10 Interview Questions About JavaScript.
Published: 2021-05-06 15:56:40

Q1. What is JavaScript?
JavaScript is an interpreted, lightweight programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages. It a client-side and server-side scripting language inserted into HTML pages and is understood by web browsers. 

Q2. What are the data types supported by JavaScript?
The data types supported by JavaScript are:

Undefined
Null
Boolean
String
Symbol
Number
Object

Q3. What are the features of JavaScript?
Following are the features of JavaScript:

It is a lightweight, interpreted programming language.
It is designed for creating network-centric applications.
It is complementary to and integrated with Java.
It is an open and cross-platform scripting language.

Q4. Is JavaScript a case-sensitive language?
Yes, JavaScript is a case sensitive language.  The language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

Q5. What are the scopes of a variable in JavaScript?
The scope of a variable is the region of your program in which it is defined. JavaScript variable will have only two scopes.
    • Global Variables − A global variable has global scope which 
      means it is visible everywhere in your JavaScript code.
    • Local Variables − A local variable will be visible only 
      within a function where it is defined. Function parameters 
      are always local to that function.

Q6. What is the purpose of ‘This’ operator in JavaScript?
The JavaScript this keyword refers to the object it belongs to. This has different values depending on where it is used. In a method, this refers to the owner object and in a function, this refers to the global object.

Q7. What is Callback?
A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘call back‘. In JavaScript, functions are objects. Because of this, functions can take functions as arguments, and can be returned by other functions.

Q8. What is Closure? Give an example.
Closures are created whenever a variable that is defined outside the current scope is accessed from within some inner scope. It gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created. To use a closure, simply define a function inside another function and expose it.

Q9. What is the difference between Local storage & Session storage?

Local Storage – The data is not sent back to the server for every HTTP request (HTML, images, JavaScript, CSS, etc) – reducing the amount of traffic between client and server. It will stay until it is manually cleared through settings or program.

Session Storage – It is similar to local storage; the only difference is while data stored in local storage has no expiration time, data stored in session storage gets cleared when the page session ends. Session Storage will leave when the browser is closed.

Q10. What is the difference between the operators ‘==‘ & ‘===‘?
The main difference between “==” and “===” operator is that formerly compares variable by making type correction e.g. if you compare a number with a string with numeric literal, == allows that, but === doesn’t allow that, because it not only checks the value but also type of two variable, if two variables are not of the same type “===” return false, while “==” return true.