Best Online Tutorials

SQL Tutorial

Free SQL Tutorial

SQL Interview Questions

SQL Practice Questions

SQL Tips & Tricks

SQL Errors

SQL Books

Angular Tutorial

Free Angular Tutorial

Angular Interview Questions

Angular Practice Questions

Angular Tips & Tricks

Angular Errors

Angular Books

Node.js Tutorial

Free Node.js Tutorial

Node.js Interview Questions

Node.js Practice Questions

Node.js Tips & Tricks

Node.js Errors

Node.js Books

React.js Tutorial

Free React.js Tutorial

React.js Interview Questions

React.js Practice Questions

React.js Tips & Tricks

React.js Errors

React.js Books

SSIS Tutorial

Free SSIS Tutorial

SSIS Interview Questions

SSIS Practice Questions

SSIS Tips & Tricks

SSIS Errors

SSIS Books

SEO Tutorial

Free SEO Tutorial

SEO Interview Questions

SEO Practice Questions

SEO Tips & Tricks

SEO Errors

SEO Books

Javascript Tutorial

Free Javascript Tutorial

Javascript Interview Questions

Javascript Practice Questions

Javascript Tips & Tricks

Javascript Errors

Javascript Books

MongoDB Tutorial

Free MongoDB Tutorial

MongoDB Interview Questions

MongoDB Practice Questions

MongoDB Tips & Tricks

MongoDB Errors

MongoDB Books

Next.js Tutorial

Free Next.js Tutorial

Next.js Interview Questions

Next.js Practice Questions

Next.js Tips & Tricks

Next.js Errors

Next.js Books

JavaScript interview questions | Part 1 

Prashant Surya  |   Oct 22, 2019  |  Views: 354


1.  What is the difference between window, document, screen in JavaScript?

Window is the main JavaScript object root.
It is the global object in a browser.
It is being treated as the root of the document object model. 
We can access it as window variable in most of the browsers.

window.screen is the small information object about the physical screen dimensions of the browser.

window.document or just document is the main object of the rendered document object model/DOM.

2.  What is the alternative of Document.ready in JavaScript?

var callbackFn = function(){
  // Scripts which needs to be run when the DOM is fully loaded

if (document.readyState === "complete" || (document.readyState !== "loading" && !document.documentElement.doScroll)) {
} else {
      document.addEventListener("DOMContentLoaded", callbackFn);

IF using older version of IE<=8

document.attachEvent("onreadystatechange", function(){

    // It checks if the DOM is fully loaded

  if(document.readyState === "complete"){

    // then remove the listener, to make sure it wouldn't be fired in the future

    document.detachEvent("onreadystatechange", arguments.callee);

    // Calling the actual handler callback...


3.  How to clone an object in JavaScript?

Object.assign() method is used for cloning an object in JavaScript.


var x = { prop: "value"};
var y = Object.assign({}, x);

4.  What is output of undefined * 5 in JavaScript ?

Output :  NaN

undefined * 5              //NaN
null * undefined          //NaN
NaN * 2                       //NaN
3 * null                             //0
3 * "string"                  //NaN
undefined * "string"    //NaN

5.  What is the difference between the substr() and substring() functions in JavaScript?

Both are used to extract a small string part from the given string using startIndex and either by giving length or the lastIndex as the parameters.


The substr() function has the form substr (startIndex , length). It returns the substring from startIndex and returns ‘length’ number of characters.

var s = "hello";
( s.substr(1,4) == "ello" )    // true


The substring() function has the form substring (startIndex , endIndex) . It returns the substring from startIndex up to endIndex – 1.

var s = "hello";
( s.substring(1,4) == "ell" )   // true

6.  How to get the primitive value of a string in JavaScript?

valueOf() method is used to get the primitive value of a string in the JavaScript.


var wlcmVar = "Welcome to DritalConnect!"

console.log( wlcmVar.valueOf() )

7.  What are JavaScript boxes and their types?

JavaScript have three types of boxes, which are used as per the requirements.

It is used to show alert messages to the users.

alert("Invalid username");


If there needs to get users confirmation, we use this.
confirm("Are you sure you want to delete?");   //It returns true/false


If we need to get the user inputs, we use prompt.

var data = prompt("Enter your username","DritalConnect"); 

8.  How to empty an array in JavaScript?

There are multiple was to empty an array in the JavaScript.

By assigning an empty array:

var arr1 =[3,4,5,6];

By assigning array length to 0:

var arr2 =[3,4,5,6];

By poping the elements of the array:

var arr2 =[3,4,5,6];
while(arr.length > 0) {

By using .splice():

var arr =[3,4,5,6];

9.  Write a JavaScript method for Factorial of n length?

function factorial(n) {
  if ((n === 0) || (n === 1))
    return 1;
    return (n * factorial(n - 1));

factorial(5)     //120

10.  What will be the output on Comparing null with undefined in JavaScript?

(null == undefined) -> true

(null === undefined) -> false

Continue reading JavaScript Interview Questions | Part 2

Content Contributor
Prashant Surya

Prashant Surya

Software developer at Uniserve Data Technologies Pvt. Ltd.  |   Noida, Uttar Pradesh, India

I am a software developer, freelancer and trainer working on technologies likes. MEAN stack, MERN Stack, ASP.NET full stack, WebRTC, ,PWA, SQL, JavaScript, jQuery, HTML, CSS, SEO, responsive UI/UX designing... having 7 years of experience in development in multiple domains like, eCommerce, RTA, capital markets, education, real estate...

Read related articles

JavaScript interview questions | Part 2

Prashant Surya  |   Oct 22, 2019  |  Views: 232

JavaScript interview questions | Part 1

Prashant Surya  |   Oct 22, 2019  |  Views: 355

Want to sell your software ?-

If you want to sell your readymade software to the genuine clients or businessman, list your software with details and demo links.

  • Sell ERP
  • Sell Website Clones
  • Sell App Clones
  • Sell Readymade Portals
  • Sell Readymade Websites
  • Sell Mobile Apps
  • Sell Window Apps

Clients will find it using our advanced search filter and will contact you directly.

No any charge for the product lsiting.

Do you own a Company ?+

Buy or Sell Commercial Property+

Are you a freelancer ?+

Looking for a job ?+

Are you a Service Provider ?+

Are you a blogger ?+

Do you own an IT Institute ?+