these functions are intended to compute approximations to the results of familiar
mathematical functions, but some latitude is allowed in the choice of approximation algorithms. The general intent is thatan implementer should be able to use the same mathematical library for ECMAScript on a given hardware platform that isavailable to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified by this standard) that
implementations use the approximation algorithms for IEEE 754 arithmetic contained in fdlibm, the freely distributable
mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).

15.8.2.1

abs (x)Returns the absolute value of x; the result has the same magnitude as x but has positive sign.

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•
•
•
15.8.2.2

If x is NaN, the result is NaN.
If x is −0, the result is +0.
If x is −∞, the result is +∞.
acos (x)Returns an implementation-dependent approximation to the arc cosine of x. The result is expressed in radians
and ranges from +0 to +π.
••••
15.8.2.3

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than −1, the result is NaN.
If x is exactly 1, the result is +0.
asin (x)Returns an implementation-dependent approximation to the arc sine of x. The result is expressed in radians
and ranges from −π/2 to +π/2.
•
•
•
•
•
15.8.2.4

If x is NaN, the result is NaN.
If x is greater than 1, the result is NaN.
If x is less than –1, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
atan (x)Returns an implementation-dependent approximation to the arc tangent of x. The result is expressed inradians and ranges from −π/2 to +π/2.
•
•
•
•
•
15.8.2.5

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is an implementation-dependent approximation to +π/2.
If x is −∞, the result is an implementation-dependent approximation to −π/2.
atan2 (y, x)

Returns an implementation-dependent approximation to the arc tangent of the quotient y/x of the arguments yand x, where the signs of y and x are used to determine the quadrant of the result. Note that it is intentional
and traditional for the two-argument arc tangent function that the argument named y be first and the argument
named x be second. The result is expressed in radians and ranges from −π to +π.
•
•
•
•
•
•
•
•
•
•
•
•
•

160

If either x or y is NaN, the result is NaN.
If y>0 and x is +0, the result is an implementation-dependent approximation to +π/2.
If y>0 and x is −0, the result is an implementation-dependent approximation to +π/2.
If y is +0 and x>0, the result is +0.
If y is +0 and x is +0, the result is +0.
If y is +0 and x is −0, the result is an implementation-dependent approximation to +π.
If y is +0 and x<0, the result is an implementation-dependent approximation to +π.
If y is −0 and x>0, the result is −0.
If y is −0 and x is +0, the result is −0.
If y is −0 and x is −0, the result is an implementation-dependent approximation to −π.
If y is −0 and x<0, the result is an implementation-dependent approximation to −π.
If y<0 and x is +0, the result is an implementation-dependent approximation to −π/2.
If y<0 and x is −0, the result is an implementation-dependent approximation to −π/2.

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•
•
•
•
•
•
•
•
•
15.8.2.6

If y>0 and y is finite and x is +∞, the result is +0.
If y>0 and y is finite and x is −∞, the result if an implementation-dependent approximation to +π.
If y<0 and y is finite and x is +∞, the result is −0.
If y<0 and y is finite and x is −∞, the result is an implementation-dependent approximation to −π.
If y is +∞ and x is finite, the result is an implementation-dependent approximation to +π/2.
If y is −∞ and x is finite, the result is an implementation-dependent approximation to −π/2.
If y is +∞ and x is +∞, the result is an implementation-dependent approximation to +π/4.
If y is +∞ and x is −∞, the result is an implementation-dependent approximation to +3π/4.
If y is −∞ and x is +∞, the result is an implementation-dependent approximation to −π/4.
If y is −∞ and x is −∞, the result is an implementation-dependent approximation to −3π/4.
ceil (x)Returns the smallest (closest to −∞) Number value that is not less than x and is equal to a mathematical
integer. If x is already an integer, the result is x.
•
•
•
•
•
•

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.
If x is less than 0 but greater than -1, the result is −0.

The value of Math.ceil(x) is the same as the value of -Math.floor(-x).
15.8.2.7

cos (x)Returns an implementation-dependent approximation to the cosine of x. The argument is expressed in radians.
•
•
•
•
•
15.8.2.8

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is −0, the result is 1.
If x is +∞, the result is NaN.
If x is −∞, the result is NaN.
exp (x)Returns an implementation-dependent approximation to the exponential function of x (e raised to the power ofx, where e is the base of the natural logarithms).
•
•
•
•
•
15.8.2.9

If x is NaN, the result is NaN.
If x is +0, the result is 1.
If x is −0, the result is 1.
If x is +∞, the result is +∞.
If x is −∞, the result is +0.
floor (x)Returns the greatest (closest to +∞) Number value that is not greater than x and is equal to a mathematical
integer. If x is already an integer, the result is x.
•
•
•
•
•

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.

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•
NOTE

If x is greater than 0 but less than 1, the result is +0.
The value of Math.floor(x) is the same as the value of -Math.ceil(-x).

15.8.2.10 log (x)
•
•
•
•
•
•

Returns an implementation-dependent approximation to the natural logarithm of x.
If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0 or −0, the result is −∞.
If x is 1, the result is +0.
If x is +∞, the result is +∞.

15.8.2.11 max ( [ value1 [ , value2 [ , … ] ] ] )Given zero or more arguments, calls ToNumber on each of the arguments and returns the largest of theresulting values.
•••

If no arguments are given, the result is −∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the largest value is done as in 11.8.5 except that +0 is considered tobe larger than −0.

The length property of the max method is 2.
15.8.2.12 min ( [ value1 [ , value2 [ , … ] ] ] )Given zero or more arguments, calls ToNumber on each of the arguments and returns the smallest of theresulting values.
•••

If no arguments are given, the result is +∞.
If any value is NaN, the result is NaN.
The comparison of values to determine the smallest value is done as in 11.8.5 except that +0 is considered
to be larger than −0.

The length property of the min method is 2.
15.8.2.13 pow (x, y)
Returns an implementation-dependent approximation to the result of raising x to the power y.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•

162

If y is NaN, the result is NaN.
If y is +0, the result is 1, even if x is NaN.
If y is −0, the result is 1, even if x is NaN.
If x is NaN and y is nonzero, the result is NaN.
If abs(x)>1 and y is +∞, the result is +∞.
If abs(x)>1 and y is −∞, the result is +0.
If abs(x)==1 and y is +∞, the result is NaN.
If abs(x)==1 and y is −∞, the result is NaN.
If abs(x)<1 and y is +∞, the result is +0.
If abs(x)<1 and y is −∞, the result is +∞.
If x is +∞ and y>0, the result is +∞.
If x is +∞ and y<0, the result is +0.
If x is −∞ and y>0 and y is an odd integer, the result is −∞.
If x is −∞ and y>0 and y is not an odd integer, the result is +∞.
If x is −∞ and y<0 and y is an odd integer, the result is −0.

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•
•
•
•
•
•
•

If x is −∞ and y<0 and y is not an odd integer, the result is +0.
If x is +0 and y>0, the result is +0.
If x is +0 and y<0, the result is +∞.
If x is −0 and y>0 and y is an odd integer, the result is −0.
If x is −0 and y>0 and y is not an odd integer, the result is +0.
If x is −0 and y<0 and y is an odd integer, the result is −∞.
If x is −0 and y<0 and y is not an odd integer, the result is +∞.
If x<0 and x is finite and y is finite and y is not an integer, the result is NaN.

15.8.2.14 random ( )Returns a Number value with positive sign, greater than or equal to 0 but less than 1, chosen randomly or
pseudo randomly with approximately uniform distribution over that range, using an implementation-dependentalgorithm or strategy. This function takes no arguments.
15.8.2.15 round (x)Returns the Number value that is closest to x and is equal to a mathematical integer. If two integer Number
values are equally close to x, then the result is the Number value that is closer to +∞. If x is already an integer,
the result is x.
•
•
•
•
•
•
•
NOTE 1

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.
If x is −∞, the result is −∞.
If x is greater than 0 but less than 0.5, the result is +0.
If x is less than 0 but greater than or equal to -0.5, the result is −0.
Math.round(3.5) returns 4, but Math.round(–3.5) returns –3.

NOTE 2The value of Math.round(x) is the same as the value of Math.floor(x+0.5), except when x is −0 or isless than 0 but greater than or equal to -0.5; for these cases Math.round(x) returns −0, but Math.floor(x+0.5)
returns +0.

15.8.2.16 sin (x)Returns an implementation-dependent approximation to the sine of x. The argument is expressed in radians.
••••

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞ or −∞, the result is NaN.

15.8.2.17 sqrt (x)Returns an implementation-dependent approximation to the square root of x.
•
•
•
•
•

If x is NaN, the result is NaN.
If x is less than 0, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞, the result is +∞.

© Ecma International 2009163

15.8.2.18 tan (x)Returns an implementation-dependent approximation to the tangent of x. The argument is expressed inradians.
••••

If x is NaN, the result is NaN.
If x is +0, the result is +0.
If x is −0, the result is −0.
If x is +∞ or −∞, the result is NaN.

15.9 Date Objects
15.9.1 Overview of Date Objects and Definitions of Abstract Operators
The following functions are abstract operations that operate on time values (defined in 15.9.1.1). Note that, inevery case, if any argument to one of these functions is NaN, the result will be NaN.
15.9.1.1

Time Values and Time Range

A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number
is called a time value. A time value may also be NaN, indicating that the Date object does not represent a
specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds
are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values
can represent all integers from –9,007,199,254,740,991 to 9,007,199,254,740,991; this range suffices tomeasure times to millisecond precision for any instant that is within approximately 285,616 years, either
forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000
days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This givesa range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0.
15.9.1.2

Day Number and Time within Day

A given time value t belongs to day number
Day(t) = floor(t / msPerDay)
where the number of milliseconds per day is
msPerDay = 86400000
The remainder is called the time within the day:
TimeWithinDay(t) = t modulo msPerDay
15.9.1.3

Year Number

ECMAScript uses an extrapolated Gregorian system to map a day number to a year number and to determine
the month and date within that year. In this system, leap years are precisely those which are (divisible by 4)and ((not divisible by 100) or (divisible by 400)). The number of days in year number y is therefore defined by
DaysInYear(y)164= 365 if (y modulo 4) ≠ 0= 366 if (y modulo 4) = 0 and (y modulo 100) ≠ 0= 365 if (y modulo 100) = 0 and (y modulo 400) ≠ 0= 366 if (y modulo 400) = 0© Ecma International 2009

All non-leap years have 365 days with the usual number of days per month and leap years have an extra day
in February. The day number of the first day of year y is given by:
DayFromYear(y) = 365 × (y−1970) + floor((y−1969)/4) − floor((y−1901)/100) + floor((y−1601)/400)
The time value of the start of a year is:
TimeFromYear(y) = msPerDay × DayFromYear(y)A time value determines a year by:
YearFromTime(t) = the largest integer y (closest to positive infinity) such that TimeFromYear(y) ≤ t
The leap-year function is 1 for a time within a leap year and otherwise is zero:
InLeapYear(t)15.9.1.4= 0 if DaysInYear(YearFromTime(t)) = 365= 1 if DaysInYear(YearFromTime(t)) = 366

Month Number

Months are identified by an integer in the range 0 to 11, inclusive. The mapping MonthFromTime(t) from a timevalue t to a month number is defined by:MonthFromTime(t) = 0
=1
=2
=3
=4
=5
=6
=7
=8
=9
= 10
= 11

if
if
if
if
if
if
if
if
if
if
if
if

0
31
59+InLeapYear(t)90+InLeapYear(t)120+InLeapYear(t)151+InLeapYear(t)181+InLeapYear(t)212+InLeapYear(t)243+InLeapYear(t)273+InLeapYear(t)304+InLeapYear(t)334+InLeapYear(t)≤ DayWithinYear(t) < 31
≤ DayWithinYear (t) < 59+InLeapYear(t)≤ DayWithinYear (t) < 90+InLeapYear(t)≤ DayWithinYear (t) < 120+InLeapYear(t)≤ DayWithinYear (t) < 151+InLeapYear(t)≤ DayWithinYear (t) < 181+InLeapYear(t)≤ DayWithinYear (t) < 212+InLeapYear(t)≤ DayWithinYear (t) < 243+InLeapYear(t)≤ DayWithinYear (t) < 273+InLeapYear(t)≤ DayWithinYear (t) < 304+InLeapYear(t)≤ DayWithinYear (t) < 334+InLeapYear(t)≤ DayWithinYear (t) < 365+InLeapYear(t)whereDayWithinYear(t) = Day(t)−DayFromYear(YearFromTime(t))
A month value of 0 specifies January; 1 specifies February; 2 specifies March; 3 specifies April; 4 specifies
May; 5 specifies June; 6 specifies July; 7 specifies August; 8 specifies September; 9 specifies October; 10
specifies November; and 11 specifies December. Note that MonthFromTime(0) = 0, corresponding to Thursday,
01 January, 1970.
15.9.1.5

Date Number

A date number is identified by an integer in the range 1 through 31, inclusive. The mapping DateFromTime(t)from a time value t to a month number is defined by:
DateFromTime(t) = DayWithinYear(t)+1= DayWithinYear(t)−30= DayWithinYear(t)−58−InLeapYear(t)= DayWithinYear(t)−89−InLeapYear(t)= DayWithinYear(t)−119−InLeapYear(t)= DayWithinYear(t)−150−InLeapYear(t)= DayWithinYear(t)−180−InLeapYear(t)= DayWithinYear(t)−211−InLeapYear(t)= DayWithinYear(t)−242−InLeapYear(t)= DayWithinYear(t)−272−InLeapYear(t)= DayWithinYear(t)−303−InLeapYear(t)= DayWithinYear(t)−333−InLeapYear(t)© Ecma International 2009

if MonthFromTime(t)=0
if MonthFromTime(t)=1
if MonthFromTime(t)=2
if MonthFromTime(t)=3
if MonthFromTime(t)=4if MonthFromTime(t)=5if MonthFromTime(t)=6
if MonthFromTime(t)=7
if MonthFromTime(t)=8if MonthFromTime(t)=9if MonthFromTime(t)=10
if MonthFromTime(t)=11165

15.9.1.6

Week Day

The weekday for a particular time value t is defined as
WeekDay(t)= (Day(t) + 4) modulo 7

A weekday value of 0 specifies Sunday; 1 specifies Monday; 2 specifies Tuesday; 3 specifies Wednesday;
4 specifies Thursday; 5 specifies Friday; and 6 specifies Saturday. Note that WeekDay(0) = 4, corresponding toThursday, 01 January, 1970.
15.9.1.7

Local Time Zone Adjustment

An implementation of ECMAScript is expected to determine the local time zone adjustment. The local timezone adjustment is a value LocalTZA measured in milliseconds which when added to UTC represents the
local standard time. Daylight saving time is not reflected by LocalTZA. The value LocalTZA does not vary withtime but depends only on the geographic location.
15.9.1.8

Daylight Saving Time Adjustment

An implementation of ECMAScript is expected to determine the daylight saving time algorithm. The algorithm
to determine the daylight saving time adjustment DaylightSavingTA(t), measured in milliseconds, must depend
only on four things:(1) the time since the beginning of the yeart – TimeFromYear(YearFromTime(t))(2) whether t is in a leap year
InLeapYear(t)(3) the week day of the beginning of the year
WeekDay(TimeFromYear(YearFromTime(t))
and (4) the geographic location.
The implementation of ECMAScript should not try to determine whether the exact time was subject to daylightsaving time, but just whether daylight saving time would have been in effect if the current daylight saving timealgorithm had been used at the time. This avoids complications such as taking into account the years that the
locale observed daylight saving time year round.
If the host environment provides functionality for determining daylight saving time, the implementation of
ECMAScript is free to map the year in question to an equivalent year (same leap-year-ness and same starting
week day for the year) for which the host environment provides daylight saving time information. The only
restriction is that all equivalent years should produce the same result.
15.9.1.9

Local Time

Conversion from UTC to local time is defined by
LocalTime(t) = t + LocalTZA + DaylightSavingTA(t)Conversion from local time to UTC is defined byUTC(t) = t – LocalTZA – DaylightSavingTA(t – LocalTZA)
Note that UTC(LocalTime(t)) is not necessarily always equal to t.
15.9.1.10 Hours, Minutes, Second, and Milliseconds
The following functions are useful in decomposing time values:
HourFromTime(t) = floor(t / msPerHour) modulo HoursPerDay

166© Ecma International 2009

MinFromTime(t) = floor(t / msPerMinute) modulo MinutesPerHour
SecFromTime(t) = floor(t / msPerSecond) modulo SecondsPerMinutemsFromTime(t)= t modulo msPerSecond

HoursPerDay

= 24

where
MinutesPerHour = 60
SecondsPerMinute = 60
msPerSecond= 1000msPerMinute= 60000

msPerHour= 3600000 = msPerMinute × MinutesPerHour= msPerSecond × SecondsPerMinute

15.9.1.11 MakeTime (hour, min, sec, ms)
The operator MakeTime calculates a number of milliseconds from its four arguments, which must be
ECMAScript Number values. This operator functions as follows:
1.
2.
3.
4.
5.
6.
7.

If hour is not finite or min is not finite or sec is not finite or ms is not finite, return NaN.
Let h be ToInteger(hour).
Let m be ToInteger(min).
Let s be ToInteger(sec).
Let milli be ToInteger(ms).
Let t be h * msPerHour + m * msPerMinute + s * msPerSecond + milli, performing the arithmetic according
to IEEE 754 rules (that is, as if using the ECMAScript operators * and +).
Return t.

15.9.1.12 MakeDay (year, month, date)
The operator MakeDay calculates a number of days from its three arguments, which must be ECMAScript
Number values. This operator functions as follows:
1.
2.
3.
4.
5.
6.
7.
8.

If year is not finite or month is not finite or date is not finite, return NaN.
Let y be ToInteger(year).
Let m be ToInteger(month).
Let dt be ToInteger(date).
Let ym be y + floor(m /12).
Let mn be m modulo 12.
Find a value t such that YearFromTime(t) == ym and MonthFromTime(t) == mn) and DateFromTime(t) ==1; but if this is not possible (because some argument is out of range), return NaN.
Return Day(t) + dt − 1.

15.9.1.13 MakeDate (day, time)
The operator MakeDate calculates a number of milliseconds from its two arguments, which must be
ECMAScript Number values. This operator functions as follows:1.
2.

If day is not finite or time is not finite, return NaN.
Return day × msPerDay + time.

15.9.1.14 TimeClip (time)
The operator TimeClip calculates a number of milliseconds from its argument, which must be an ECMAScript
Number value. This operator functions as follows:1.

If time is not finite, return NaN.

© Ecma International 2009

167

2.
3.

If abs(time) > 8.64 x 10 15, return NaN.
Return an implementation-dependent choice of either ToInteger(time) or ToInteger(time) + (+0). (Adding a
positive zero converts −0 to +0.)

NOTE
The point of step 3 is that an implementation is permitted a choice of internal representations of time values,for example as a 64-bit signed integer or as a 64-bit floating-point value. Depending on the implementation, this internal
representation may or may not distinguish −0 and +0.

15.9.1.15 Date Time String Format
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601
Extended Format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the fields are as follows:
YYYY is the decimal digits of the year in the Gregorian calendar.
“:” (hyphon) appears literally twice in the string.
MM
is the month of the year from 01 (January) to 12 (December).
DD
is the day of the month from 01 to 31.
T“T” appears literally in the string, to indicate the beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two decimal digits.
:“:” (colon) appears literally twice in the string.
mmis the number of complete minutes since the start of the hour as two decimal digits.
ssis the number of complete seconds since the start of the minute as two decimal digits.
.
“.” (dot) appears literally in the string.
sss is the number of complete milliseconds since the start of the second as three decimal digits.
Both the “.” and the milliseconds field may be omitted.
Z
is the time zone offset specified as “Z” (for UTC) or either “+” or “-” followed by a time
expression hh:mm
This format includes date-only forms:
YYYY
YYYY-MMYYYY-MM-DD
It also includes time-only forms with an optional time zone offset appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
Also included are “date-times” which may be any combination of the above.
All numbers must be base 10.
Illegal values (out-of-bounds as well as syntax errors) in a format string means that the format string is not avalid instance of this format.
NOTE 1As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available todistinguish the two midnights that can be associated with one date. This means that the following two notations refer toexactly the same point in time: 1995-02-04T24:00 and 1995-02-05T00:00
NOTE 2
There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. andsometimes the same abbreviation is even used for two very different time zones. For this reason, ISO 8601 and thisformat specifies numeric representations of date and time.

168© Ecma International 2009

15.9.1.15.1 Extended years
ECMAScript requires the ability to specify 6 digit years (extended years); approximately 285,616 years, either
forward or backward, from 01 January, 1970 UTC. To represent years before 0 or after 9999, ISO 8601
permits the expansion of the year representation, but only by prior agreement between the sender and thereceiver. In the simplified ECMAScript format such an expanded year representation shall have 2 extra year
digits and is always prefixed with a + or – sign. The year 0 is considered positive and hence prefixed with a +sign.
15.9.2 The Date Constructor Called as a Function
When Date is called as a function rather than as a constructor, it returns a String representing the current
time (UTC).
NOTE
The function call Date(…) is not equivalent to the object creation expression new Date(…) with the same
arguments.

15.9.2.1

Date ( [ year [, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] ] ] )All of the arguments are optional; any arguments supplied are accepted but are completely ignored. A Stringis created and returned as if by the expression (new Date()).toString() where Date is the standard
built-in constructor with that name and toString is the standard built-in method
Date.prototype.toString.
15.9.3 The Date Constructor
When Date is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.9.3.1

new Date (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )When Date is called with two to seven arguments, it computes the date from year, month, and (optionally)
date, hours, minutes, seconds and ms.
The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object,
the one that is the initial value of Date.prototype (15.9.4.1).
The [[Class]] internal property of the newly constructed object is set to "Date".
The [[Extensible]] internal property of the newly constructed object is set to true.
The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:
1. Let y be ToNumber(year).
2. Let m be ToNumber(month).
3. If date is supplied then let dt be ToNumber(date); else let dt be 1.
4. If hours is supplied then let h be ToNumber(hours); else let h be 0.
5. If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
6. If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
7. If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
8. If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
9. Let finalDate be MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli)).
10. Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(UTC(finalDate)).
15.9.3.2

new Date (value)

The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object,
the one that is the initial value of Date.prototype (15.9.4.1).

© Ecma International 2009169

The [[Class]] internal property of the newly constructed object is set to "Date".
The [[Extensible]] internal property of the newly constructed object is set to true.
The [[PrimitiveValue]] internal property of the newly constructed object is set as follows:
1.
2.
3.
4.

Let v be ToPrimitive(value).
If Type(v) is String, thena. Parse v as a date, in exactly the same manner as for the parse method (15.9.4.2); let V be the timevalue for this date.
Else, let V be ToNumber(v).
Set the [[PrimitiveValue]] internal property of the newly constructed object to TimeClip(V) and return.

15.9.3.3

new Date ( )The [[Prototype]] internal property of the newly constructed object is set to the original Date prototype object,
the one that is the initial value of Date.prototype (15.9.4.1).
The [[Class]] internal property of the newly constructed object is set to "Date".
The [[Extensible]] internal property of the newly constructed object is set to true.
The [[PrimitiveValue]] internal property of the newly constructed object is set to the time value (UTC)
identifying the current time.
15.9.4 Properties of the Date Constructor
The value of the [[Prototype]] internal property of the Date constructor is the Function prototype object (15.3.4).
Besides the internal properties and the length property (whose value is 7), the Date constructor has thefollowing properties:
15.9.4.1

Date.prototype

The initial value of Date.prototype is the built-in Date prototype object (15.9.5).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.9.4.2

Date.parse (string)

The parse function applies the ToString operator to its argument and interprets the resulting String as a date
and time; it returns a Number, the UTC time value corresponding to the date and time. The String may beinterpreted as a local time, a UTC time, or a time in some other time zone, depending on the contents of theString. The function first attempts to parse the format of the String according to the rules called out in Date
Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any
implementation-specific heuristics or implementation-specific date formats. Unrecognizable Strings or dates
containing illegal element values in the format String shall cause Date.parse to return NaN.
If x is any Date object whose milliseconds amount is zero within a particular implementation of ECMAScript,
then all of the following expressions should produce the same numeric value in that implementation, if all theproperties referenced have their initial values:
x.valueOf()
Date.parse(x.toString())
Date.parse(x.toUTCString())
Date.parse(x.toISOString())
However, the expression
Date.parse(x.toLocaleString())

170© Ecma International 2009

is not required to produce the same Number value as the preceding three expressions and, in general, thevalue produced by Date.parse is implementation-dependent when given any String value that does notconform to the Date Time String Format (15.9.1.15) and that could not be produced in that implementation by
the toString or toUTCString method.
15.9.4.3

Date.UTC (year, month [, date [, hours [, minutes [, seconds [, ms ] ] ] ] ] )When the UTC function is called with fewer than two arguments, the behaviour is implementation-dependent.
When the UTC function is called with two to seven arguments, it computes the date from year, month and(optionally) date, hours, minutes, seconds and ms. The following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.
9.

Let y be ToNumber(year).
Let m be ToNumber(month).
If date is supplied then let dt be ToNumber(date); else let dt be 1.
If hours is supplied then let h be ToNumber(hours); else let h be 0.
If minutes is supplied then let min be ToNumber(minutes); else let min be 0.
If seconds is supplied then let s be ToNumber(seconds); else let s be 0.
If ms is supplied then let milli be ToNumber(ms); else let milli be 0.
If y is not NaN and 0 ≤ ToInteger(y) ≤ 99, then let yr be 1900+ToInteger(y); otherwise, let yr be y.
Return TimeClip(MakeDate(MakeDay(yr, m, dt), MakeTime(h, min, s, milli))).

The length property of the UTC function is 7.
NOTE
The UTC function differs from the Date constructor in two ways: it returns a time value as a Number, rather
than creating a Date object, and it interprets the arguments in UTC rather than as local time.

15.9.4.4

Date.now ( )The now function return a Number value that is the time value designating the UTC date and time of theoccurrence of the call to now.
15.9.5 Properties of the Date Prototype Object
The Date prototype object is itself a Date object (its [[Class]] is "Date") whose [[PrimitiveValue]] is NaN.
The value of the [[Prototype]] internal property of the Date prototype object is the standard built-in Object
prototype object (15.2.4).
In following descriptions of functions that are properties of the Date prototype object, the phrase “this Dateobject” refers to the object that is the this value for the invocation of the function. Unless explicitly noted
otherwise, none of these functions are generic; a TypeError exception is thrown if the this value is not an
object for which the value of the [[Class]] internal property is "Date". Also, the phrase “this time value” refers
to the Number value for the time represented by this Date object, that is, the value of the [[PrimitiveValue]]internal property of this Date object.
15.9.5.1

Date.prototype.constructor

The initial value of Date.prototype.constructor is the built-in Date constructor.
15.9.5.2

Date.prototype.toString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the Date in the current time zone in a convenient, human-readable form.
NOTE
For any Date value d whose milliseconds amount is zero, the result of Date.parse(d.toString()) isequal to d.valueOf(). See 15.9.4.2.

© Ecma International 2009171

15.9.5.3

Date.prototype.toDateString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readableform.
15.9.5.4

Date.prototype.toTimeString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readableform.
15.9.5.5

Date.prototype.toLocaleString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment’s current locale.
NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.

15.9.5.6

Date.prototype.toLocaleDateString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the “date” portion of the Date in the current time zone in a convenient, human-readable
form that corresponds to the conventions of the host environment’s current locale.
NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.

15.9.5.7

Date.prototype.toLocaleTimeString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the “time” portion of the Date in the current time zone in a convenient, human-readable
form that corresponds to the conventions of the host environment’s current locale.
NOTE
The first parameter to this function is likely to be used in a future version of this standard; it is recommended
that implementations do not use this parameter position for anything else.

15.9.5.8

Date.prototype.valueOf ( )The valueOf function returns a Number, which is this time value.
15.9.5.9
1.

Date.prototype.getTime ( )Return this time value.

15.9.5.10 Date.prototype.getFullYear ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)).

15.9.5.11 Date.prototype.getUTCFullYear ( )1.
2.

Let t be this time value.
If t is NaN, return NaN.

172

© Ecma International 2009

3.

Return YearFromTime(t).

15.9.5.12 Date.prototype.getMonth ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return MonthFromTime(LocalTime(t)).

15.9.5.13 Date.prototype.getUTCMonth ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return MonthFromTime(t).

15.9.5.14 Date.prototype.getDate ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return DateFromTime(LocalTime(t)).

15.9.5.15 Date.prototype.getUTCDate ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return DateFromTime(t).

15.9.5.16 Date.prototype.getDay ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return WeekDay(LocalTime(t)).

15.9.5.17 Date.prototype.getUTCDay ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return WeekDay(t).

15.9.5.18 Date.prototype.getHours ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return HourFromTime(LocalTime(t)).

15.9.5.19 Date.prototype.getUTCHours ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return HourFromTime(t).

15.9.5.20 Date.prototype.getMinutes ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return MinFromTime(LocalTime(t)).

© Ecma International 2009173

15.9.5.21 Date.prototype.getUTCMinutes ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return MinFromTime(t).

15.9.5.22 Date.prototype.getSeconds ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return SecFromTime(LocalTime(t)).

15.9.5.23 Date.prototype.getUTCSeconds ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return SecFromTime(t).

15.9.5.24 Date.prototype.getMilliseconds ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return msFromTime(LocalTime(t)).

15.9.5.25 Date.prototype.getUTCMilliseconds ( )
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return msFromTime(t).

15.9.5.26 Date.prototype.getTimezoneOffset ( )Returns the difference between local time and UTC time in minutes.
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return (t − LocalTime(t)) / msPerMinute.

15.9.5.27 Date.prototype.setTime (time)
1.
2.
3.

Let v be TimeClip(ToNumber(time)).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

15.9.5.28 Date.prototype.setMilliseconds (ms)
1.
2.
3.
4.
5.

Let t be the result of LocalTime(this time value).
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
Let u be TimeClip(UTC(MakeDate(Day(t), time))).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

15.9.5.29 Date.prototype.setUTCMilliseconds (ms)
1.
2.
3.
4.

Let t be this time value.
Let time be MakeTime(HourFromTime(t), MinFromTime(t), SecFromTime(t), ToNumber(ms)).
Let v be TimeClip(MakeDate(Day(t), time)).
Set the [[PrimitiveValue]] internal property of this Date object to v.

174© Ecma International 2009

5.

Return v.

15.9.5.30 Date.prototype.setSeconds (sec [, ms ] )If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().
1.
2.
3.
4.
5.
6.
7.

Let t be the result of LocalTime(this time value).
Let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

The length property of the setSeconds method is 2.
15.9.5.31 Date.prototype.setUTCSeconds (sec [, ms ] )If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().
1.
2.
3.
4.
5.
6.
7.

Let t be this time value.
Let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), MinFromTime(t), s, milli)).
Let v be TimeClip(date).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

The length property of the setUTCSeconds method is 2.
15.9.5.32 Date.prototype.setMinutes (min [, sec [, ms ] ] )If sec is not specified, this behaves as if sec were specified with the value getSeconds().
If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().
1.
2.
3.
4.
5.
6.
7.
8.

Let t be the result of LocalTime(this time value).
Let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

The length property of the setMinutes method is 3.
15.9.5.33 Date.prototype.setUTCMinutes (min [, sec [, ms ] ] )If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().
If ms is not specified, this function behaves as if ms were specified with the value return by
getUTCMilliseconds().
1.
2.
3.
4.

Let t be this time value.
Let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).

© Ecma International 2009

175

5.
6.
7.
8.

Let date be MakeDate(Day(t), MakeTime(HourFromTime(t), m, s, milli)).
Let v be TimeClip(date).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

The length property of the setUTCMinutes method is 3.
15.9.5.34 Date.prototype.setHours (hour [, min [, sec [, ms ] ] ] )If min is not specified, this behaves as if min were specified with the value getMinutes().
If sec is not specified, this behaves as if sec were specified with the value getSeconds().
If ms is not specified, this behaves as if ms were specified with the value getMilliseconds().
1.
2.
3.
4.
5.
6.
7.
8.
9.

Let t be the result of LocalTime(this time value).
Let h be ToNumber(hour).
If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
If If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let date be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let u be TimeClip(UTC(date)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

The length property of the setHours method is 4.
15.9.5.35 Date.prototype.setUTCHours (hour [, min [, sec [, ms ] ] ] )If min is not specified, this behaves as if min were specified with the value getUTCMinutes().
If sec is not specified, this behaves as if sec were specified with the value getUTCSeconds().
If ms is not specified, this behaves as if ms were specified with the value getUTCMilliseconds().
1.
2.
3.
4.
5.
6.
7.
8.
9.

Let t be this time value.
Let h be ToNumber(hour).
If min is not specified, then let m be MinFromTime(t); otherwise, let m be ToNumber(min).
If sec is not specified, then let s be SecFromTime(t); otherwise, let s be ToNumber(sec).
If ms is not specified, then let milli be msFromTime(t); otherwise, let milli be ToNumber(ms).
Let newDate be MakeDate(Day(t), MakeTime(h, m, s, milli)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

The length property of the setUTCHours method is 4.
15.9.5.36 Date.prototype.setDate (date)
1.
2.
3.
4.
5.
6.

Let t be the result of LocalTime(this time value).
Let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

176© Ecma International 2009

15.9.5.37 Date.prototype.setUTCDate (date)
1.
2.
3.
4.
5.
6.

Let t be this time value.
Let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), MonthFromTime(t), dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

15.9.5.38 Date.prototype.setMonth (month [, date ] )If date is not specified, this behaves as if date were specified with the value getDate().
1.
2.
3.
4.
5.
6.
7.

Let t be the result of LocalTime(this time value).
Let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

The length property of the setMonth method is 2.
15.9.5.39 Date.prototype.setUTCMonth (month [, date ] )If date is not specified, this behaves as if date were specified with the value getUTCDate().
1.
2.
3.
4.
5.
6.
7.

Let t be this time value.
Let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(YearFromTime(t), m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

The length property of the setUTCMonth method is 2.
15.9.5.40 Date.prototype.setFullYear (year [, month [, date ] ] )If month is not specified, this behaves as if month were specified with the value getMonth().
If date is not specified, this behaves as if date were specified with the value getDate().
1.
2.
3.
4.
5.
6.
7.
8.

Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
Let y be ToNumber(year).
If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let u be TimeClip(UTC(newDate)).
Set the [[PrimitiveValue]] internal property of this Date object to u.
Return u.

The length property of the setFullYear method is 3.
15.9.5.41 Date.prototype.setUTCFullYear (year [, month [, date ] ] )If month is not specified, this behaves as if month were specified with the value getUTCMonth().

© Ecma International 2009177

If date is not specified, this behaves as if date were specified with the value getUTCDate().
1.
2.
3.
4.
5.
6.
7.
8.

Let t be this time value; but if this time value is NaN, let t be +0.
Let y be ToNumber(year).
If month is not specified, then let m be MonthFromTime(t); otherwise, let m be ToNumber(month).
If date is not specified, then let dt be DateFromTime(t); otherwise, let dt be ToNumber(date).
Let newDate be MakeDate(MakeDay(y, m, dt), TimeWithinDay(t)).
Let v be TimeClip(newDate).
Set the [[PrimitiveValue]] internal property of this Date object to v.
Return v.

The length property of the setUTCFullYear method is 3.
15.9.5.42 Date.prototype.toUTCString ( )This function returns a String value. The contents of the String are implementation-dependent, but are
intended to represent the Date in a convenient, human-readable form in UTC.
NOTE
The intent is to produce a String representation of a date that is more readable than the format specified in
15.9.1.15. It is not essential that the chosen format be unambiguous or easily machine parsable. If an implementation
does not have a preferred human-readable format it is recommended to use the format defined in 15.9.1.15 but with aspace rather than a “T” used to separate the date and time elements.

15.9.5.43 Date.prototype.toISOString ( )This function returns a String value represent the instance in time represented by this Date object. The formatof the String is the Date Time string format defined in 15.9.1.15. All fields are present in the String. The timezone is always UTC, denoted by the suffix Z. If the time value of this object is not a finite Number aRangeError exception is thrown.
15.9.5.44 Date.prototype.toJSON ( key )This function provides a String representation of a Date object for use by JSON.stringify (15.12.3).
When the toJSON method is called with argument key, the following steps are taken:
1.
2.
3.
4.
5.
6.

Let O be the result of calling ToObject, giving it the this value as its argument.
Let tv be ToPrimitive(O, hint Number).
If tv is a Number and is not finite, return null.
Let toISO be the result of calling the [[Get]] internal method of O with argument "toISOString".
If IsCallable(toISO) is false, throw a TypeError exception.
Return the result of calling the [[Call]] internal method of toISO with O as the this value and an empty
argument list.

NOTE 1

The argument is ignored.

NOTE 2The toJSON function is intentionally generic; it does not require that its this value be a Date object. Therefore,
it can be transferred to other kinds of objects for use as a method. However, it does require that any such object have atoISOString method. An object is free to use the argument key to filter its stringification.

15.9.6 Properties of Date Instances
Date instances inherit properties from the Date prototype object and their [[Class]] internal property value is"Date". Date instances also have a [[PrimitiveValue]] internal property.
The [[PrimitiveValue]] internal property is time value represented by this Date object.

178© Ecma International 2009

15.10 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
NOTE
The form and functionality of regular expressions is modelled after the regular expression facility in the Perl 5
programming language.

15.10.1

Patterns

The RegExp constructor applies the following grammar to the input pattern String. An error occurs if thegrammar cannot interpret the String as an expansion of Pattern.
Syntax
Pattern ::Disjunction
Disjunction ::Alternative
Alternative | Disjunction
Alternative ::[empty]Alternative Term
Term ::
Assertion
Atom
Atom Quantifier
Assertion ::
^
$
\ b
\ B
( ? = Disjunction )( ? ! Disjunction )Quantifier ::QuantifierPrefix
QuantifierPrefix ?
QuantifierPrefix ::*+?
{ DecimalDigits }{ DecimalDigits , }{ DecimalDigits , DecimalDigits }Atom ::
PatternCharacter.
\ AtomEscapeCharacterClass( Disjunction )( ? : Disjunction )© Ecma International 2009179

PatternCharacter :: SourceCharacter but not any of:^ $ \ . * + ? ( ) [ ] {}|AtomEscape ::DecimalEscapeCharacterEscapeCharacterClassEscapeCharacterEscape ::ControlEscapec ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscapeControlEscape :: one off n r t v
ControlLetter :: one ofa b c d e
A B C D E

f
F

g
G

h
H

i
I

j
J

k
K

l
L

m
M

n
N

o
O

p
P

q
Q

r
R

s
S

t
T

u
U

v
V

w
W

x
X

y
Y

z
Z

IdentityEscape ::SourceCharacter but not IdentifierPart<ZWJ><ZWNJ>
DecimalEscape ::DecimalIntegerLiteral [lookahead ∉ DecimalDigit]
CharacterClassEscape :: one ofd D s S w W
CharacterClass ::[ [lookahead ∉ {^}] ClassRanges ][ ^ ClassRanges ]ClassRanges ::[empty]NonemptyClassRanges
NonemptyClassRanges ::ClassAtomClassAtom NonemptyClassRangesNoDash
ClassAtom - ClassAtom ClassRanges
NonemptyClassRangesNoDash ::ClassAtomClassAtomNoDash NonemptyClassRangesNoDash
ClassAtomNoDash - ClassAtom ClassRanges
ClassAtom ::ClassAtomNoDash
ClassAtomNoDash ::SourceCharacter but not one of \ or ] or \ ClassEscape180© Ecma International 2009

ClassEscape ::DecimalEscapebCharacterEscapeCharacterClassEscape15.10.2

Pattern Semantics

A regular expression pattern is converted into an internal procedure using the process described below. An
implementation is encouraged to use more efficient algorithms than the ones listed below, as long as theresults are the same. The internal procedure is used as the value of a RegExp object’s [[Match]] internal
property.
15.10.2.1 Notation
The descriptions below use the following variables:•

Input is the String being matched by the regular expression pattern. The notation input[n] means
the nth character of input, where n can range between 0 (inclusive) and InputLength (exclusive).

•

InputLength is the number of characters in the Input String.

•

NcapturingParens is the total number of left capturing parentheses (i.e. the total number of times
the Atom :: ( Disjunction ) production is expanded) in the pattern. A left capturing parenthesis isany ( pattern character that is matched by the ( terminal of the Atom :: ( Disjunction ) production.

•

IgnoreCase is the setting of the RegExp object's ignoreCase property.

•

Multiline is the setting of the RegExp object’s multiline property.

Furthermore, the descriptions below use the following internal data structures:•

A CharSet is a mathematical set of characters.

•

A State is an ordered pair (endIndex, captures) where endIndex is an integer and captures is aninternal array of NcapturingParens values. States are used to represent partial match states in theregular expression matching algorithms. The endIndex is one plus the index of the last input
character matched so far by the pattern, while captures holds the results of capturing parentheses.
The nth element of captures is either a String that represents the value obtained by the nth set of
capturing parentheses or undefined if the nth set of capturing parentheses hasn’t been reachedyet. Due to backtracking, many States may be in use at any time during the matching process.

•

A MatchResult is either a State or the special token failure that indicates that the match failed.

•

A Continuation procedure is an internal closure (i.e. an internal procedure with some arguments
already bound to values) that takes one State argument and returns a MatchResult result. If aninternal closure references variables bound in the function that creates the closure, the closure
uses the values that these variables had at the time the closure was created. The Continuation
attempts to match the remaining portion (specified by the closure's already-bound arguments) of
the pattern against the input String, starting at the intermediate state given by its State argument. If
the match succeeds, the Continuation returns the final State that it reached; if the match fails, theContinuation returns failure.

•

A Matcher procedure is an internal closure that takes two arguments -- a State and a Continuation -and returns a MatchResult result. A Matcher attempts to match a middle subpattern (specified by
the closure's already-bound arguments) of the pattern against the input String, starting at theintermediate state given by its State argument. The Continuation argument should be a closure thatmatches the rest of the pattern. After matching the subpattern of a pattern to obtain a new State,
the Matcher then calls Continuation on that new State to test if the rest of the pattern can match aswell. If it can, the matcher returns the State returned by Continuation; if not, the Matcher may try
different choices at its choice points, repeatedly calling Continuation until it either succeeds or all
possibilities have been exhausted.

•

An AssertionTester procedure is an internal closure that takes a State argument and returns a
Boolean result. The assertion tester tests a specific condition (specified by the closure's already-© Ecma International 2009181

bound arguments) against the current place in the input String and returns true if the condition
matched or false if not.
•An EscapeValue is either a character or an integer. An EscapeValue is used to denote theinterpretation of a DecimalEscape escape sequence: a character ch means that the escapesequence is interpreted as the character ch, while an integer n means that the escape sequenceis interpreted as a backreference to the nth set of capturing parentheses.

15.10.2.2 Pattern
The production Pattern :: Disjunction evaluates as follows:
1.
2.
3.
4.
5.
6.
7.
8.

Evaluate Disjunction to obtain a Matcher m.
Return an internal closure that takes two arguments, a String str and an integer index, and performs thefollowing:
Let Input be the given String str. This variable will be used throughout the algorithms in 15.10.2.
Let InputLength be the length of Input. This variable will be used throughout the algorithms in 15.10.2.
Let c be a Continuation that always returns its State argument as a successful MatchResult.
Let cap be an internal array of NcapturingParens undefined values, indexed 1 through NcapturingParens.
Let x be the State (index, cap).
Call m(x, c) and return its result.

NOTE
A Pattern evaluates ("compiles") to an internal procedure value. RegExp.prototype.exec can then apply
this procedure to a String and an offset within the String to determine whether the pattern would match starting at exactlythat offset within the String, and, if it does match, what the values of the capturing parentheses would be. The algorithms
in 15.10.2 are designed so that compiling a pattern may throw a SyntaxError exception; on the other hand, once thepattern is successfully compiled, applying its result internal procedure to find a match in a String cannot throw an
exception (except for any host-defined exceptions that can occur anywhere such as out-of-memory).

15.10.2.3 Disjunction
The production Disjunction :: Alternative evaluates by evaluating Alternative to obtain a Matcher and returningthat Matcher.
The production Disjunction :: Alternative | Disjunction evaluates as follows:
1.
2.
3.

Evaluate Alternative to obtain a Matcher m1.
Evaluate Disjunction to obtain a Matcher m2.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following:
1. Call m1(x, c) and let r be its result.
2. If r isn't failure, return r.
3. Call m2(x, c) and return its result.

NOTE
The | regular expression operator separates two alternatives. The pattern first tries to match the left Alternative(followed by the sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the sequel ofthe regular expression). If the left Alternative, the right Disjunction, and the sequel all have choice points, all choices in thesequel are tried before moving on to the next choice in the left Alternative. If choices in the left Alternative are exhausted,
the right Disjunction is tried instead of the left Alternative. Any capturing parentheses inside a portion of the pattern skippedby | produce undefined values instead of Strings. Thus, for example,/a|ab/.exec("abc")
returns the result "a" and not "ab". Moreover,/((a)|(ab))((c)|(bc))/.exec("abc")
returns the array["abc", "a", "a", undefined, "bc", undefined, "bc"]and not["abc", "ab", undefined, "ab", "c", "c", undefined]

182© Ecma International 2009

15.10.2.4 Alternative
The production Alternative :: [empty] evaluates by returning a Matcher that takes two arguments, a State x and aContinuation c, and returns the result of calling c(x).
The production Alternative :: Alternative Term evaluates as follows:
1.
2.
3.

Evaluate Alternative to obtain a Matcher m1.
Evaluate Term to obtain a Matcher m2.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following:
1. Create a Continuation d that takes a State argument y and returns the result of calling m2(y,c).
2. Call m1(x, d) and return its result.

NOTE
Consecutive Terms try to simultaneously match consecutive portions of the input String. If the left Alternative,
the right Term, and the sequel of the regular expression all have choice points, all choices in the sequel are tried before
moving on to the next choice in the right Term, and all choices in the right Term are tried before moving on to the next
choice in the left Alternative.

15.10.2.5 Term
The production Term :: Assertion evaluates by returning an internal Matcher closure that takes two arguments,
a State x and a Continuation c, and performs the following:
1.
2.
3.
4.

Evaluate Assertion to obtain an AssertionTester t.
Call t(x) and let r be the resulting Boolean value.
If r is false, return failure.
Call c(x) and return its result.

The production Term :: Atom evaluates by evaluating Atom to obtain a Matcher and returning that Matcher.
The production Term :: Atom Quantifier evaluates as follows:
1.
2.
3.
4.

5.
6.

Evaluate Atom to obtain a Matcher m.
Evaluate Quantifier to obtain the three results: an integer min, an integer (or ∞) max, and Boolean greedy.
If max is finite and less than min, then throw a SyntaxError exception.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to theleft of this production expansion's Term. This is the total number of times the Atom :: ( Disjunction )production is expanded prior to this production's Term plus the total number of Atom :: ( Disjunction )productions enclosing this Term.
Let parenCount be the number of left capturing parentheses in the expansion of this production's Atom. Thisis the total number of Atom :: ( Disjunction ) productions enclosed by this production's Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following:
1. Call RepeatMatcher(m, min, max, greedy, x, c, parenIndex, parenCount) and return its
result.

The abstract operation RepeatMatcher takes eight parameters, a Matcher m, an integer min, an integer (or ∞)max, a Boolean greedy, a State x, a Continuation c, an integer parenIndex, and an integer parenCount, andperforms the following:
1.
2.

If max is zero, then call c(x) and return its result.
Create an internal Continuation closure d that takes one State argument y and performs the following:
1. If min is zero and y's endIndex is equal to x's endIndex, then return failure.
2. If min is zero then let min2 be zero; otherwise let min2 be min–1.
3. If max is ∞, then let max2 be ∞; otherwise let max2 be max–1.

© Ecma International 2009

183

4.

Call RepeatMatcher(m, min2, max2, greedy, y, c, parenIndex, parenCount) and return its
result.
3. Let cap be a fresh copy of x's captures internal array.
4. For every integer k that satisfies parenIndex < k and k ≤ parenIndex+parenCount, set cap[k] to undefined.
5. Let e be x's endIndex.
6. Let xr be the State (e, cap).
7. If min is not zero, then call m(xr, d) and return its result.
8. If greedy is false, thena. Call c(x) and let z be its result.
b. If z is not failure, return z.
c. Call m(xr, d) and return its result.
9. Call m(xr, d) and let z be its result.
10. If z is not failure, return z.
11. Call c(x) and return its result.
NOTE
An Atom followed by a Quantifier is repeated the number of times specified by the Quantifier. A Quantifier can
be non-greedy, in which case the Atom pattern is repeated as few times as possible while still matching the sequel, or it
can be greedy, in which case the Atom pattern is repeated as many times as possible while still matching the sequel. The
Atom pattern is repeated rather than the input String that it matches, so different repetitions of the Atom can match different
input substrings.
NOTE 2
If the Atom and the sequel of the regular expression all have choice points, the Atom is first matched as many(or as few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the next choice in thethlast repetition of Atom. All choices in the last (n ) repetition of Atom are tried before moving on to the next choice in thestnext-to-last (n–1) repetition of Atom; at which point it may turn out that more or fewer repetitions of Atom are now possible;
these are exhausted (again, starting with either as few or as many as possible) before moving on to the next choice in thest(n-1) repetition of Atom and so on.
Compare/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array["aaba", "ba"]and not any of:["aabaac", "aabaac"]["aabaac", "c"]The above ordering of choice points can be used to write a regular expression that calculates the greatest common divisorof two numbers (represented in unary notation). The following example calculates the gcd of 10 and 15:"aaaaaaaaaa,aaaaaaaaaaaaaaa".replace(/^(a+)\1*,\1+$/,"$1")
which returns the gcd in unary notation "aaaaa".
NOTE 3
Step 4 of the RepeatMatcher clears Atom's captures each time Atom is repeated. We can see its behaviour inthe regular expression/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")which returns the array["zaacbbbcac", "z", "ac", "a", undefined, "c"]and not["zaacbbbcac", "z", "ac", "a", "bbb", "c"]because each iteration of the outermost * clears all captured Strings contained in the quantified Atom, which in this caseincludes capture Strings numbered 2, 3, and 4.

184© Ecma International 2009

NOTE 4
Step 1 of the RepeatMatcher's d closure states that, once the minimum number of repetitions has beensatisfied, any more expansions of Atom that match the empty String are not considered for further repetitions. This
prevents the regular expression engine from falling into an infinite loop on patterns such as:/(a*)*/.exec("b")
or the slightly more complicated:/(a*)b\1+/.exec("baaaac")
which returns the array["b", ""]

15.10.2.6 Assertion
The production Assertion :: ^ evaluates by returning an internal AssertionTester closure that takes a Stateargument x and performs the following:
1.
2.
3.
4.
5.

Let e be x's endIndex.
If e is zero, return true.
If Multiline is false, return false.
If the character Input[e–1] is one of LineTerminator, return true.
Return false.

The production Assertion :: $ evaluates by returning an internal AssertionTester closure that takes a Stateargument x and performs the following:
1.
2.
3.
4.
5.

Let e be x's endIndex.
If e is equal to InputLength, return true.
If multiline is false, return false.
If the character Input[e] is one of LineTerminator, return true.
Return false.

The production Assertion :: \ b evaluates by returning an internal AssertionTester closure that takes a Stateargument x and performs the following:
1.
2.
3.
4.
5.
6.

Let e be x's endIndex.
Call IsWordChar(e–1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
If a is true and b is false, return true.
If a is false and b is true, return true.
Return false.

The production Assertion :: \ B evaluates by returning an internal AssertionTester closure that takes a Stateargument x and performs the following:
1.
2.
3.
4.
5.
6.

Let e be x's endIndex.
Call IsWordChar(e–1) and let a be the Boolean result.
Call IsWordChar(e) and let b be the Boolean result.
If a is true and b is false, return false.
If a is false and b is true, return false.
Return true.

The production Assertion :: ( ? = Disjunction ) evaluates as follows:1.
2.

Evaluate Disjunction to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r is failure, return failure.
4. Let y be r's State.
5. Let cap be y's captures internal array.

© Ecma International 2009

185

6.
7.
8.

Let xe be x's endIndex.
Let z be the State (xe, cap).
Call c(z) and return its result.

The production Assertion :: ( ? ! Disjunction ) evaluates as follows:1.
2.

Evaluate Disjunction to obtain a Matcher m.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following steps:
1. Let d be a Continuation that always returns its State argument as a successful MatchResult.
2. Call m(x, d) and let r be its result.
3. If r isn't failure, return failure.
4. Call c(x) and return its result.

The abstract operation IsWordChar takes an integer parameter e and performs the following:
1.
2.
3.

If e == –1 or e == InputLength, return false.
Let c be the character Input[e].
If c is one of the sixty-three characters below, return true.
A b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 _4.

Return false.

15.10.2.7 Quantifier
The production Quantifier :: QuantifierPrefix evaluates as follows:1.
2.

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min , max, and true.

The production Quantifier :: QuantifierPrefix ? evaluates as follows:1.
2.

Evaluate QuantifierPrefix to obtain the two results: an integer min and an integer (or ∞) max.
Return the three results min , max, and false.

The production QuantifierPrefix :: * evaluates by returning the two results 0 and ∞.
The production QuantifierPrefix :: + evaluates by returning the two results 1 and ∞.
The production QuantifierPrefix :: ? evaluates by returning the two results 0 and 1.
The production QuantifierPrefix :: { DecimalDigits } evaluates as follows:1.
2.

Let i be the MV of DecimalDigits (see 7.8.3).
Return the two results i and i.

The production QuantifierPrefix :: { DecimalDigits , } evaluates as follows:1.
2.

Let i be the MV of DecimalDigits.
Return the two results i and ∞.

The production QuantifierPrefix :: { DecimalDigits , DecimalDigits } evaluates as follows:
1.
2.
3.

Let i be the MV of the first DecimalDigits.
Let j be the MV of the second DecimalDigits.
Return the two results i and j.

186© Ecma International 2009

15.10.2.8 AtomThe production Atom :: PatternCharacter evaluates as follows:
1.
2.
3.

Let ch be the character represented by PatternCharacter.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: . evaluates as follows:
1.
2.

Let A be the set of all characters except LineTerminator.
Call CharacterSetMatcher(A, false) and return its Matcher result.

The production Atom :: \ AtomEscape evaluates by evaluating AtomEscape to obtain a Matcher and returningthat Matcher.
The production Atom :: CharacterClass evaluates as follows:1.
2.

Evaluate CharacterClass to obtain a CharSet A and a Boolean invert.
Call CharacterSetMatcher(A, invert) and return its Matcher result.

The production Atom :: ( Disjunction ) evaluates as follows:
1.
2.

3.

Evaluate Disjunction to obtain a Matcher m.
Let parenIndex be the number of left capturing parentheses in the entire regular expression that occur to theleft of this production expansion's initial left parenthesis. This is the total number of times the
Atom :: ( Disjunction ) production is expanded prior to this production's Atom plus the total number ofAtom :: ( Disjunction ) productions enclosing this Atom.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following steps:
1. Create an internal Continuation closure d that takes one State argument y and performs thefollowing steps:
1. Let cap be a fresh copy of y's captures internal array.
2. Let xe be x's endIndex.
3. Let ye be y's endIndex.
4. Let s be a fresh String whose characters are the characters of Input at
positions xe (inclusive) through ye (exclusive).
5. Set cap[parenIndex+1] to s.
6. Let z be the State (ye, cap).
7. Call c(z) and return its result.
2. Call m(x, d) and return its result.

The production Atom :: ( ? : Disjunction ) evaluates by evaluating Disjunction to obtain a Matcher andreturning that Matcher.
The abstract operation CharacterSetMatcher takes two arguments, a CharSet A and a Boolean flag invert, andperforms the following:
1.

Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following steps:
1. Let e be x's endIndex.
2. If e == InputLength, return failure.
3. Let c be the character Input[e].
4. Let cc be the result of Canonicalize(c).
5. If invert is false, thena If there does not exist a member a of set A such that Canonicalize(a) == cc, returnfailure.
6. Else invert is true,
a If there exists a member a of set A such that Canonicalize(a) == cc, return failure.

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7.
8.
9.

Let cap be x's captures internal array.
Let y be the State (e+1, cap).
Call c(y) and return its result.

The abstract operation Canonicalize takes a character parameter ch and performs the following steps:
1.
2.
3.
4.
5.
6.

If IgnoreCase is false, return ch.
Let u be ch converted to upper case as if by calling the standard built-in method
String.prototype.toUpperCase on the one-character String ch.
If u does not consist of a single character, return ch.
Let cu be u's character.
If ch's code unit value is greater than or equal to decimal 128 and cu's code unit value is less than decimal
128, then return ch.
Return cu.

NOTE 1
Parentheses of the form ( Disjunction ) serve both to group the components of the Disjunction pattern
together and to save the result of the match. The result can be used either in a backreference (\ followed by a nonzero
decimal number), referenced in a replace String, or returned as part of an array from the regular expression matchinginternal procedure. To inhibit the capturing behaviour of parentheses, use the form (?: Disjunction ) instead.
NOTE 2
The form (?= Disjunction ) specifies a zero-width positive lookahead. In order for it to succeed, the patterninside Disjunction must match at the current position, but the current position is not advanced before matching the sequel.
If Disjunction can match at the current position in several ways, only the first one is tried. Unlike other regular expression
operators, there is no backtracking into a (?= form (this unusual behaviour is inherited from Perl). This only matters when
the Disjunction contains capturing parentheses and the sequel of the pattern contains backreferences to those captures.
For example,/(?=(a+))/.exec("baaabac")matches the empty String immediately after the first b and therefore returns the array:["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:/(?=(a+))a*b\1/.exec("baaabac")This expression returns["aba", "a"]and not:["aaaba", "a"]NOTE 3
The form (?! Disjunction ) specifies a zero-width negative lookahead. In order for it to succeed, the patterninside Disjunction must fail to match at the current position. The current position is not advanced before matching thesequel. Disjunction can contain capturing parentheses, but backreferences to them only make sense from within
Disjunction itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return undefined
because the negative lookahead must fail for the pattern to succeed. For example,/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a b, another n a's (specified by the first \2) anda c. The second \2 is outside the negative lookahead, so it matches against undefined and therefore always succeeds.
The whole expression returns the array:["baaabaac", "ba", undefined, "abaac"]
In case-insignificant matches all characters are implicitly converted to upper case immediately before they are compared.
However, if converting a character to upper case would expand that character into more than one character (such as
converting "ß" (\u00DF) into "SS"), then the character is left as-is instead. The character is also left as-is if it is not an
ASCII character but converting it to upper case would make it into an ASCII character. This prevents Unicode characterssuch as \u0131 and \u017F from matching regular expressions such as /[a-z]/i, which are only intended to match
ASCII letters. Furthermore, if these conversions were allowed, then /[^\W]/i would match each of a, b, …, h, but not i
or s.

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15.10.2.9 AtomEscape
The production AtomEscape :: DecimalEscape evaluates as follows:
1.
2.

3.
4.
5.

Evaluate DecimalEscape to obtain an EscapeValue E.
If E is a character, thena. Let ch be E's character.
b. Let A be a one-element CharSet containing the character ch.
c. Call CharacterSetMatcher(A, false) and return its Matcher result.
E must be an integer. Let n be that integer.
If n=0 or n>NCapturingParens then throw a SyntaxError exception.
Return an internal Matcher closure that takes two arguments, a State x and a Continuation c, and performs
the following:
1. Let cap be x's captures internal array.
2. Let s be cap[n].
3. If s is undefined, then call c(x) and return its result.
4. Let e be x's endIndex.
5. Let len be s's length.
6. Let f be e+len.
7. If f>InputLength, return failure.
8. If there exists an integer i between 0 (inclusive) and len (exclusive) such that
Canonicalize(s[i]) is not the same character as Canonicalize(Input [e+i]), then returnfailure.
9. Let y be the State (f, cap).
10. Call c(y) and return its result.

The production AtomEscape :: CharacterEscape evaluates as follows:
1.
2.
3.

Evaluate CharacterEscape to obtain a character ch.
Let A be a one-element CharSet containing the character ch.
Call CharacterSetMatcher(A, false) and return its Matcher result.

The production AtomEscape :: CharacterClassEscape evaluates as follows:1.
2.

Evaluate CharacterClassEscape to obtain a CharSet A.
Call CharacterSetMatcher(A, false) and return its Matcher result.

NOTE
An escape sequence of the form \ followed by a nonzero decimal number n matches the result of the nth set
of capturing parentheses (see 15.10.2.11). It is an error if the regular expression has fewer than n capturing parentheses.
If the regular expression has n or more capturing parentheses but the nth one is undefined because it has not captured
anything, then the backreference always succeeds.

15.10.2.10 CharacterEscape
The production CharacterEscape :: ControlEscape evaluates by returning the character according to Table 23
Table 23 — ControlEscape Character Values
ControlEscape
t
n
v
f
r

Code Unit

Name

Symbol\u0009\u000A\u000B\u000C\u000D

horizontal tab
line feed (new line)
vertical tab
form feed
carriage return

<HT>
<LF>
<VT>
<FF>
<CR>

The production CharacterEscape :: c ControlLetter evaluates as follows:© Ecma International 2009

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1.
2.
3.
4.

Let ch be the character represented by ControlLetter.
Let i be ch's code unit value.
Let j be the remainder of dividing i by 32.
Return the code unit numbered j.

The production CharacterEscape :: HexEscapeSequence evaluates
HexEscapeSequence (see 7.8.4) and returning its character result.

byevaluating

the

CV

of

the

The production CharacterEscape :: UnicodeEscapeSequence evaluates by evaluating the CV of theUnicodeEscapeSequence (see 7.8.4) and returning its character result.
The production CharacterEscape :: IdentityEscape evaluates by returning the character represented by
IdentityEscape.
15.10.2.11 DecimalEscape
The production DecimalEscape :: DecimalIntegerLiteral [lookahead ∉ DecimalDigit] evaluates as follows.
1.
2.
3.

Let i be the MV of DecimalIntegerLiteral.
If i is zero, return the EscapeValue consisting of a <NUL> character (Unicode value 0000).
Return the EscapeValue consisting of the integer i.

The definition of “the MV of DecimalIntegerLiteral” is in 7.8.3.
NOTE
If \ is followed by a decimal number n whose first digit is not 0, then the escape sequence is considered to bea backreference. It is an error if n is greater than the total number of left capturing parentheses in the entire regularexpression. \0 represents the <NUL> character and cannot be followed by a decimal digit.

15.10.2.12 CharacterClassEscape
The production CharacterClassEscape :: d evaluates by returning the ten-element set of characters containing
the characters 0 through 9 inclusive.
The production CharacterClassEscape :: D evaluates by returning the set of all characters not included in the set
returned by CharacterClassEscape :: d.
The production CharacterClassEscape :: s evaluates by returning the set of characters containing thecharacters that are on the right-hand side of the WhiteSpace (7.2) or LineTerminator (7.3) productions.
The production CharacterClassEscape :: S evaluates by returning the set of all characters not included in the set
returned by CharacterClassEscape :: s.
The production CharacterClassEscape :: w evaluates by returning the set of characters containing the sixtythree characters:a b c d e f g h i j k l m n o p q r s t u v w x y z
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
0 1 2 3 4 5 6 7 8 9 _The production CharacterClassEscape :: W evaluates by returning the set of all characters not included in the set
returned by CharacterClassEscape :: w.
15.10.2.13 CharacterClass
The production CharacterClass :: [ [lookahead ∉ {^}] ClassRanges ] evaluates by evaluating ClassRanges toobtain a CharSet and returning that CharSet and the Boolean false.

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The production CharacterClass :: [ ^ ClassRanges ] evaluates by evaluating ClassRanges to obtain a CharSet
and returning that CharSet and the Boolean true.
15.10.2.14 ClassRanges
The production ClassRanges :: [empty] evaluates by returning the empty CharSet.
The production ClassRanges :: NonemptyClassRanges evaluates by evaluating NonemptyClassRanges to obtain a
CharSet and returning that CharSet.
15.10.2.15 NonemptyClassRanges
The production NonemptyClassRanges :: ClassAtom evaluates by evaluating ClassAtom to obtain a CharSet andreturning that CharSet.
The production NonemptyClassRanges :: ClassAtom NonemptyClassRangesNoDash evaluates as follows:
1.
2.
3.

Evaluate ClassAtom to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRanges :: ClassAtom - ClassAtom ClassRanges evaluates as follows:
1.
2.
3.
4.
5.

Evaluate the first ClassAtom to obtain a CharSet A.
Evaluate the second ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.
Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

The abstract operation CharacterRange takes two CharSet parameters A and B and performs the following:
1.
2.
3.
4.
5.
6.
7.

If A does not contain exactly one character or B does not contain exactly one character then throw a
SyntaxError exception.
Let a be the one character in CharSet A.
Let b be the one character in CharSet B.
Let i be the code unit value of character a.
Let j be the code unit value of character b.
If i > j then throw a SyntaxError exception.
Return the set containing all characters numbered i through j, inclusive.

15.10.2.16 NonemptyClassRangesNoDash
The production NonemptyClassRangesNoDash :: ClassAtom evaluates by evaluating ClassAtom to obtain a
CharSet and returning that CharSet.
The production NonemptyClassRangesNoDash :: ClassAtomNoDash NonemptyClassRangesNoDash evaluates asfollows:
1.
2.
3.

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate NonemptyClassRangesNoDash to obtain a CharSet B.
Return the union of CharSets A and B.

The production NonemptyClassRangesNoDash :: ClassAtomNoDash - ClassAtom ClassRanges evaluates asfollows:
1.
2.
3.

Evaluate ClassAtomNoDash to obtain a CharSet A.
Evaluate ClassAtom to obtain a CharSet B.
Evaluate ClassRanges to obtain a CharSet C.

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4.
5.

Call CharacterRange(A, B) and let D be the resulting CharSet.
Return the union of CharSets D and C.

NOTE 1
ClassRanges can expand into single ClassAtoms and/or ranges of two ClassAtoms separated by dashes. In thelatter case the ClassRanges includes all characters between the first ClassAtom and the second ClassAtom, inclusive; anerror occurs if either ClassAtom does not represent a single character (for example, if one is \w) or if the first ClassAtom's
code unit value is greater than the second ClassAtom's code unit value.
NOTE 2
Even if the pattern ignores case, the case of the two ends of a range is significant in determining whichcharacters belong to the range. Thus, for example, the pattern /[E-F]/i matches only the letters E, F, e, and f, while thepattern /[E-f]/i matches all upper and lower-case ASCII letters as well as the symbols [, \, ], ^, _, and `.
NOTE 3 A - character can be treated literally or it can denote a range. It is treated literally if it is the first or last characterof ClassRanges, the beginning or end limit of a range specification, or immediately follows a range specification.

15.10.2.17 ClassAtom
The production ClassAtom :: - evaluates by returning the CharSet containing the one character -.
The production ClassAtom :: ClassAtomNoDash evaluates by evaluating ClassAtomNoDash to obtain a CharSet
and returning that CharSet.
15.10.2.18 ClassAtomNoDash
The production ClassAtomNoDash :: SourceCharacter but not one of \ or ] or - evaluates by returning a oneelement CharSet containing the character represented by SourceCharacter.
The production ClassAtomNoDash :: \ ClassEscape evaluates by evaluating ClassEscape to obtain a CharSet
and returning that CharSet.
15.10.2.19 ClassEscape
The production ClassEscape :: DecimalEscape evaluates as follows:
1.
2.
3.
4.

Evaluate DecimalEscape to obtain an EscapeValue E.
If E is not a character then throw a SyntaxError exception.
Let ch be E's character.
Return the one-element CharSet containing the character ch.

The production ClassEscape :: b evaluates by returning the CharSet containing the one character <BS>(Unicode value 0008).
The production ClassEscape :: CharacterEscape evaluates by evaluating CharacterEscape to obtain a characterand returning a one-element CharSet containing that character.
The production ClassEscape :: CharacterClassEscape evaluates by evaluating CharacterClassEscape to obtain a
CharSet and returning that CharSet.
NOTE
A ClassAtom can use any of the escape sequences that are allowed in the rest of the regular expression
except for \b, \B, and backreferences. Inside a CharacterClass, \b means the backspace character, while \B andbackreferences raise errors. Using a backreference inside a ClassAtom causes an error.

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15.10.3

The RegExp Constructor Called as a Function

15.10.3.1 RegExp(pattern, flags)
If pattern is an object R whose [[Class]] internal property is "RegExp" and flags is undefined, then return R
unchanged. Otherwise call the standard built-in RegExp constructor (15.10.4.1) as if by the expression new
RegExp(pattern, flags) and return the object constructed by that constructor.
15.10.4

The RegExp Constructor

When RegExp is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.10.4.1 new RegExp(pattern, flags)
If pattern is an object R whose [[Class]] internal property is "RegExp" and flags is undefined, then let P be thepattern used to construct R and let F be the flags used to construct R. If pattern is an object R whose [[Class]]
internal property is "RegExp" and flags is not undefined, then throw a TypeError exception. Otherwise, let P
be the empty String if pattern is undefined and ToString(pattern) otherwise, and let F be the empty String if
flags is undefined and ToString(flags) otherwise.
If the characters of P do not have the syntactic form Pattern, then throw a SyntaxError exception. Otherwise
let the newly constructed object have a [[Match]] internal property obtained by evaluating ("compiling") the
characters of P as a Pattern as described in 15.10.2.
If F contains any character other than "g", "i", or "m", or if it contains the same character more than once,
then throw a SyntaxError exception.
If a SyntaxError exception is not thrown, then:
Let S be a String in the form of a Pattern equivalent to P, in which certain characters are escaped as described
below. S may or may not be identical to P or pattern; however, the internal procedure that would result from
evaluating S as a Pattern must behave identically to the internal procedure given by the constructed object's[[Match]] internal property.
The characters / or backslash \ occurring in the pattern shall be escaped in S as necessary to ensure that theString value formed by concatenating the Strings "/", S, "/", and F can be parsed (in an appropriate lexical
context) as a RegularExpressionLiteral that behaves identically to the constructed regular expression. For
example, if P is "/", then S could be "\/" or "\u002F", among other possibilities, but not "/", because ///followed by F would be parsed as a SingleLineComment rather than a RegularExpressionLiteral. If P is the empty
String, this specification can be met by letting S be "(?:)".
The following properties of the newly constructed object are data properties with the attributes that are
specified in 15.10.7. The [[Value]] of each property is set as follows:
The source property of the newly constructed object is set to S.
The global property of the newly constructed object is set to a Boolean value that is true if F contains thecharacter "g" and false otherwise.
The ignoreCase property of the newly constructed object is set to a Boolean value that is true if F contains
the character "i" and false otherwise.
The multiline property of the newly constructed object is set to a Boolean value that is true if F contains
the character "m" and false otherwise.
The lastIndex property of the newly constructed object is set to 0.

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The [[Prototype]] internal property of the newly constructed object is set to the standard built-in RegExp
prototype object as specified in 15.10.6.
The [[Class]] internal property of the newly constructed object is set to "RegExp".
NOTE
If pattern is a StringLiteral, the usual escape sequence substitutions are performed before the String is
processed by RegExp. If pattern must contain an escape sequence to be recognised by RegExp, any backslash\ characters must be escaped within the StringLiteral to prevent them being removed when the contents of the StringLiteral
are formed.

15.10.5

Properties of the RegExp Constructor

The value of the [[Prototype]] internal property of the RegExp constructor is the standard built-in Function
prototype object (15.3.4).
Besides the internal properties and the length property (whose value is 2), the RegExp constructor has thefollowing properties:
15.10.5.1 RegExp.prototype
The initial value of RegExp.prototype is the RegExp prototype object (15.10.6).
This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.10.6

Properties of the RegExp Prototype Object

The value of the [[Prototype]] internal property of the RegExp prototype object is the standard built-in Object
prototype object (15.2.4). The RegExp prototype object is itself a regular expression object; its [[Class]] is"RegExp". The initial values of the RegExp prototype object’s data properties (15.10.7) are set as if theobject was created by the expression new RegExp() where RegExp is that standard built-in constructor withthat name.
The RegExp prototype object does not have a valueOf property of its own; however, it inherits the valueOfproperty from the Object prototype object.
In the following descriptions of functions that are properties of the RegExp prototype object, the phrase “thisRegExp object” refers to the object that is the this value for the invocation of the function; a TypeError
exception is thrown if the this value is not an object or an object for which the value of the [[Class]] internal
property is not "RegExp".
15.10.6.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is the standard built-in RegExp constructor.
15.10.6.2 RegExp.prototype.exec(string)
Performs a regular expression match of string against the regular expression and returns an Array object
containing the results of the match, or null if string did not match.
The String ToString(string) is searched for an occurrence of the regular expression pattern as follows:
1.
2.
3.
4.
5.
6.

Let R be this RegExp object.
Let S be the value of ToString(string).
Let length be the length of S.
Let lastIndex be the result of calling the [[Get]] internal method of R with argument "lastIndex"..
Let i be the value of ToInteger(lastIndex).
Let global be the result of calling the [[Get]] internal method of R with argument "global".

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7.
8.
9.

If global is false, then let i = 0.
Let matchSucceeded be false.
Repeat, while matchSucceeded is false
a. If i < 0 or i > length, theni.
Call the [[Put]] internal method of R with arguments "lastIndex", 0, and true.
ii.
Return null.
b. Call the [[Match]] internal method of R with arguments S and i.
c. If [[Match]] returned failure, theni.
Let i = i+1.
d. elsei.
Let r be the State result of the call to [[Match]].
ii.
Set matchSucceeded to true.
e. Let i = i+1.
10. Let e be r's endIndex value.
11. If global is true,
a. Call the [[Put]] internal method of R with arguments "lastIndex", e, and true.
12. Let n be the length of r's captures array. (This is the same value as 15.10.2.1's NCapturingParens.)
13. Let A be a new array created as if by the expression new Array() where Array is the standard built-in
constructor with that name.
14. Let matchIndex be the position of the matched substring within the complete String S.
15. Call the [[DefineOwnProperty]] internal method of A with arguments "index", Property Descriptor{[[Value]]: matchIndex, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and true.
16. Call the [[DefineOwnProperty]] internal method of A with arguments "input", Property Descriptor{[[Value]]: S, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and true.
17. Call the [[DefineOwnProperty]] internal method of A with arguments "length", Property Descriptor{[[Value]]: n + 1}, and true.
18. Let matchedSubstr be the matched substring (i.e. the portion of S between offset i inclusive and offset e
exclusive).
19. Call the [[DefineOwnProperty]] internal method of A with arguments "0", Property Descriptor {[[Value]]:matchedSubstr, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true}, and true.
20. For each integer i such that I > 0 and I ≤ na. Let captureI be i th element of r's captures array.
b. Call the [[DefineOwnProperty]] internal method of A with arguments ToString(i), Property
Descriptor {[[Value]]: captureI, [[Writable]: true, [[Enumerable]]: true, [[Configurable]]: true},and true.
21. Return A.
15.10.6.3 RegExp.prototype.test(string)
The following steps are taken:
1.
2.

Let match be the result of evaluating the RegExp.prototype.exec (15.10.6.3) algorithm upon thisRegExp object using string as the argument.
If match is not null, then return true; else return false.

15.10.6.4 RegExp.prototype.toString()
Return the String value formed by concatenating the Strings "/", the String value of the source property ofthis RegExp object, and "/"; plus "g" if the global property is true, "i" if the ignoreCase property is true,and "m" if the multiline property is true.
NOTE
The returned String has the form of a RegularExpressionLiteral that evaluates to another RegExp object withthe same behaviour as this object.

15.10.7

Properties of RegExp Instances

RegExp instances inherit properties from the RegExp prototype object and their [[Class]] internal property
value is "RegExp". RegExp instances also have a [[Match]] internal property and a length property.

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The value of the [[Match]] internal property is an implementation dependent representation of the Pattern of theRegExp object.
RegExp instances also have the following properties.
15.10.7.1 source
The value of the source property is a String in the form of a Pattern representing the current regularexpression. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:false }.
15.10.7.2 global
The value of the global property is a Boolean value indicating whether the flags contained the character “g”.
This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.10.7.3 ignoreCase
The value of the ignoreCase property is a Boolean value indicating whether the flags contained thecharacter “i”. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false,[[Configurable]]: false }.
15.10.7.4 multiline
The value of the multiline property is a Boolean value indicating whether the flags contained the character“m”. This property shall have the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.10.7.5 lastIndex
The value of the lastIndex property specifies the String position at which to start the next match. It is
coerced to an integer when used (see 15.10.6.2). This property shall have the attributes { [[Writable]]: true,[[Enumerable]]: false, [[Configurable]]: false }.
NOTE

Unlike the other standard built-in properties of RegExp instances, lastIndex is writable.

15.11 Error Objects
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also
serve as base objects for user-defined exception classes.
15.11.1

The Error Constructor Called as a Function

When Error is called as a function rather than as a constructor, it creates and initialises a new Error object.
Thus the function call Error(…) is equivalent to the object creation expression new Error(…) with thesame arguments.
15.11.1.1 Error (message)
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object,
the one that is the initial value of Error.prototype (15.11.3.1).
The [[Class]] internal property of the newly constructed object is set to "Error".
The [[Extensible]] internal property of the newly constructed object is set to true.

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If the argument message is not undefined, the message own property of the newly constructed object is set to
ToString(message). Otherwise, the message own property is set to the empty String.
15.11.2

The Error Constructor

When Error is called as part of a new expression, it is a constructor: it initialises the newly created object.
15.11.2.1 new Error (message)
The [[Prototype]] internal property of the newly constructed object is set to the original Error prototype object,
the one that is the initial value of Error.prototype (15.11.3.1).
The [[Class]] internal property of the newly constructed Error object is set to "Error".
The [[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message own property of the newly constructed object is set to
ToString(message). Otherwise, the message own property is set to the empty String.
15.11.3

Properties of the Error Constructor

The value of the [[Prototype]] internal property of the Error constructor is the Function prototype object (15.3.4).
Besides the internal properties and the length property (whose value is 1), the Error constructor has thefollowing property:
15.11.3.1 Error.prototype
The initial value of Error.prototype is the Error prototype object (15.11.4).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
15.11.4

Properties of the Error Prototype Object

The Error prototype object is itself an Error object (its [[Class]] is "Error").
The value of the [[Prototype]] internal property of the Error prototype object is the standard built-in Object
prototype object (15.2.4).
15.11.4.1 Error.prototype.constructor
The initial value of Error.prototype.constructor is the built-in Error constructor.
15.11.4.2 Error.prototype.name
The initial value of Error.prototype.name is "Error".
15.11.4.3 Error.prototype.message
The initial value of Error.prototype.message is the empty String.
15.11.4.4 Error.prototype.toString ( )The following steps are taken:© Ecma International 2009

197

1.
2.
3.
4.
5.
6.
7.
8.

Let O be the this value.
If Type(O) is not Object, throw a TypeError exception.
Let name be the result of calling the [[Get]] internal method of O with argument "name".
If name is undefined, then let name be "Error"; else let name be ToString(name).
Let msg be the result of calling the [[Get]] internal method of O with argument "message".
If msg is undefined, then let R be msg.
Else, let R be the result of concatenating name, ":", a single space character, and ToString(msg).
Return R.

15.11.5

Properties of Error Instances

Error instances inherit properties from the Error prototype object and their [[Class]] internal property value is"Error". Error instances have no special properties.
15.11.6

Native Error Types Used in This Standard

One of the NativeError objects below is thrown when a runtime error is detected. All of these objects share thesame structure, as described in 15.11.7.
15.11.6.1 EvalError
This exception is not currently used within this specification. This object remains for compatibility with previous
editions of this specification.
15.11.6.2 RangeError
Indicates a numeric value has exceeded the allowable range. See 15.4.2.2, 15.4.5.1, 15.7.4.2, 15.7.4.5,
15.7.4.6, and 15.7.4.7, 15.9.5.43.
15.11.6.3 ReferenceError
Indicate that an invalid reference value has been detected. See 8.7.1, 8.7.2, 10.2.1, 10.2.1.1.4, 10.2.1.2.4,
and 11.13.1.
15.11.6.4 SyntaxError
Indicates that a parsing error has occurred. See 11.1.5, 11.3.1, 11.3.2, 11.4.1, 11.4.4, 11.4.5, 11.13.1, 11.13.2,
12.2.1, 12.10.1, 12.14.1, 13.1, 15.1.2.1, 15.3.2.1, 15.10.2.2, 15.10.2.5, 15.10.2.9, 15.10.2.15, 15.10.2.19,
15.10.4.1, and 15.12.2.
15.11.6.5 TypeError
Indicates the actual type of an operand is different than the expected type. See 8.6.2, 8.7.2, 8.10.5, 8.12.5,
8.12.7, 8.12.8, 8.12.9, 9.9, 9.10, 10.2.1, 10.2.1.1.3, 10.6, 11.2.2, 11.2.3, 11.4.1, 11.8.6, 11.8.7, 11.3.1, 13.2,
13.2.3, 15, 15.2.3.2, 15.2.3.3, 15.2.3.4, 15.2.3.5, 15.2.3.6, 15.2.3.7, 15.2.3.8, 15.2.3.9, 15.2.3.10, 15.2.3.11,
15.2.3.12, 15.2.3.13, 15.2.3.14, 15.2.4.3, 15.3.4.2, 15.3.4.3, 15.3.4.4, 15.3.4.5, 15.3.4.5.2, 15.3.4.5.3, 15.3.5,
15.3.5.3, 15.3.5.4, 15.4.4.3, 15.4.4.11, 15.4.4.16, 15.4.4.17, 15.4.4.18, 15.4.4.19, 15.4.4.20, 15.4.4.21,
15.4.4.22, 15.4.5.1, 15.5.4.2, 15.5.4.3, 15.6.4.2, 15.6.4.3, 15.7.4, 15.7.4.2, 15.7.4.4, 15.7.4.8, 15.9.5,
15.9.5.44, 15.10.4.1, 15.10.6, 15.11.4.4 and 15.12.3.
15.11.6.6 URIError
Indicates that one of the global URI handling functions was used in a way that is incompatible with its
definition. See 15.1.3.

198© Ecma International 2009

15.11.7

NativeError Object Structure

When an ECMAScript implementation detects a runtime error, it throws an instance of one of the NativeError
objects defined in 15.11.6. Each of these objects has the structure described below, differing only in the nameused as the constructor name instead of NativeError, in the name property of the prototype object, and in theimplementation-defined message property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the appropriate error
object name from 15.11.6.
15.11.7.1 NativeError Constructors Called as Functions
When a NativeError constructor is called as a function rather than as a constructor, it creates and initialises a
new object. A call of the object as a function is equivalent to calling it as a constructor with the same
arguments.
15.11.7.2 NativeError (message)
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for this error
constructor. The [[Class]] internal property of the newly constructed object is set to "Error". The[[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message own property of the newly constructed object is set to
ToString(message).
15.11.7.3 The NativeError Constructors
When a NativeError constructor is called as part of a new expression, it is a constructor: it initialises the newly
created object.
15.11.7.4 New NativeError (message)
The [[Prototype]] internal property of the newly constructed object is set to the prototype object for thisNativeError constructor. The [[Class]] internal property of the newly constructed object is set to "Error". The[[Extensible]] internal property of the newly constructed object is set to true.
If the argument message is not undefined, the message property of the newly constructed object is set to
ToString(message). Otherwise, the message own property is set to the empty String.
15.11.7.5 Properties of the NativeError Constructors
The value of the [[Prototype]] internal property of a NativeError constructor is the Function prototype object(15.3.4).
Besides the internal properties and the length property (whose value is 1), each NativeError constructor has
the following property:
15.11.7.6 NativeError.prototype
The initial value of NativeError.prototype is a NativeError prototype object (15.11.7.7). Each NativeError
constructor has a separate prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

© Ecma International 2009199

15.11.7.7 Properties of the NativeError Prototype Objects
Each NativeError prototype object is an Error object (its [[Class]] is "Error").
The value of the [[Prototype]] internal property of each NativeError prototype object is the standard built-in
Error prototype object (15.11.4).
15.11.7.8 NativeError.prototype.constructor
The initial value of the constructor property of the prototype for a given NativeError constructor is theNativeError constructor function itself (15.11.7).
15.11.7.9 NativeError.prototype.name
The initial value of the name property of the prototype for a given NativeError constructor is the name of theconstructor (the name used instead of NativeError).
15.11.7.10 NativeError.prototype.message
The initial value of the message property of the prototype for a given NativeError constructor is the empty
String.
NOTE
The prototypes for the NativeError constructors do not themselves provide a toString function, but
instances of errors will inherit it from the Error prototype object.

15.11.7.11 Properties of NativeError Instances
NativeError instances inherit properties from their NativeError prototype object and their [[Class]] internal
property value is "Error". NativeError instances have no special properties.

15.12 The JSON Object
The JSON object is a single object that contains two functions, parse and stringify, that are used to parseand construct JSON texts. The JSON Data Interchange Format is described in RFC 4627
<http://www.ietf.org/rfc/rfc4627.txt>. The JSON interchange format used in this specification is exactly that
described by RFC 4627 with two exceptions:•

The top level JSONText production of the ECMAScript JSON grammar may consist of any JSONValuerather than being restricted to being a JSONObject or a JSONArray as specified by RFC 4627.

•

Conforming implementations of JSON.parse and JSON.stringify must support the exact interchange
format described in this specification without any deletions or extensions to the format. This differsfrom RFC 4627 which permits a JSON parser to accept non-JSON forms and extensions.

The value of the [[Prototype]] internal property of the JSON object is the standard built-in Object prototype
object (15.2.4). The value of the [[Class]] internal property of the JSON object is "JSON". The value of the[[Extensible]] internal property of the JSON object is set to true.
The JSON object does not have a [[Construct]] internal property; it is not possible to use the JSON object as aconstructor with the new operator.
The JSON object does not have a [[Call]] internal property; it is not possible to invoke the JSON object as afunction.

200© Ecma International 2009

15.12.1

The JSON Grammar

JSON.stringify produces a String that conforms to the following JSON grammar. JSON.parse accepts a Stringthat conforms to the JSON grammar.
15.12.1.1 The JSON Lexical Grammar
JSON is similar to ECMAScript source text in that it consists of a sequence of characters conforming to therules of SourceCharacter. The JSON Lexical Grammar defines the tokens that make up a JSON text similar to
the manner that the ECMAScript lexical grammar defines the tokens of an ECMAScript source test. The
JSON Lexical grammar only recognizes the white space character specified by the production JSONWhiteSpace.
The JSON lexical grammar shares some productions with the ECMAScript lexical grammar. All nonterminal
symbols of the grammar that do not begin with the characters “JSON” are defined by productions of the
ECMAScript lexical grammar.
SyntaxJSONWhiteSpace ::
<TAB>
<CR>
<LF>
<SP>
JSONString ::"JSONStringCharactersopt "JSONStringCharacters ::JSONStringCharacter JSONStringCharactersopt
JSONStringCharacter ::SourceCharacter but not double-quote " or backslash \ or U+0000 thru U+001F\ JSONEscapeSequence
JSONEscapeSequence ::JSONEscapeCharacter
UnicodeEscapeSequence
JSONEscapeCharacter :: one of" / \ b f n r t
JSONNumber ::-opt DecimalIntegerLiteral JSONFractionopt ExponentPartopt
JSONFraction ::. DecimalDigits
JSONNullLiteral ::NullLiteral
JSONBooleanLiteral ::BooleanLiteral
15.12.1.2 The JSON Syntactic Grammar
The JSON Syntactic Grammar defines a valid JSON text in terms of tokens defined by the JSON lexical
grammar. The goal symbol of the grammar is JSONText.

© Ecma International 2009201

Syntax
JSONText :JSONValue
JSONValue :JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber
JSONObject :{ }{ JSONMemberList }JSONMember :JSONString : JSONValue
JSONMemberList :JSONMember
JSONMemberList , JSONMember
JSONArray :[[ JSONElementList ]]JSONElementList :JSONValue
JSONElementList , JSONValue
15.12.2

parse ( text [ , reviver ] )The parse function parses a JSON text (a JSON-formatted String) and produces an ECMAScript value. The
JSON format is a restricted form of ECMAScript literal. JSON objects are realized as ECMAScript objects.
JSON arrays are realized as ECMAScript arrays. JSON strings, numbers, booleans, and null are realized as
ECMAScript Strings, Numbers, Booleans, and null. JSON uses a more limited set of white space charactersthan WhiteSpace and allows Unicode code points U+2028 and U+2029 to directly appear in JSONString literals
without using an escape sequence. The process of parsing is similar to 11.1.4 and 11.1.5 as constrained by
the JSON grammar.
The optional reviver parameter is a function that takes two parameters, (key and value). It can filter andtransform the results. It is called with each of the key/value pairs produced by the parse, and its return value isused instead of the original value. If it returns what it received, the structure is not modified. If it returns
undefined then the property is deleted from the result.
1.
2.
3.

4.

Let JText be ToString(text).
Parse JText using the grammars in 15.12.1. Throw a SyntaxError exception if JText did not conform to the
JSON grammar for the goal symbol JSONText.
Let unfiltered be the result of parsing and evaluating JText as if it was the source text of an ECMAScript
program but using JSONString in place of StringLiteral. Note that since JText conforms to the JSON
grammar this result will be either a primitive value or an object that is defined by either an ArrayLiteral oran ObjectLiteral.
If IsCallable(reviver) is true, thena. Let root be a new object created as if by the expression new Object(), where Object is thestandard built-in constructor with that name.
b. Call the [[DefineOwnProperty]] internal method of root with the empty String, thePropertyDescriptor {[[Value]]: unfiltered, [[Writable]]: true, [[Enumerable]]: true,[[Configurable]]: true}, and false as arguments.

202© Ecma International 2009

c.
5.

Else
a.

Return the result of calling the abstract operation Walk, passing root and the empty String. The
abstract operation Walk is described below.
Return unfiltered.

The abstract operation Walk is a recursive abstract operation that takes two parameters: a holder object andthe String name of a property in that object. Walk uses the value of reviver that was originally passed to theabove parse function.
1.
2.

3.

Let val be the result of calling the [[Get]] internal method of holder with argument name.
If val is an object, thena. If the [[Class]] internal property of val is "Array"i.
Set I to 0.
ii.
Let len be the result of calling the [[Get]] internal method of val with argument "length".
iii.
Repeat while I < len,
1. Let newElement be the result of calling the abstract operation Walk, passing val and
ToString(I).
2. If newElement is undefined, thena Call the [[Delete]] internal method of val with ToString(I) and false asarguments.
3. Elsea Call the [[DefineOwnProperty]] internal method of val with arguments
ToString(I), the Property Descriptor {[[Value]]: newElement, [[Writable]]:true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
4. Add 1 to I.
b. Elsei.
Let keys be an internal List of String values consisting of the names of all the own
properties of val whose [[Enumerable]] attribute is true. The ordering of the Strings shouldbe the same as that used by the Object.keys standard built-in function.
ii.
For each String P in keys do,
1. Let newElement be the result of calling the abstract operation Walk, passing val andP.
2. If newElement is undefined, thena Call the [[Delete]] internal method of val with P and false as arguments.
3. Elsea Call the [[DefineOwnProperty]] internal method of val with arguments P,
the Property Descriptor {[[Value]]: newElement, [[Writable]]: true,[[Enumerable]]: true, [[Configurable]]: true}, and false.
Return the result of calling the [[Call]] internal method of reviver passing holder as the this value and withan argument list consisting of name and val.

It is not permitted for a conforming implementation of JSON.parse to extend the JSON grammars. If an
implementation wishes to support a modified or extended JSON interchange format it must do so by defining a
different parse function.
NOTE
In the case where there are duplicate name Strings within an object, lexically preceding values for the same
key shall be overwritten.

15.12.3

stringify ( value [ , replacer [ , space ] ] )The stringify function returns a String in JSON format representing an ECMAScript value. It can take threeparameters. The first parameter is required. The value parameter is an ECMAScript value, which is usually anobject or array, although it can also be a String, Boolean, Number or null. The optional replacer parameter iseither a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers thatacts as a white list for selecting the object properties that will be stringified. The optional space parameter is a
String or Number that allows the result to have white space injected into it to improve human readability.

© Ecma International 2009203

These are the steps in stringifying an object:
1.
2.
3.
4.

Let stack be an empty List.
Let indent be the empty String.
Let PropertyList and ReplacerFunction be undefined.
If Type(replacer) is Object, thena. If IsCallable(replacer) is true, theni.
Let ReplacerFunction be replacer.
b. Else if the [[Class]] internal property of replacer is "Array", theni.
Let PropertyList be an empty internal Listii.
For each value v of a property of replacer that has an array index property name. The
properties are enumerated in the ascending array index order of their names.
1. Let item be undefined.
2. If Type(v) is String then let item be v.
3. Else if Type(v) is Number then let item be ToString(v).
4. Else if Type(v) is Object then,a If the [[Class]] internal property of v is "String" or "Number" then let
item be ToString(v).
5. If item is not undefined and item is not currently an element of PropertyList then,a Append item to the end of PropertyList.
5. If Type(space) is Object then,a. If the [[Class]] internal property of space is "Number" then,i.
Let space be ToNumber(space).
b. Else if the [[Class]] internal property of space is "String" then,i.
Let space be ToString(space).
6. If Type(space) is Number
a. Let space be min(10, ToInteger(space)).
b. Set gap to a String containing space space characters. This will be the empty String if space is lessthan 1.
7. Else if Type(space) is Stringa. If the number of characters in space is 10 or less, set gap to space otherwise set gap to a String
consisting of the first 10 characters of space.
8. Elsea. Set gap to the empty String.
9. Let wrapper be a new object created as if by the expression new Object(), where Object is thestandard built-in constructor with that name.
10. Call the [[DefineOwnProperty]] internal method of wrapper with arguments the empty String, the Property
Descriptor {[[Value]]: value, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true}, and false.
11. Return the result of calling the abstract operation Str with the empty String and wrapper.
The abstract operation Str(key, holder) has access to ReplacerFunction from the invocation of the stringify
method. Its algorithm is as follows:
1.
2.

3.
4.

Let value be the result of calling the [[Get]] internal method of holder with argument key.
If Type(value) is Object, thena. Let toJSON be the result of calling the [[Get]] internal method of value with argument "toJSON".
b. If IsCallable(toJSON) is truei.
Let value be the result of calling the [[Call]] internal method of toJSON passing value as thethis value and with an argument list consisting of key.
If ReplacerFunction is not undefined, thena. Let value be the result of calling the [[Call]] internal method of ReplacerFunction passing holder as
the this value and with an argument list consisting of key and value.
If Type(value) is Object then,a. If the [[Class]] internal property of value is "Number" then,i.
Let value be ToNumber(value).
b. Else if the [[Class]] internal property of value is "String" then,i.
Let value be ToString(value).
c. Else if the [[Class]] internal property of value is "Boolean" then,i.
Let value be the value of the [[PrimitiveValue]] internal property of value.

204© Ecma International 2009

5.
6.
7.
8.
9.

If value is null then return "null".
If value is true then return "true".
If value is false then return "false".
If Type(value) is String, then return the result of calling the abstract operation Quote with argument value.
If Type(value) is Number
a. If value is finite then return ToString(value).
b. Else, return "null".
10. If Type(value) is Object, and IsCallable(value) is false
a. If the [[Class]] internal property of value is "Array" theni.
Return the result of calling the abstract operation JA with argument value.
b. Else, return the result of calling the abstract operation JO with argument value.
11. Return undefined.
The abstract operation Quote(value) wraps a String value in double quotes and escapes characters within it.
1.
2.

3.
4.

Let product be the double quote character.
For each character C in value
a. If C is the double quote character or the backslash characteri.
Let product be the concatenation of product and the backslash character.
ii.
Let product be the concatenation of product and C.
b. Else if C is backspace, formfeed, newline, carriage return, or tabi.
Let product be the concatenation of product and the backslash character.
ii.
Let abbrev be the character corresponding to the value of C as follows:backspace"b"formfeed"f"newline"n"carriage return
"r"
tab
"t"
iii.
Let product be the concatenation of product and abbrev.
c. Else if C is a control character having a code unit value less than the space characteri.
Let product be the concatenation of product and the backslash character.
ii.
Let product be the concatenation of product and "u".
iii.
Let hex be the result of converting the numeric code unit value of C to a String of four
hexadecimal digits.
iv.
Let product be the concatenation of product and hex.
d. Elsei.
Let product be the concatenation of product and C.
Let product be the concatenation of product and the double quote character.
Return product.

The abstract operation JO(value) serializes an object. It has access to the stack, indent, gap, PropertyList,
ReplacerFunction, and space of the invocation of the stringify method.
1.
2.
3.
4.
5.
6.

7.
8.

If stack contains value then throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
If PropertyList is not undefined, thena. Let K be PropertyList.
Elsea. Let K be an internal List of Strings consisting of the names of all the own properties of value whose[[Enumerable]] attribute is true. The ordering of the Strings should be the same as that used by the
Object.keys standard built-in function.
Let partial be an empty List.
For each element P of K.
a. Let strP be the result of calling the abstract operation Str with arguments P and value.
b. If strP is not undefinedi.
Let member be the result of calling the abstract operation Quote with argument P.
ii.
Let member be the concatenation of member and the colon character.
iii.
If gap is not the empty String© Ecma International 2009205

1. Let member be the concatenation of member and the space character.
iv.
Let member be the concatenation of member and strP.
v.
Append member to partial.
9. If partial is empty, thena. Let final be "{}".
10. Else
a. If gap is the empty Stringi.
Let properties be a String formed by concatenating all the element Strings of partial witheach adjacent pair of Strings separated with the comma character. A comma is not inserted
either before the first String or after the last String.
ii.
Let final be the result of concatenating "{", properties, and "}".
b. Else gap is not the empty Stringi.
Let separator be the result of concatenating the comma character, the line feed character,and indent.
ii.
Let properties be a String formed by concatenating all the element Strings of partial witheach adjacent pair of Strings separated with separator. The separator String is not inserted
either before the first String or after the last String.
iii.
Let final be the result of concatenating "{", the line feed character, indent, properties, theline feed character, stepback, and "}".
11. Remove the last element of stack.
12. Let indent be stepback.
13. Return final.
The abstract operation JA(value) serializes an array. It has access to the stack, indent, gap, and space of the
invocation of the stringify method. The representation of arrays includes only the elements between zero andarray.length – 1 inclusive. Named properties are excluded from the stringification. An array is stringified as
an open left bracket, elements separated by comma, and a closing right bracket.
1.
2.
3.
4.
5.
6.
7.
8.

If stack contains value then throw a TypeError exception because the structure is cyclical.
Append value to stack.
Let stepback be indent.
Let indent be the concatenation of indent and gap.
Let partial be an empty List.
Let len be the result of calling the [[Get]] internal method of value with argument "length".
Let index be 0.
Repeat while index < lena. Let strP be the result of calling the abstract operation Str with arguments ToString(index) and value.
b. If strP is undefinedi.
Append "null" to partial.
c. Elsei.
Append strP to partial.
d. Increment index by 1.
9. If partial is empty ,thena. Let final be "[]".
10. Else
a. If gap is the empty Stringi.
Let properties be a String formed by concatenating all the element Strings of partial witheach adjacent pair of Strings separated with the comma character. A comma is not inserted
either before the first String or after the last String.
ii.
Let final be the result of concatenating "[", properties, and "]".
b. Elsei.
Let separator be the result of concatenating the comma character, the line feed character,and indent.
ii.
Let properties be a String formed by concatenating all the element Strings of partial witheach adjacent pair of Strings separated with separator. The separator String is not inserted
either before the first String or after the last String.
iii.
Let final be the result of concatenating "[", the line feed character, indent, properties, theline feed character, stepback, and "[".
11. Remove the last element of stack.

206© Ecma International 2009

12. Let indent be stepback.
13. Return final.
NOTE 1
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is or contains a cyclic
structure, then the stringify function must throw a TypeError exception. This is an example of a value that cannot be
stringified:a = [];
a[0] = a;my_text = JSON.stringify(a); // This must throw an TypeError.
NOTE 2••••

Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String null.
The undefined value is not rendered.
The true value is rendered in JSON text as the String true.
The false value is rendered in JSON text as the String false.

NOTE 3String values are wrapped in double quotes. The characters " and \ are escaped with \ prefixes. Control
characters are replaced with escape sequences \uHHHH, or with the shorter forms, \b (backspace), \f (formfeed), \n(newline), \r (carriage return), \t (tab).
NOTE 4
Finite numbers are stringified as if by calling ToString(number). NaN and Infinity regardless of sign are
represented as the String null.
NOTE 5
Values that do not have a JSON representation (such as undefined and functions) do not produce a String.
Instead they produce the undefined value. In arrays these values are represented as the String null. In objects an
unrepresentable value causes the property to be excluded from stringification.
NOTE 6
An object is rendered as an opening left brace followed by zero or more properties, separated with commas,
closed with a right brace. A property is a quoted String representing the key or property name, a colon, and then thestringified property value. An array is rendered as an opening left bracket followed by zero or more values, separated withcommas, closed with a right bracket.

16 Errors
An implementation must report most errors at the time the relevant ECMAScript language construct isevaluated. An early error is an error that can be detected and reported prior to the evaluation of any constructin the Program containing the error. An implementation must report early errors in a Program prior to the firstevaluation of that Program. Early errors in eval code are reported at the time eval is called but prior toevaluation of any construct within the eval code. All errors that are not early errors are runtime errors.
An implementation must treat any instance of the following kinds of errors as an early error:
•
•
•
•
•
•
•
•
•

Any syntax error.
Attempts to define an ObjectLiteral that has multiple get property assignments with the same name or
multiple set property assignments with the same name.
Attempts to define an ObjectLiteral that has both a data property assignment and a get or set property
assignment with the same name.
Errors in regular expression literals that are not implementation-defined syntax extensions.
Attempts in strict mode code to define an ObjectLiteral that has multiple data property assignmentswith the same name.
The occurrence of a WithStatement in strict mode code.
The occurrence of an Identifier value appearing more than once within a FormalParameterList of anindividual strict mode FunctionDeclaration or FunctionExpression.
Improper uses of return, break, and continue.
Attempts to call PutValue on any value for which an early determination can be made that the value is
not a Reference (for example, executing the assignment statement 3=4).

© Ecma International 2009207

An implementation shall not treat other kinds of errors as early errors even if the compiler can prove that a
construct cannot execute without error under any circumstances. An implementation may issue an early
warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
•

•

•

208

An implementation may extend program syntax and regular expression pattern or flagsyntax. To permit this, all operations (such as calling eval, using a regular expression
literal, or using the Function or RegExp constructor) that are allowed to throw
SyntaxError are permitted to exhibit implementation-defined behaviour instead of throwing
SyntaxError when they encounter an implementation-defined extension to the program
syntax or regular expression pattern or flag syntax.
An implementation may provide additional types, values, objects, properties, and functions
beyond those described in this specification. This may cause constructs (such as looking up
a variable in the global scope) to have implementation-defined behaviour instead ofthrowing an error (such as ReferenceError).
An implementation may define behaviour other than throwing RangeError for toFixed,
toExponential, and toPrecision when the fractionDigits or precision argument is
outside the specified range.

© Ecma International 2009

Annex A(informative)
Grammar Summary

A.1 Lexical Grammar
SourceCharacter ::any Unicode code unit

See clause 6

InputElementDiv ::WhiteSpace
LineTerminator
Comment
Token
DivPunctuator

See clause 7

InputElementRegExp ::WhiteSpace
LineTerminator
Comment
Token
RegularExpressionLiteral

See clause 7

WhiteSpace ::
<TAB>
<VT>
<FF>
<SP>
<NBSP><BOM><USP>

See 7.2

LineTerminator ::
<LF>
<CR>
<LS>
<PS>

See 7.3

LineTerminatorSequence ::<LF><CR> [lookahead ∉ <LF> ]
<LS>
<PS>
<CR> <LF>

See 7.3

Comment ::MultiLineComment
SingleLineComment

See 7.4© Ecma International 2009209

MultiLineComment ::/* MultiLineCommentCharsopt */See 7.4

MultiLineCommentChars ::MultiLineNotAsteriskChar MultiLineCommentCharsopt* PostAsteriskCommentCharsopt

See 7.4

PostAsteriskCommentChars ::MultiLineNotForwardSlashOrAsteriskChar MultiLineCommentCharsopt* PostAsteriskCommentCharsopt

See 7.4

MultiLineNotAsteriskChar ::SourceCharacter but not asterisk *See 7.4

MultiLineNotForwardSlashOrAsteriskChar ::SourceCharacter but not forward-slash / or asterisk *See 7.4

SingleLineComment ::// SingleLineCommentCharsopt

See 7.4

SingleLineCommentChars ::SingleLineCommentChar SingleLineCommentCharsopt

See 7.4

SingleLineCommentChar ::SourceCharacter but not LineTerminatorSee 7.4

Token ::
IdentifierName
Punctuator
NumericLiteralStringLiteralSee 7.5

Identifier ::IdentifierName but not ReservedWord

See 7.6

IdentifierName ::IdentifierStart
IdentifierName IdentifierPart

See 7.6

IdentifierStart ::UnicodeLetter$_\ UnicodeEscapeSequence

See 7.6

210© Ecma International 2009

IdentifierPart ::IdentifierStart
UnicodeCombiningMark
UnicodeDigit
UnicodeConnectorPunctuation<ZWNJ><ZWJ>

See 7.6

UnicodeLetter
See 7.6
any character in the Unicode categories “Uppercase letter (Lu)”, “Lowercase letter(Ll)”, “Titlecase letter (Lt)”, “Modifier letter (Lm)”, “Other letter (Lo)”, or “Letter
number (Nl)”.
UnicodeCombiningMark
See 7.6
any character in the Unicode categories “Non-spacing mark (Mn)” or “Combining
spacing mark (Mc)”UnicodeDigit
any character in the Unicode category “Decimal number (Nd)”See 7.6

UnicodeConnectorPunctuation
any character in the Unicode category “Connector punctuation (Pc)”

See 7.6

ReservedWord ::Keyword
FutureReservedWord
NullLiteral
BooleanLiteralSee 7.6.1

Keyword :: one of
break
case
catch
continue
debuggerdefault
deletedo
else
finallyforfunction
ifininstanceof
newreturnswitch
this
throw
try

typeof
var
void
while
with

See 7.6.1.1

FutureReservedWord :: one of
class
const

enum
exportextends
importsuper

privateprotected

publicstaticSee 7.6.1.2

or in strict mode code one of
implements
letinterfacepackageyield© Ecma International 2009211

Punctuator :: one of{}See 7.7
(

)

[

]
<=

.

;

,

<

>

>=

==

!=

===

!==

+

-

*

%

++

--

<<

>>

>>>

&

|

^

!

~

&&

||

?

:

=

+=

-=

*=

%=

<<=

>>=

>>>=

&=

|=

^=

DivPunctuator :: one of//=See 7.7

Literal ::NullLiteral
BooleanLiteral
NumericLiteralStringLiteralSee 7.8

NullLiteral ::null

See 7.8.1

BooleanLiteral ::
true
false

See 7.8.2

NumericLiteral ::DecimalLiteral
HexIntegerLiteralSee 7.8.3

DecimalLiteral ::DecimalIntegerLiteral . DecimalDigitsopt ExponentPartopt. DecimalDigits ExponentPartopt
DecimalIntegerLiteral ExponentPartopt

See 7.8.3

DecimalIntegerLiteral ::0
NonZeroDigit DecimalDigitsopt

See 7.8.3

DecimalDigits ::DecimalDigit
DecimalDigits DecimalDigit

See 7.8.3

DecimalDigit :: one of
0 1 2 3 4212

See 7.8.3
5

6

7

8

9

© Ecma International 2009

ExponentIndicator :: one ofe E

See 7.8.3

SignedInteger ::DecimalDigits+ DecimalDigits- DecimalDigits

See 7.8.3

HexIntegerLiteral ::0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit

See 7.8.3

HexDigit :: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

See 7.8.3

StringLiteral ::" DoubleStringCharactersopt "' SingleStringCharactersopt 'See 7.8.4

DoubleStringCharacters ::DoubleStringCharacter DoubleStringCharactersopt

See 7.8.4

SingleStringCharacters ::SingleStringCharacter SingleStringCharactersopt

See 7.8.4

DoubleStringCharacter ::SourceCharacter but not double-quote " or backslash \ or LineTerminator\ EscapeSequence
LineContinuation

See 7.8.4

SingleStringCharacter ::SourceCharacter but not single-quote ' or backslash \ or LineTerminator\ EscapeSequence
LineContinuation

See 7.8.4

LineContinuation ::\ LineTerminatorSequence

See 7.8.4

EscapeSequence ::CharacterEscapeSequence
0 [lookahead ∉ DecimalDigit]
HexEscapeSequence
UnicodeEscapeSequence

See 7.8.4

CharacterEscapeSequence ::SingleEscapeCharacter
NonEscapeCharacterSee 7.8.4

SingleEscapeCharacter :: one of' " \ b f n r

See 7.8.4
tv

NonEscapeCharacter ::SourceCharacter but not EscapeCharacter or LineTerminator© Ecma International 2009

See 7.8.4

213

EscapeCharacter ::SingleEscapeCharacter
DecimalDigitxu

See 7.8.4

HexEscapeSequence ::x HexDigit HexDigit

See 7.8.4

UnicodeEscapeSequence ::u HexDigit HexDigit HexDigit HexDigit

See 7.8.4

RegularExpressionLiteral ::/ RegularExpressionBody / RegularExpressionFlags

See 7.8.5

RegularExpressionBody ::RegularExpressionFirstChar RegularExpressionCharsSee 7.8.5

RegularExpressionChars ::See 7.8.5[empty]RegularExpressionChars RegularExpressionChar
RegularExpressionFirstChar ::RegularExpressionNonTerminator but not * or \ or / or [RegularExpressionBackslashSequence
RegularExpressionClassSee 7.8.5

RegularExpressionChar ::RegularExpressionNonTerminator but not \ or / or [RegularExpressionBackslashSequence
RegularExpressionClassSee 7.8.5

RegularExpressionBackslashSequence ::\ NonTerminatorSee 7.8.5

RegularExpressionNonTerminator ::SourceCharacter but not LineTerminatorSee 7.8.5

RegularExpressionClass ::[ RegularExpressionClassChars ]See 7.8.5

RegularExpressionClassChars ::See 7.8.5[empty]RegularExpressionClassChars RegularExpressionClassChar
RegularExpressionClassChar ::RegularExpressionNonTerminator but not ] or \RegularExpressionBackslashSequence

See 7.8.5

214© Ecma International 2009

RegularExpressionFlags ::See 7.8.5[empty]RegularExpressionFlags IdentifierPart

A.2 Number Conversions
StringNumericLiteral :::StrWhiteSpaceopt
StrWhiteSpaceopt StrNumericLiteral StrWhiteSpaceopt

See 9.3.1

StrWhiteSpace :::StrWhiteSpaceChar StrWhiteSpaceopt

See 9.3.1

StrWhiteSpaceChar :::WhiteSpace
LineTerminatorSee 9.3.1

StrNumericLiteral :::StrDecimalLiteral
HexIntegerLiteralSee 9.3.1

StrDecimalLiteral :::StrUnsignedDecimalLiteral+ StrUnsignedDecimalLiteral- StrUnsignedDecimalLiteralSee 9.3.1

StrUnsignedDecimalLiteral :::Infinity
DecimalDigits . DecimalDigitsopt ExponentPartopt. DecimalDigits ExponentPartopt
DecimalDigits ExponentPartopt

See 9.3.1

DecimalDigits :::DecimalDigit
DecimalDigits DecimalDigit

See 9.3.1

DecimalDigit ::: one of
0 1 2 3 4

See 9.3.1
5

6

7

8

9

ExponentPart :::ExponentIndicator SignedInteger

See 9.3.1

ExponentIndicator ::: one ofe E

See 9.3.1

SignedInteger :::DecimalDigits+ DecimalDigits- DecimalDigits

See 9.3.1© Ecma International 2009

215

HexIntegerLiteral :::0x HexDigit
0X HexDigit
HexIntegerLiteral HexDigit

See 9.3.1

HexDigit ::: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

See 9.3.1

A.3 Expressions
PrimaryExpression :this
Identifier
LiteralArrayLiteral
ObjectLiteral( Expression )See 11.1

ArrayLiteral :[ Elisionopt ][ ElementList ][ ElementList , Elisionopt ]See 11.1.4

ElementList :Elisionopt AssignmentExpression
ElementList , Elisionopt AssignmentExpression

See 11.1.4

Elision :,Elision ,See 11.1.4

ObjectLiteral :{ }{ PropertyNameAndValueList }{ PropertyNameAndValueList , }See 11.1.5

PropertyNameAndValueList :PropertyAssignment
PropertyNameAndValueList , PropertyAssignment

See 11.1.5

PropertyAssignment :PropertyName : AssignmentExpression
get PropertyName() { FunctionBody }set PropertyName( PropertySetParameterList ) { FunctionBody }See 11.1.5

PropertyName :IdentifierName
StringLiteral
NumericLiteralSee 11.1.5

PropertySetParameterList :Identifier

See 11.1.5

216© Ecma International 2009

MemberExpression :PrimaryExpression
FunctionExpression
MemberExpression [ Expression ]MemberExpression . IdentifierName
new MemberExpression Arguments

See 11.2

NewExpression :MemberExpression
new NewExpression

See 11.2

CallExpression :MemberExpression Arguments
CallExpression Arguments
CallExpression [ Expression ]CallExpression . IdentifierName

See 11.2

Arguments :()( ArgumentList )See 11.2

ArgumentList :AssignmentExpression
ArgumentList , AssignmentExpression

See 11.2

LeftHandSideExpression :NewExpression
CallExpression

See 11.2

PostfixExpression :LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++LeftHandSideExpression [no LineTerminator here] --See 11.3

UnaryExpression :PostfixExpression
delete UnaryExpression
void UnaryExpression
typeof UnaryExpression++ UnaryExpression-- UnaryExpression+ UnaryExpression- UnaryExpression~ UnaryExpression! UnaryExpression

See 11.4

MultiplicativeExpression :UnaryExpression
MultiplicativeExpression * UnaryExpression
MultiplicativeExpression / UnaryExpression
MultiplicativeExpression % UnaryExpression

See 11.5© Ecma International 2009217

AdditiveExpression :MultiplicativeExpression
AdditiveExpression + MultiplicativeExpression
AdditiveExpression - MultiplicativeExpression

See 11.6

ShiftExpression :AdditiveExpression
ShiftExpression << AdditiveExpression
ShiftExpression >> AdditiveExpression
ShiftExpression >>> AdditiveExpression

See 11.7

RelationalExpression :ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ShiftExpression
RelationalExpression in ShiftExpression

See 11.8

RelationalExpressionNoIn :ShiftExpression
RelationalExpressionNoIn < ShiftExpression
RelationalExpressionNoIn > ShiftExpression
RelationalExpressionNoIn <= ShiftExpression
RelationalExpressionNoIn >= ShiftExpression
RelationalExpressionNoIn instanceof ShiftExpression

See 11.8

EqualityExpression :RelationalExpression
EqualityExpression == RelationalExpression
EqualityExpression != RelationalExpression
EqualityExpression === RelationalExpression
EqualityExpression !== RelationalExpression

See 11.9

EqualityExpressionNoIn :RelationalExpressionNoIn
EqualityExpressionNoIn == RelationalExpressionNoIn
EqualityExpressionNoIn != RelationalExpressionNoIn
EqualityExpressionNoIn === RelationalExpressionNoIn
EqualityExpressionNoIn !== RelationalExpressionNoIn

See 11.9

BitwiseANDExpression :EqualityExpression
BitwiseANDExpression & EqualityExpression

See 11.10

BitwiseANDExpressionNoIn :EqualityExpressionNoIn
BitwiseANDExpressionNoIn & EqualityExpressionNoIn

See 11.10

218© Ecma International 2009

BitwiseXORExpression :BitwiseANDExpression
BitwiseXORExpression ^ BitwiseANDExpression

See 11.10

BitwiseXORExpressionNoIn :BitwiseANDExpressionNoIn
BitwiseXORExpressionNoIn ^ BitwiseANDExpressionNoIn

See 11.10

BitwiseORExpression :BitwiseXORExpression
BitwiseORExpression | BitwiseXORExpression

See 11.10

BitwiseORExpressionNoIn :BitwiseXORExpressionNoIn
BitwiseORExpressionNoIn | BitwiseXORExpressionNoIn

See 11.10

LogicalANDExpression :BitwiseORExpression
LogicalANDExpression && BitwiseORExpression

See 11.11

LogicalANDExpressionNoIn :BitwiseORExpressionNoIn
LogicalANDExpressionNoIn && BitwiseORExpressionNoIn

See 11.11

LogicalORExpression :LogicalANDExpression
LogicalORExpression || LogicalANDExpression

See 11.11

LogicalORExpressionNoIn :LogicalANDExpressionNoIn
LogicalORExpressionNoIn || LogicalANDExpressionNoIn

See 11.11

ConditionalExpression :LogicalORExpression
LogicalORExpression ? AssignmentExpression : AssignmentExpression

See 11.12

ConditionalExpressionNoIn :LogicalORExpressionNoIn
LogicalORExpressionNoIn ? AssignmentExpressionNoIn : AssignmentExpressionNoIn

See 11.12

AssignmentExpression :ConditionalExpression
LeftHandSideExpression AssignmentOperator AssignmentExpression

See 11.13

AssignmentExpressionNoIn :ConditionalExpressionNoIn
LeftHandSideExpression AssignmentOperator AssignmentExpressionNoIn

See 11.13

AssignmentOperator : one of=*=/=© Ecma International 2009

%=

+=

-=

<<=

>>=

>>>= &=^=

See 11.13|=

219

Expression :AssignmentExpression
Expression , AssignmentExpression

See 11.14

ExpressionNoIn :AssignmentExpressionNoIn
ExpressionNoIn , AssignmentExpressionNoIn

See 11.14

A.4 Statements
Statement :Block
VariableStatement
EmptyStatement
ExpressionStatement
IfStatement
IterationStatement
ContinueStatement
BreakStatement
ReturnStatement
WithStatement
LabelledStatement
SwitchStatement
ThrowStatement
TryStatement
DebuggerStatement

See clause 12

Block :

See 12.1{ StatementListopt }StatementList :Statement
StatementList Statement

See 12.1

VariableStatement :var VariableDeclarationList ;See 12.2

VariableDeclarationList :VariableDeclaration
VariableDeclarationList , VariableDeclarationSee 12.2

VariableDeclarationListNoIn :VariableDeclarationNoIn
VariableDeclarationListNoIn , VariableDeclarationNoIn

See 12.2

VariableDeclaration :Identifier Initialiseropt

See 12.2

VariableDeclarationNoIn :Identifier InitialiserNoInopt

See 12.2

220© Ecma International 2009

Initialiser := AssignmentExpression

See 12.2

InitialiserNoIn := AssignmentExpressionNoIn

See 12.2

EmptyStatement :;See 12.3

ExpressionStatement :See 12.4[lookahead ∉ {{, function}] Expression ;IfStatement :if ( Expression ) Statement else Statement
if ( Expression ) Statement

See 12.5

IterationStatement :do Statement while ( Expression );while ( Expression ) Statementfor (ExpressionNoInopt; Expressionopt ; Expressionopt ) Statementfor ( var VariableDeclarationListNoIn; Expressionopt ; Expressionopt ) Statementfor ( LeftHandSideExpression in Expression ) Statementfor ( var VariableDeclarationNoIn in Expression ) Statement

See 12.6

ContinueStatement :continue [no LineTerminator here] Identifieropt ;See 12.7

BreakStatement :break [no LineTerminator here] Identifieropt ;See 12.8

ReturnStatement :return [no LineTerminator here] Expressionopt ;See 12.9

WithStatement :with ( Expression ) Statement

See 12.10

SwitchStatement :switch ( Expression ) CaseBlockSee 12.11

CaseBlock :{ CaseClausesopt }{ CaseClausesopt DefaultClause CaseClausesopt }See 12.11

CaseClauses :CaseClause
CaseClauses CaseClauseSee 12.11© Ecma International 2009221

CaseClause :case Expression : StatementListopt

See 12.11

DefaultClause :default : StatementListopt

See 12.11

LabelledStatement :Identifier : Statement

See 12.12

ThrowStatement :throw [no LineTerminator here] Expression ;See 12.13

TryStatement :try Block Catchtry Block Finallytry Block Catch FinallySee 12.14

Catch :
catch ( Identifier ) BlockSee 12.14

Finally :finally BlockSee 12.14

DebuggerStatement :debugger ;See 12.15

A.5 Functions and Programs
FunctionDeclaration :function Identifier ( FormalParameterListopt ) { FunctionBody }See clause 13

FunctionExpression :function Identifieropt ( FormalParameterListopt ) { FunctionBody }See clause 13

FormalParameterList :Identifier
FormalParameterList , Identifier

See clause 13

FunctionBody :SourceElementsopt

See clause 13

Program :SourceElementsopt

See clause 14

SourceElements :SourceElement
SourceElements SourceElement

See clause 14

222© Ecma International 2009

SourceElement :Statement
FunctionDeclaration

See clause 14

A.6 Universal Resource Identifier Character Classes
uri :::

See 15.1.3
uriCharactersopt

uriCharacters :::uriCharacter uriCharactersopt

See 15.1.3

uriCharacter :::uriReserved
uriUnescaped
uriEscapedSee 15.1.3

uriReserved ::: one of; / ? : @See 15.1.3
&

=

+

$

,

uriUnescaped :::uriAlpha
DecimalDigit
uriMarkSee 15.1.3

uriEscaped :::% HexDigit HexDigit

See 15.1.3

uriAlpha ::: one ofa b c dA B C D

e
E

f
F

g
G

h
H

i
I

uriMark ::: one of- _ . !

~

*

‘

(

)

j
J

k
K

l
L

m
M

n
N

o
O

p
P

q
Q

r
R

s
S

t
T

u
U

See 15.1.3
v w x
V W X

y
Y

z
Z

See 15.1.3

A.7 Regular Expressions
Pattern ::Disjunction

See 15.10.1

Disjunction ::Alternative
Alternative | Disjunction

See 15.10.1

Alternative ::See 15.10.1[empty]Alternative Term

© Ecma International 2009223

Term ::
Assertion
Atom
Atom Quantifier

See 15.10.1

Assertion ::
^
$
\ b
\ B
( ? = Disjunction )( ? ! Disjunction )See 15.10.1

Quantifier ::QuantifierPrefix
QuantifierPrefix ?

See 15.10.1

QuantifierPrefix ::*+?
{ DecimalDigits }{ DecimalDigits , }{ DecimalDigits , DecimalDigits }See 15.10.1

Atom ::
PatternCharacter.
\ AtomEscapeCharacterClass( Disjunction )( ? : Disjunction )See 15.10.1

PatternCharacter :: SourceCharacter but not any of:^ $ \ . * + ? ( ) [ ] {See 15.10.1}|

AtomEscape ::DecimalEscapeCharacterEscapeCharacterClassEscapeSee 15.10.1

CharacterEscape ::ControlEscapec ControlLetter
HexEscapeSequence
UnicodeEscapeSequence
IdentityEscapeSee 15.10.1

ControlEscape :: one off n r t v

See 15.10.1

224© Ecma International 2009

ControlLetter :: one ofa b c d e
A B C D E

f
F

g
G

h
H

i
I

j
J

k
K

l
L

m
M

n
N

o
O

p
P

q
Q

r
R

s
S

t
T

u
U

See 15.10.1
v w x
V W X

IdentityEscape ::SourceCharacter but not IdentifierPart<ZWJ><ZWNJ>

See 15.10.1

DecimalEscape ::DecimalIntegerLiteral [lookahead ∉ DecimalDigit]

See 15.10.1

CharacterClassEscape :: one ofd D s S w W

See 15.10.1

CharacterClass ::[ [lookahead ∉ {^}] ClassRanges ][ ^ ClassRanges ]See 15.10.1

ClassRanges ::See 15.10.1

y
Y

z
Z

[empty]NonemptyClassRanges
NonemptyClassRanges ::ClassAtomClassAtom NonemptyClassRangesNoDash
ClassAtom – ClassAtom ClassRanges

See 15.10.1

NonemptyClassRangesNoDash ::ClassAtomClassAtomNoDash NonemptyClassRangesNoDash
ClassAtomNoDash – ClassAtom ClassRanges

See 15.10.1

ClassAtom ::ClassAtomNoDash

See 15.10.1

ClassAtomNoDash ::SourceCharacter but not one of \ or ] or \ ClassEscapeSee 15.10.1

ClassEscape ::DecimalEscapebCharacterEscapeCharacterClassEscapeSee 15.10.1© Ecma International 2009225

A.8 JSON
A.8.1 JSON Lexical Grammar
JSONWhiteSpace ::
<TAB>
<CR>
<LF>
<SP>

See 15.12.1.1

JSONString ::" JSONStringCharactersopt "See 15.12.1.1

JSONStringCharacters ::JSONStringCharacter JSONStringCharactersopt

See 15.12.1.1

JSONStringCharacter ::See 15.12.1.1
JSONSourceCharacter but not double-quote " or backslash \ or U+0000 thru U+001F\ JSONEscapeSequence
JSONEscapeSequence ::JSONEscapeCharacter
UnicodeEscapeSequence

See 15.12.1.1

JSONEscapeCharacter :: one of" / \ b f n r t

See 15.12.1.1

JSONNumber ::-opt DecimalIntegerLiteral JSONFractionopt ExponentPartopt

See 15.12.1.1

JSONFraction ::. DecimalDigits

See 15.12.1.1

JSONNullLiteral ::NullLiteralSee 15.12.1.1

JSONBooleanLiteral ::BooleanLiteralSee 15.12.1.1

A.8.2 JSON Syntactic Grammar
JSONText :JSONValueSee 15.12.1.2

JSONValue :JSONNullLiteral
JSONBooleanLiteral
JSONObject
JSONArray
JSONString
JSONNumber

See 15.12.1.2

JSONObject :{ }{ JSONMemberList }See 15.12.1.2

JSONMember :JSONString : JSONValueSee 15.12.1.2

226© Ecma International 2009

JSONMemberList :JSONMember
JSONMemberList , JSONMember

See 15.12.1.2

JSONArray :[ ][ JSONElementList ]See 15.12.1.2

JSONElementList :JSONValue
JSONElementList , JSONValueSee 15.12.1.2© Ecma International 2009

227

228

© Ecma International 2009

Annex B(informative)
Compatibility

B.1 Additional Syntax
Past editions of ECMAScript have included additional syntax and semantics for specifying octal literals andoctal escape sequences. These have been removed from this edition of ECMAScript. This non-normative
annex presents uniform syntax and semantics for octal literals and octal escape sequences for compatibility
with some older ECMAScript programs.

B.1.1 Numeric Literals
The syntax and semantics of 7.8.3 can be extended as follows except that this extension is not allowed forstrict mode code:
SyntaxNumericLiteral ::DecimalLiteral
HexIntegerLiteral
OctalIntegerLiteral
OctalIntegerLiteral ::0 OctalDigit
OctalIntegerLiteral OctalDigit
OctalDigit :: one of
0 1 2 3 4 5 6 7
Semantics
•
•
•
•
•
•
•
•
•
•
•

The MV of NumericLiteral :: OctalIntegerLiteral is the MV of OctalIntegerLiteral.
The MV of OctalDigit :: 0 is 0.
The MV of OctalDigit :: 1 is 1.
The MV of OctalDigit :: 2 is 2.
The MV of OctalDigit :: 3 is 3.
The MV of OctalDigit :: 4 is 4.
The MV of OctalDigit :: 5 is 5.
The MV of OctalDigit :: 6 is 6.
The MV of OctalDigit :: 7 is 7.
The MV of OctalIntegerLiteral :: 0 OctalDigit is the MV of OctalDigit.
The MV of OctalIntegerLiteral :: OctalIntegerLiteral OctalDigit is (the MV of OctalIntegerLiteral
times 8) plus the MV of OctalDigit.

B.1.2 String Literals
The syntax and semantics of 7.8.4 can be extended as follows except that this extension is not allowed forstrict mode code:© Ecma International 2009229

Syntax
EscapeSequence ::CharacterEscapeSequence
OctalEscapeSequence
HexEscapeSequence
UnicodeEscapeSequence
OctalEscapeSequence ::OctalDigit [lookahead ∉ DecimalDigit]
ZeroToThree OctalDigit [lookahead ∉ DecimalDigit]
FourToSeven OctalDigit
ZeroToThree OctalDigit OctalDigit
ZeroToThree :: one of
0 1 2 3
FourToSeven :: one of
4 5 6 7
Semantics• The CV of EscapeSequence :: OctalEscapeSequence is the CV of the OctalEscapeSequence.
• The CV of OctalEscapeSequence :: OctalDigit [lookahead ∉ DecimalDigit] is the character whose code
unit value is the MV of the OctalDigit.
• The CV of OctalEscapeSequence :: ZeroToThree OctalDigit [lookahead ∉ DecimalDigit] is the character
whose code unit value is (8 times the MV of the ZeroToThree) plus the MV of the OctalDigit.
• The CV of OctalEscapeSequence :: FourToSeven OctalDigit is the character whose code unit valueis (8 times the MV of the FourToSeven) plus the MV of the OctalDigit.
• The CV of OctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is the character whose code
unit value is (64 (that is, 82) times the MV of the ZeroToThree) plus (8 times the MV of the first
OctalDigit) plus the MV of the second OctalDigit.
• The MV of ZeroToThree :: 0 is 0.
• The MV of ZeroToThree :: 1 is 1.
• The MV of ZeroToThree :: 2 is 2.
• The MV of ZeroToThree :: 3 is 3.
• The MV of FourToSeven :: 4 is 4.
• The MV of FourToSeven :: 5 is 5.
• The MV of FourToSeven :: 6 is 6.
• The MV of FourToSeven :: 7 is 7.

B.2 Additional Properties
Some implementations of ECMAScript have included additional properties for some of the standard native
objects. This non-normative annex suggests uniform semantics for such properties without making theproperties or their semantics part of this standard.

B.2.1 escape (string)
The escape function is a property of the global object. It computes a new version of a String value in which
certain characters have been replaced by a hexadecimal escape sequence.
For those characters being replaced whose code unit value is 0xFF or less, a two-digit escape sequence ofthe form %xx is used. For those characters being replaced whose code unit value is greater than 0xFF, a fourdigit escape sequence of the form %uxxxx is used

230

© Ecma International 2009

When the escape function is called with one argument string, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.

Call ToString(string).
Compute the number of characters in Result(1).
Let R be the empty string.
Let k be 0.
If k equals Result(2), return R.
Get the character (represented as a 16-bit unsigned integer) at position k within Result(1).
If Result(6) is one of the 69 nonblank characters“ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./”
then go to step 13.
8. If Result(6), is less than 256, go to step 11.
9. Let S be a String containing six characters “%uwxyz” where wxyz are four hexadecimal digits encoding thevalue of Result(6).
10. Go to step 14.
11. Let S be a String containing three characters “%xy” where xy are two hexadecimal digits encoding the valueof Result(6).
12. Go to step 14.
13. Let S be a String containing the single character Result(6).
14. Let R be a new String value computed by concatenating the previous value of R and S.
15. Increase k by 1.
16. Go to step 5.
NOTE
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified in thisstandard is described above without regard to the contents of RFC 1738.

B.2.2 unescape (string)
The unescape function is a property of the global object. It computes a new version of a String value in which
each escape sequence of the sort that might be introduced by the escape function is replaced with thecharacter that it represents.
When the unescape function is called with one argument string, the following steps are taken:
1. Call ToString(string).
2. Compute the number of characters in Result(1).
3. Let R be the empty String.
4. Let k be 0.
5. If k equals Result(2), return R.
6. Let c be the character at position k within Result(1).
7. If c is not %, go to step 18.
8. If k is greater than Result(2)−6, go to step 14.
9. If the character at position k+1 within Result(1) is not u, go to step 14.
10. If the four characters at positions k+2, k+3, k+4, and k+5 within Result(1) are not all hexadecimal digits, go
to step 14.
11. Let c be the character whose code unit value is the integer represented by the four hexadecimal digits at
positions k+2, k+3, k+4, and k+5 within Result(1).
12. Increase k by 5.
13. Go to step 18.
14. If k is greater than Result(2)−3, go to step 18.
15. If the two characters at positions k+1 and k+2 within Result(1) are not both hexadecimal digits, go to step18.
16. Let c be the character whose code unit value is the integer represented by two zeroes plus the twohexadecimal digits at positions k+1 and k+2 within Result(1).
17. Increase k by 2.
18. Let R be a new String value computed by concatenating the previous value of R and c.
19. Increase k by 1.
20. Go to step 5.

© Ecma International 2009231

B.2.3 String.prototype.substr (start, length)
The substr method takes two arguments, start and length, and returns a substring of the result of converting
the this object to a String, starting from character position start and running for length characters (or throughthe end of the String if length is undefined). If start is negative, it is treated as (sourceLength+start) where
sourceLength is the length of the String. The result is a String value, not a String object. The following steps aretaken:
1.
2.
3.
4.
5.
6.
7.
8.

Call ToString, giving it the this value as its argument.
Call ToInteger(start).
If length is undefined, use +∞; otherwise call ToInteger(length).
Compute the number of characters in Result(1).
If Result(2) is positive or zero, use Result(2); else use max(Result(4)+Result(2),0).
Compute min(max(Result(3),0), Result(4)–Result(5)).
If Result(6) ≤ 0, return the empty String “”.
Return a String containing Result(6) consecutive characters from Result(1) beginning with the character at
position Result(5).

The length property of the substr method is 2.
NOTE
The substr function is intentionally generic; it does not require that its this value be a String object.
Therefore it can be transferred to other kinds of objects for use as a method.

B.2.4 Date.prototype.getYear ( )NOTEThe getFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the getYear method is called with no arguments, the following steps are taken:
1.
2.
3.

Let t be this time value.
If t is NaN, return NaN.
Return YearFromTime(LocalTime(t)) − 1900.

B.2.5 Date.prototype.setYear (year)
NOTE

The setFullYear method is preferred for nearly all purposes, because it avoids the “year 2000 problem.”

When the setYear method is called with one argument year, the following steps are taken:
1.
2.
3.
4.
5.
6.
7.
8.

Let t be the result of LocalTime(this time value); but if this time value is NaN, let t be +0.
Call ToNumber(year).
If Result(2) is NaN, set the [[PrimitiveValue]] internal property of the this value to NaN and return NaN.
If Result(2) is not NaN and 0 ≤ ToInteger(Result(2)) ≤ 99 then Result(4) is ToInteger(Result(2)) + 1900.
Otherwise, Result(4) is Result(2).
Compute MakeDay(Result(4), MonthFromTime(t), DateFromTime(t)).
Compute UTC(MakeDate(Result(5), TimeWithinDay(t))).
Set the [[PrimitiveValue]] internal property of the this value to TimeClip(Result(6)).
Return the value of the [[PrimitiveValue]] internal property of the this value.

B.2.6 Date.prototype.toGMTString ( )NOTEThe property toUTCString is preferred. The toGMTString property is provided principally for compatibility
with old code. It is recommended that the toUTCString property be used in new ECMAScript code.

The Function object that is the initial value of Date.prototype.toGMTString is the same Function object
that is the initial value of Date.prototype.toUTCString.

232© Ecma International 2009

Annex C(informative)
The Strict Mode of ECMAScript

The strict mode restriction and exceptions•

A conforming implementation, when processing strict mode code, may not extend the syntax
of NumericLiteral (7.8.3) to include OctalIntegerLiteral as described in B.1.1.

•

A conforming implementation, when processing strict mode code (see 10.1.1), may notextend the syntax of EscapeSequence to include OctalEscapeSequence as described in B.1.2.

•

Assignment to an undeclared identifier or otherwise unresolvable reference does not create aproperty in the global object. When a simple assignment occurs within strict mode code, its
LeftHandSide must not evaluate to an unresolvable Reference. If it does a ReferenceError
exception is thrown (8.7.2). The LeftHandSide also may not be a reference to a data property
with the attribute value {[[Writable]]:false}, to an accessor property with the attribute value{[[Set]]:undefined}, nor to a non-existent property of an object whose [[Extensible]] internal
property has the value false. In these cases a TypeError exception is thrown (11.13.1).

•

The identifier eval or arguments may not appear as the LeftHandSideExpression of an
Assignment operator (11.13) or of a PostfixExpression (11.3) or as the UnaryExpression
operated upon by a Prefix Increment (11.4.4) or a Prefix Decrement (11.4.5) operator.

•

Arguments objects for strict mode functions define non-configurable accessor properties
named "caller" and "callee" which throw a TypeError exception on access (10.6).

•

Arguments objects for strict mode functions do not dynamically share their array indexed
property values with the corresponding formal parameter bindings of their functions. (10.6).

•

For strict mode functions, if an arguments object is created the binding of the local identifierarguments to the arguments object is immutable and hence may not be the target of anassignment expression. (10.5).

•

It is a SyntaxError if strict mode code contains an ObjectLiteral with more than one definitionof any data property (11.1.5).

•

It is a SyntaxError if the Identifier "eval" or the Identifier "arguments" occurs as the
Identifier in a PropertySetParameterList of a PropertyAssignment that is contained in strict code
or if its FunctionBody is strict code (11.1.5).

•

Strict mode eval code cannot instantiate variables or functions in the variable environment of
the caller to eval. Instead, a new variable environment is created and that environment isused for declaration binding instantiation for the eval code (10.4.2).

•

If this is evaluated within strict mode code, then the this value is not coerced to an object. A
this value of null or undefined is not converted to the global object and primitive values are
not converted to wrapper objects. The this value passed via a function call (including calls
made using Function.prototype.apply and Function.prototype.call) do notcoerce the passed this value to an object (10.4.3, 11.1.1, 15.3.4.3, 15.3.4.4).

•

When a delete operator occurs within strict mode code, a SyntaxError is thrown if its
UnaryExpression is a direct reference to a variable, function argument, or function
name(11.4.1).

•

When a delete operator occurs within strict mode code, a TypeError is thrown if theproperty to be deleted has the attribute { [[Configurable]]:false } (11.4.1).

•

It is a SyntaxError if a VariableDeclaration or VariableDeclarationNoIn occurs within strict codeand its Identifier is eval or arguments (12.2.1).

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•

Strict mode code may not include a WithStatement. The occurrence of a WithStatement in sucha context is an SyntaxError (12.10).

•

It is a SyntaxError if a TryStatement with a Catch occurs within strict code and the Identifier of theCatch production is eval or arguments (12.14.1)•

It is a SyntaxError if the identifier eval or arguments appears within a FormalParameterList of astrict mode FunctionDeclaration or FunctionExpression (13.1)•

A strict mode function may not have two or more formal parameters that have the same
name. An attempt to create such a function using a FunctionDeclaration, FunctionExpression, orFunction constructor is a SyntaxError (13.1, 15.3.2).

•

An implementation may not associate special meanings within strict mode functions toproperties named caller or arguments of function instances. ECMAScript code may notcreate or modify properties with these names on function objects that correspond to strictmode functions (13.2).

•

It is a SyntaxError to use within strict mode code the identifiers eval or arguments as the
Identifier of a FunctionDeclaration or FunctionExpression or as a formal parameter name (13.1).
Attempting to dynamically define such a strict mode function using the Function constructor(15.3.2) will throw a SyntaxError exception.

© Ecma International 2009

Annex D(informative)
Corrections and Clarifications in the 5th Edition
with Possible 3rd Edition Compatibility Impact

Throughout: In the Edition 3 specification the meaning of phrases such as “as if by the expression new
Array()” are subject to misinterpretation. In the Edition 5 specification text for all internal references andinvocations of standard built-in objects and methods has been clarified by making it explicit that the intent isthat the actual built-in object is to be used rather than the current dynamic value of the correspondingly named
property.
11.8.2, 11.8.3, 11.8.5: ECMAScript generally uses a left to right evaluation order, however the Edition 3
specification language for the > and <= operators resulted in a partial right to left order. The specification has
been corrected for these operators such that it now specifies a full left to right evaluation order. However, thischange of order is potentially observable if side-effects occur during the evaluation process.
11.1.4: Edition 5 clarifies the fact that a trailing comma at the end of an ArrayInitialiser does not add to thelength of the array. This is not a semantic change from Edition 3 but some implementations may havepreviously misinterpreted this.
11.2.3: Edition 5 reverses the order of steps 2 and 3 of the algorithm. The original order as specified in
Editions 1 through 3 was incorrectly specified such that side-effects of evaluating Arguments could affect theresult of evaluating MemberExpression.
12.4: In Edition 3, an object is created, as if by new Object()to serve as the scope for resolving the name ofthe exception parameter passed to a catch clause of a try statement. If the actual exception object is afunction and it is called from within the catch clause, the scope object will be passed as the this value of thecall. The body of the function can then define new properties on its this value and those property names
become visible identifiers bindings within the scope of the catch clause after the function returns. In Edition 5,
when an exception parameter is called as a function, undefined is passed as the this value.
13:. In Edition 3, the algorithm for the production FunctionExpression with an Identifier adds an object createdas if by new Object() to the scope chain to serve as a scope for looking up the name of the function. The
identifier resolution rules (10.1.4 in Edition 3) when applied to such an object will, if necessary, follow theobject’s prototype chain when attempting to resolve an identifier. This means all the properties of
Object.prototype are visible as identifiers within that scope. In practice most implementations of Edition 3
have not implemented this semantics. Edition 5 changes the specified semantics by using a Declarative
Environment Record to bind the name of the function.
14:. In Edition 3, the algorithm for the production SourceElements : SourceElements SourceElement did notcorrectly propagate statement result values in the same manner as Block. This could result in the evalfunction producing an incorrect result when evaluating a Program text. In practice most implementations of
Edition 3 have implemented the correct propagation rather than what was specified in Edition 3.
15.10.6: RegExp.prototype is now a RegExp object rather than an instance of Object. The value of its [[Class]]
internal property which is observable using Object.prototype.toString is now “RegExp” rather than “Object”.

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© Ecma International 2009

Annex E(informative)
Additions and Changes in the 5th Edition that
Introduce Incompatibilities with the 3rd Edition

7.1: Unicode format control characters are no longer stripped from ECMAScript source text before processing.
In Edition 5, if such a character appears in a StringLiteral or RegularExpressionLiteral the character will beincorporated into the literal where in Edition 3 the character would not be incorporated into the literal.
7.2: Unicode character <BOM> is now treated as whitespace and its presence in the middle of what appears
to be an identifier could result in a syntax error which would not have occurred in Edition 3
7.3: Line terminator characters that are preceded by an escape sequence are now allowed within a string
literal token. In Edition 3 a syntax error would have been produced.
7.8.5: Regular expression literals now return a unique object each time the literal is evaluated. This change is
detectable by any programs that test the object identity of such literal values or that are sensitive to the sharedside effects.
7.8.5: Edition 5 requires early reporting of any possible RegExp constructor errors that would be produced
when converting a RegularExpressionLiteral to a RegExp object. Prior to Edition 5 implementations werepermitted to defer the reporting of such errors until the actual execution time creation of the object.
7.8.5: In Edition 5 unescaped “/” characters may appear as a CharacterClass in a regular expression literal. In
Edition 3 such a character would have been interpreted as the final character of the literal.
10.4.2: In Edition 5, indirect calls to the eval function use the global environment as both the variable
environment and lexical environment for the eval code. In Edition 3, the variable and lexical environments ofthe caller of an indirect eval was used as the environments for the eval code.
15.4.4: In Edition 5 all methods of Array.prototype are intentionally generic. In Edition 3 toString andtoLocaleString were not generic and would throw a TypeError exception if applied to objects that were
not instances of Array.
10.6: In Edition 5 the array indexed properties of argument objects that correspond to actual formal
parameters are enumerable. In Edition 3, such properties were not enumerable.
10.6: In Edition 5 the value of the [[Class]] internal property of an arguments object is "Arguments". In
Edition 3, it was "Object". This is observable if toString is called as a method of an arguments object.
12.6.4: for-in statements no longer throw a TypeError if the in expression evaluates to null or undefined.
Instead, the statement behaves as if the value of the expression was an object with no enumerable properties.
15: In Edition 5, the following new properties are defined on built-in objects that exist in Edition 3:
Object.getPrototypeOf, Object.getOwnPropertyDescriptor, Object.getOwnPropertyNames,
Object.create, Object.defineProperty, Object.defineProperties, Object.seal,
Object.freeze, Object.preventExtensions, Object.isSealed, Object.isFrozen,
Object.isExtensible, Object.keys, Function.prototype.bind, Array.prototype.indexOf,
Array.prototype.lastIndexOf, Array.prototype.every, Array.prototype.some,Array.prototype.forEach, Array.prototype.map, Array.prototype.filter,
Array.prototype.reduce, Array.prototype.reduceRight, String.prototype.trim, Date.now,
Date.prototype.toISOString, Date.prototype.toJSON.

© Ecma International 2009237

15: Implementations are now required to ignore extra arguments to standard built-in methods unlessotherwise explicitly specified. In Edition 3 the handling of extra arguments was unspecified andimplementations were explicitly allowed to throw a TypeError exception.
15.1.1: The value properties NaN, Infinity, and undefined of the Global Object have been changed to beread-only properties.
15.1.2.1. Implementations are no longer permitted to restrict the use of eval in ways that are not a direct call.
In addition, any invocation of eval that is not a direct call uses the global environment as its variable
environment rather than the caller’s variable environment.
15.1.2.2: The specification of the function parseInt no longer allows implementations to treat Strings
beginning with a 0 character as octal values.
15.3.4.3: In Edition 3, a TypeError is thrown if the second argument passed toFunction.prototype.apply is neither an array object nor an arguments object. In Edition 5, the second
argument may be any kind of generic array-like object that has a valid length property.
15.3.4.3, 15.3.4.4: In Edition 3 passing undefined or null as the first argument to either
Function.prototype.apply or Function.prototype.call causes the global object to be passed tothe indirectly invoked target function as the this value. If the first argument is a primitive value the result of
calling ToObject on the primitive value is passed as the this value. In Edition 5, these transformations are not
performed and the actual first argument value is passed as the this value. This difference will normally beunobservable to existing ECMAScript Edition 3 code because a corresponding transformation takes place
upon activation of the target function. However, depending upon the implementation, this difference may beobservable by host object functions called using apply or call. In addition, invoking a standard built-infunction in this manner with null or undefined passed as the this value will in many cases cause behaviour in
Edition 5 implementations that differ from Edition 3 behaviour. In particular, in Edition 5 built-in functions thatare specified to actually use the passed this value as an object typically throw a TypeError exception if
passed null or undefined as the this value.
15.3.5.2: In Edition 5, the prototype property of Function instances is not enumerable. In Edition 3, thisproperty was enumerable.
15.5.5.2: In Edition 5, the individual characters of a String object’s [[PrimitiveValue] may be accessed as arrayindexed properties of the String object. These properties are non-writable and non-configurable and shadow
any inherited properties with the same names. In Edition 3, these properties did not exist and ECMAScript
code could dynamically add and remove writable properties with such names and could access inheritedproperties with such names.
15.9.4.2: Date.parse is now required to first attempt to parse its argument as an ISO format string.
Programs that use this format but depended upon implementation specific behaviour (including failure) may
behave differently.
15.10.2.12: In Edition 5, \s now additionally matches <BOM>.
15.10.4.1: In Edition 3, the exact form of the String value of the source property of an object created by the
RegExp constructor is implementation defined. In Edition 5, the String must conform to certain specified
requirements and hence may be different from that produced by an Edition 3 implementation.
15.10.6.4: In Edition 3, the result of RegExp.prototype.toString need not be derived from the value of
the RegExp object’s source property. In Edition 5 the result must be derived from the source property in a
specified manner and hence may be different from the result produced by an Edition 3 implementation.
15.11.2.1, 15.11.4.3: In Edition 5, if an initial value for the message property of an Error object is not
specified via the Error constructor the initial value of the property is the empty String. In Edition 3, such an
initial value is implementation defined.

238© Ecma International 2009

15.11.4.4: In Edition 3, the result of Error.prototype.toString is implementation defined. In Edition 5,
the result is fully specified and hence may differ from some Edition 3 implementations.
15.12: In Edition 5, the name JSON is defined in the global environment. In Edition 3, testing for the presenceof that name will show it to be undefined unless it is defined by the program or implementation.

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© Ecma International 2009

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ANSI/IEEE Std 754-1985: IEEE Standard for Binary Floating-Point Arithmetic. Institute of Electricaland Electronic Engineers, New York (1985)[2]

The Unicode Consortium. The Unicode Standard, Version 3.0, defined by: The Unicode Standard,
Version 3.0 (Boston, MA, Addison-Wesley, 2000. ISBN 0-201-61635-5)[3]

Unicode Inc. (1998), Unicode Technical Report #15: Unicode Normalization Forms© Ecma International 2009241© Ecma International 2009