JavaScript Interview Questions and Answers

Last update: 05 Sep 2022, 19 Questions

JavaScript is a high-level programming language that is known for its dynamic, prototype-based and weakly typed characteristics. It conforms to the ECMAScript specification and is one of the core technologies of the World Wide Web. Initially only implemented client-side, JavaScript is now embedded in other types of software, including server-side in web servers and non-web programs such as PDF software and word processors. This information is often asked in JavaScript interview questions.


JS Interview Questions

Here in this article, we will be listing frequently asked JavaScript Interview Questions and Answers with the belief that they will be helpful for you to gain higher marks. Also, to let you know that this article has been written under the guidance of industry professionals and covered all the current competencies.

1. What is the difference between let, var and const?


With JavaScript, the user can declare variables with three keywords, which are let, var, and const. The terms var let and const are somewhat confusing. We will explain all 3 variables with the help of examples.

The difference between the variables var, let as well as const is described below in different ways.

  • Scope
  • Hoisting
  • Reassign the value
  • Redeclaration of the variable

NOTE: If you want to read more about the difference between var, let as well as const then you can visit here.

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2. How to check if value exists in array JavaScript?


1. Using includes()

const fruits = ["Banana", "Orange", "Apple", "Mango"];

2. Using indexof()

const fruits = ["Banana", "Orange", "Apple", "Mango"];
if(fruits.indexOf("Orange") !== -1) {  
   alert("Yes, the value exists!")  
} else {
   alert("No, the value is absent.")  

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3. What is the difference between arrow and normal functions?


The main differences between arrow function and normal function are based on some parameters like.

  • Syntax
  • Arguments that are binding.
  • "this" keyword is used to describe the use.
  • Utilizing a keyword that is new.
  • No duplicates of identified parameters.

Syntax of normal functions

// Function declaration
function printHello(name) {
   return `Hey ${name}`;

NOTE: If you want to read more about Regular vs Arrow functions then you can visit here.

// Function expression
const printHello = function(name) {
  return `Hey ${name}`;

Syntax of arrow functions

const printHello = (name) => `Hey ${name}`;

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4. What would be the output of the code below?

var variable1 = ["red", "green", "blue", "yellow"];
var variable2 = null;
console.log('This is a type of - ', typeof(variable1));
console.log('This is a type of - ', typeof(variable2));


This is a type of -  object
This is a type of -  object

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5. What are the data types available in JavaScript?


In JavaScript data types is an important concept that helps to operate on variables,JavaScript supports mainly two different kinds of data types-

  • Primitives
  • Objects
1. Primitives Data Type

It supports six data types that’s are given below.

  • Boolean: It represents only true or false based on condition.
  • Number: It can be written with or without decimals. Example var num1 = 32;
  • String: It is used to store texts and inside of either double or single quotes. Example var str_one = 'Best Interview Question';
  • Null: It is "nothing" but supposed to be something that does not exist.
  • Undefined: It has no value but variable is exists. Example var MyVariable_1;
    console.log(MyVariable_1); // undefined
  • Symbol: It is new in ES6. It is an immutable data type that have unique value.Example : const BestInterviewQuestion = Symbol(‘BestInterviewQuestion’);
2. Objects Data Type

It is a type of not primitive data Type. It is a collection of properties and these properties are stored in key and value pairs.

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6. How to create an object in Javascript?


There are various ways of creating an Object in JavaScript.

  • With the Object():
    Example : var name = new Object();
  • With Object.create():
    Example : var name = Object.create(null);
  • With the bracket syntactig sugar:
    Example : var name = {};
  • With a function constructor:
    Example :
    var ObjectName = function(Vname) {
        this. Vname = Vname
    var NewVar = new ObjectName("BestInterviewQuestion.com");
  • With function constructor + prototype:
    Example :
    function myObj(){};
    myObj.prototype.name = "bestinterviewquestion.com";
    var Newk = new myObj();
  • With ES6 class:
    Example :
    class myObject {
        constructor(name) {
            this.name = name;
    var e = new myObject("bestinterviewquestion.com");
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7. What is the output of below code snippet?

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; 
const bigNumbers = numbers.map(number => {
  return number * 2;


[ 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 ]

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8. Is JavaScript client side or server side?


JavaScript is a client-side language it gets executed at the user side. Whenever we browse the web, all the files from JavaScript are first fetched from the server and then performed at our side by the browser.

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9. What is a closure JavaScript?


It’s a JavaScript feature where an inner function has access to the enclosing or outer function’s variables, which is defined as a scope chain.

There are three types of scope chains:
  • Access to its own scope
  • Access to variables of outer functions
  • Access to global variables
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10. What is JavaScript and why it is used?


JavaScript is a high-level programming language that is used to calculate, manipulate and validate data. With the help of JavaScript, we can update and change both HTML and CSS.

We can write JavaScript code in an HTML page. When a HTML page loaded with JavaScript code, the script is sent to the browser for manipulate then HTML or CSS code will be rendered. It is a client-side technology.

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11. What are the difference between undefined and not defined in JavaScript?


  undefined Not defined
1. A variable was declared with a keyword "var" but not assigned with a value. A variable have not been declared without assignment:
2. Example :
var a = 1, b;
console.log(a) //1
console.log(b) //undefined
Example :
var a = 1, b;
console.log(f) // not defined
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12. What is console.log() and why it is used?


It is not a jQuery feature but a feature for debugging purposes used by developers. It is used to writes a message to the console.

console.log() accepts a parameter which can be an object, an array or any message.

Syntax : console.log(name); // here name is object, an array or variable.


$('#form').submit(function() {
     console.log(‘Your form is submitted successfully!’);
     // do something

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13. Explain the difference between call() and apply()?


S.no Call() Apply()
1. In JavaScript, call() is a predefined method used to invoke (call) methods with the use of an owner object as an argument (parameter). apply() is a method to write another way which can be used on different objects. It’s usually action similar to call() action.
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14. What is singleton class in JavaScript? Explain


Singleton is a JavaScript object which can be instantiated one time, whereas the singleton pattern is a design pattern that restricts the reinstallation of a class to one object. It allows only a single instance of itself to be created and can give access to that created instance.

Point to be noted:- Don’t let this JavaScript Interview Question take your job opportunity. Read it twice and then go for the interview.

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15. What is design pattern in JavaScript? Explain


It’s an advance object-oriented solution designed to solve commonly occurring software problems. These are reusable designs and interact with objects.

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16. How do you check the type of a variable?




var x = 12345; 
console.log(typeof x)
x = 'string'; 
console.log(typeof x)
x = { key: 'value' };
console.log(typeof x)



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17. What is the output of below code?

if (Math.round(null) > 0.5) {
  var x = 1;
} else {
  var x = 2;



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18. What is the output of code snippet?

const x = [];
x = 5;


It will give an error like "TypeError: Assignment to constant variable".

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19. What are the difference between JavaScript and jQuery?


On the basis of functioning

JavaScript is a scripting language which is compatible with all the major browsers and used to create interactive web-based applications. While jQuery is a framework which is the fastest JavaScript library used for the simplification of HTML document.

Time and simplicity

JavaScript is complex because it consumes the time of developers as it is important for the developers if they are using JavaScript then they have to create their own script. In the case of jQuery, developers need not write scripting as it already exists in the libraries.

Animation support

With JavaScript, it cannot be possible for the developer to use animation. On the other hand, jQuery allows the developers to use unbuild animations to make the applications interactive and attractive.

Featural aspects

JavaScript programming language is a combination of the European Manufacturing Association Script (ECMA) and Document Object Model (DOM). While on the other hand jQuery has a Document Object Model (DOM).

Interactivity feature

With the use of JavaScript, developers can make web pages more useful by suppling immediate feedback. While in case of jQuery developer can create web-based applications which becomes easier for the developer to customize the application as per the user's requirement.

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Development History of JavaScript

Invented by Brendan Eich in 1995, JavaScript became an ECMA standard in 1997. JavaScript was first to run on the Netscape 2 browser.

Latest Version: ECMAScript 2018 or ES9 is the latest version and was released in October 2018.

Advantages of JavaScript

  • Client-side JavaScript is quite fast.
  • The syntax is simple and flexible.
  • It develops well with most of the other popular languages.
  • Drag and drop components give a rich interface to your site.
  • Third party add-ons enable developers to write rich snippets.
About Umesh Singh
Umesh Singh
Full Stack Developer

My name is Umesh Singh having 7+ experience in Node.JS, React JS, PHP, Laravel, WordPress, MySQL, JavaScript, HTML and CSS etc. I have worked on around 20+ projects. I love programming, coding and most of my time goes in learning the best ways to write clean code. When I am not found coding, I can be seen helping out tech geeks. I am always open to help you.

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