JavaScript Data Types

JavaScript variables can hold different data types: numbers, 
strings, objects and more:
In programming, data types is an important concept.
To be able to operate on variables, it is important to know something about 
the type.
Without data types, a computer cannot safely solve this:
Does it make any sense to add "Volvo" to sixteen? Will it produce an 
error or will it produce a result?
JavaScript will treat the example above as:
When adding a number and a string, JavaScript will treat the number as a 
string.
JavaScript evaluates expressions from left to right. Different sequences can 
produce different results:
Result:
Result:
In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".
In the second example, since the first operand is a string, all operands are 
treated as strings.
JavaScript has dynamic types. This means that the same variable can be used 
to hold 
different data types:
A string (or a text string) is a series of characters like "John Doe".
Strings are written with quotes. You can use single or double quotes:

You can use quotes inside a string, as long as they don't match the quotes 
surrounding the string:
You will learn more about strings later in this 
tutorial.
JavaScript has only one type of numbers.
Numbers can be written with, or without decimals:
Extra large or extra small numbers can be written with scientific 
(exponential) notation:
You will learn more about numbers later in this 
tutorial.
Booleans can only have two values: true or false.
Booleans are often used in conditional testing.
You will learn more about 
conditional testing later in this tutorial.
JavaScript arrays are written with square brackets.
Array items are separated by commas.
The following code declares (creates) an array called cars, containing three 
items (car names):
Array indexes are zero-based, which means the first item is [0], second is 
[1], and so on.
You will learn more about arrays later in this tutorial.
JavaScript objects are written with curly braces {}.
Object 
properties are written as name:value pairs, separated by commas.
The object (person) in the example above has 4 properties: firstName, 
lastName, age, and eyeColor.
You will learn more about objects later in this tutorial.
You can use the JavaScript typeof operator to find the type 
of a JavaScript variable.
The typeof operator returns the type of a variable or an expression:
You will learn more about typeof later in this tutorial.
In JavaScript, a variable without a value, has the value undefined. 
The type is also undefined.
Any variable can be emptied, by setting the value to undefined. 
The type will also be undefined.
An empty value has nothing to do with undefined.
An empty string has both a legal value and a type.
Use comments to describe the correct data type of the following variables:
let length = 16;            // 
let lastName = "Johnson";   // 
const x = {
  firstName: "John",
  lastName: "Doe"
};                          // 

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