Javascript Interview Questions and Answers


To prepare yourself for these job positions you must be aware of the Commonly asked JavaScript Interview Questions and Answers. Here in this blog, we have compiled a set of 101 Important JavaScript Interview Questions and Answers and these questions have been compiled in the perspectiveto give equal emphasis for both fresher and experienced candidates

JavaScript was created to give little more pep to the web pages and improve them as the basic <blink> tag was not fascinating enough. Ever since its evolution, JavaScript has proven its capabilities and hence, big IT giants like Google and Facebook are using JavaScript. Now, it is used to make desktop-like and complex web-applications. And in addition to it, Node.js has made it more popular to design server-side software. And since, it’s just the beginning, there is going to be a huge growth in the requirement of JavaScript professionals in the industry. 

We know it is hard to face an interview, even when you have the desired knowledge and skill set. So for that we have brought up a complete reference for you to have confidence on your JavaScript skills. 

So go through this collection of a 101 most common and predicted interview question and answer series that will help you clear your basics and will help you face the interview with confidence. 


JavaScript is an interpreted programming language with great object-oriented capabilities and is very lightweight. It is used to manipulate HTML as well as CSS to make web pages interactive. The functionality of JavaScript lets you do calculations, validate, and manipulate data. 

JavaScript was founded by Brandan Eich in 1995.

Here are some of the data types supported by JavaScript:


Although JavaScript has a lot of features, a few of the important ones are:

JavaScript is interpreted and a very lightweight programming language
JavaScript is an open and a cross-platform scripting language
By using JavaScript, it is easy to design network-centric applications
It is complementary to Java

Yes, JavaScript is a case-sensitive language. The variable names, keywords, function names, and other phrases must be written in a correct combination of capitals and smalls.

Some of the advantages of working with JavaScript are:

Increased interactivity
Rich interfaces
Less interaction with the server
Visitors can see immediate feedback

No, there is no similarity between Java and JavaScript. Even though their names are similar, there is nothing in common for their working. JavaScript is just coded programs that can be added to HTML pages whereas Java is a complete full-stack programming language.

JavaScript can be debugged using various methods like:

By using JavaScript debuggers
The console.log() method
By using the ‘debugger’ keyword

We can easily create an object in JavaScript as it is also an object-oriented language. We use the following syntax for creating an object: 

Var emp = {

Name: “Fita”


Creating an array is very simple in JavaScript, you can use the array literals for creating arrays.

var x = [];

var y = [1, 2, 3, 4, 5];

Yes, we can assign an anonymous function to a variable. We can also pass it as an argument for a different function.

The scope of any variable is the defined region in which that particular variable can be used. 

JavaScript has two scopes of any variable, local and global

Local Scope: A variable having local scope can be assessed only within the function where it is defined. Parameters of the functions are local for the variable.
Global Scope: A variable having global scope in visible and accessible anywhere in the JavaScript code.

These are commonly asked JavaScript Interview Questions and Answers for Fresher and Experienced candidates.

‘this’ is a keyword in JavaScript and it is used to refer to the object where it belongs. Depending on where it is used, it can have different values. For example, when used in a function, it refers to the global object and when used in a method, it refers to the owner object.

JavaScript interview questions and answers

The ‘Callback’ function is a very simple JavaScript function that is passed as an argument to a method. This function is executed after another function has completed its execution. In JavaScript, functions are objects and hence they can accept other functions as arguments and also, they can be returned as the output of any function.

Closures are automatically created whenever there is an incident in which a variable defined outside the scope is being accessed. It is used to give access to the outer function’s scope from the inner function. In JavaScript, a closure is created every time a function is created.

The prime use of the ‘TypeOf’ operator is to fetch the data type of its operand. It is a unary operator and is placed before the operand to fetch its type. The operand is allowed to be any data structure or literal. It returns a string that indicates the data type of the operand.

In JavaScript, a cookie can be created easily with the help of document.cookie object, syntax of which is as follows:

document.cookie = “key1 = value1; key2 = value2, key3 = value3; expires = date”;

Very similar to writing, reading a cookie in JavaScript is also quite simple, as the cookie is the value returned by document.cookie, and this string is useful every time you want to access a cookie.

For deleting a cookie in JavaScript, you just need to set the timer of the cookie to any time in the past and it will be deleted automatically. Also, you should carefully define the path of the cookie to ensure that the right cookie is being deleted.

There are 3 ways of doing so:


Here are 3 ways to define a variable:

Var – The JavaScript variables statement is employed to declare a variable and, optionally, we will initialize the worth of that variable. For instance: var x =60; Before executing the code, all the variable declarations are processed at first.
Const – the thought of const functions doesn’t allow them to switch the thing on which they’re called. When a function is said as const, it is often called on any sort of object.
Let – It is a signal that the variable may be reassigned, such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable is going to be used only within the block it’s defined in.

Try-catch-finally is the keywords that are used for exception handling in JavaScript. Here is the syntax:

Try {

Try to execute this piece of code


Catch (Exception) {

Run this code if an exception occurs


Finally {

This piece of code runs irrespective of the exception.


Some of the common JavaScript frameworks are:


The window in JavaScript is a global object that is used to hold functions, variables, location, and history. Document in JavaScript is also a kind of window that is considerably called a property of the window.

Basically, if an HTML tag is found in a string, innerHTML will process it and innerTEXT would not be able to process it.

NaN is the abbreviation of Not a Number. Generally, NaN appears when we compare unequal to a number. It is used for showing the error condition that occurs when a function returns an absurd value when it is expected to return a valid numeral. NaN is also seen in conditions where we convert a string or anything to a number and the process cannot be completed and results in error. 

In JavaScript, objects are passed by reference into a function and primitive data types are passed by value into the function.

Strict mode is enabled to ensure better error checking of the code. You can enable this strict mode by adding the ‘use strict’ keyword at the beginning of any function, program, or file.

The prompt box is just like a text box which allows the user to give inputs. It appears when the user uses the execute prompt() method. After entering the desired values, the user needs to click on either the ‘Ok’ or ‘Cancel’ button to continue further.

The apply() method is used to call any function provided with ‘this’ value along with an array that contains the arguments, whereas the call() methods are used to call any function with a mentioned ‘this’ value and individual arguments.

JavaScript interview questions for freshers

A very simple command document.write(“Hello”) is used to print Hello on the screen.

These are commonly asked JavaScript Interview Questions and Answers for Fresher candidates in an interview

The ‘blur’ function in JavaScript is used to disconnect the focus from a particular object.

Variable typing is done in JavaScript when first a number is assigned to a variable and then a string is assigned to that same variable.


a = 4;

a = ‘Fita’;

 There is a special command in JavaScript for doing this job. It’s the ‘Navigator.appversion’ command.

The types of errors are:

Load time errors
Logical errors
Run-time errors

The unshift methods work at the starting of an array. It is very similar to the push method. Using these methods, we can prepend some elements from the beginning of an array.

The push method works at the endpoint of an array. It appends some elements at the very end of an array. Also, multiple arguments will append multiple elements onto the array by using this push command.

Yes, both of them are very similar, the main difference is that JavaScript was developed and released by Netscape, and Jscript is developed by Microsoft.

There are two ways to assign properties to any object:

obj.class = 15;


obj[“class”] =15;

These are commonly asked JavaScript Interview Questions and Answers for Fresher candidates in an interview


The above query is used to get the status of the checkbox. By running this query, if the checkbox is checked, the query returns TRUE.

The break statement is used to break the current loop and jump onto the next consecutive line. A continue statement is used to continue within the same loop but with a new recurrence. 

This action can be performed by using the mentioned JavaScript extensions, which are available through JavaScript editors. 

For opening a file, the extension used is: 

fh = fopen(getScriptPath(), 0);

To get and read the information from a client’s screen, various screen objects are used. 


EncodeURI() is the method used for converting the URL into Hex coding

DecodeURI() is the methods used to convert the Hex code or the encoded URL back to normal 

The following code is used

Var array1 = [[[]]];

Since there is no scope of validation in innerHTML and the content of innerHTML is refreshed each time, making it much slower, it is advised to not use innerHTML. Also, it makes the webpage unstable.

In JavaScript, timers are higher-order functions that are supposed to be used to delay the function or to repeat the execution of other functions, which are received as the very first argument. 

There are two methods of writing comments in JavaScript.

Inline comments (//)
Multiline comments (/*……*/)

It’s an equity operator. And is used without type coercion. In simple words, if triple equal is used, then the compared values must be equal in their types also. 

Yes, automatic type conversion is done in JavaScript.

There are multiple methods used to convert a string to an integer function
Unary plus operator
Using number object

THere, to represent the absence of the value of any object, NULL is used. 

The void (0) operator in JavaScript is used to evaluate any given expression and returns an undefined value as the result. Generally, it is used when there is a certainty of having a side effect while inserting any expression into the web page. 

These are commonly asked JavaScript Interview Questions and Answers for Fresher and experienced candidates in an interview

Namespacing is a JavaScript feature that is used for grouping the required variables and functions with unique names. It just provides the names to attach to particular object properties or even functions. it is useful for using the specific piece of code and is also important to maintain the modularity of the code. 

Basically, the escape characters that are used in JavaScript are a special feature that allows the coder to write special characters in the code without disturbing the application. To make the special characters like double quotes, singles quotes, apostrophes, etc. visible in your text just write the escape character which is the backslash character (/) before the characters to display them. 

There are multiple methods that can be used to empty the array:

arraylist = [] 
arrayList.length = 0;
arrayList.splice(0, arrayList.length);
while (arrayList.length)




Event propagation is also known as event bubbling in HTML DOM API. It is a process when an event is targeted to the innermost element and then further triggered until it reaches the outermost layer of DOM. 

The primary difference is that the keyword ‘undefined’ is used to mention that a variable has been declared earlier but no value has been assigned to it yet. And ‘null’ is a value in itself that can be assigned to any variable. The other difference between ‘undefined’ and ‘null’ is that undefined is a type and null is said to be an object. 

lared somewhere but no value has been assigned to it and the ‘undeclared’ variables are the variable that is not even declared and they do not exist in a particular program. A runtime error occurs when the program reads the value of the undeclared variable whereas an undefined value is returned when the program reads the value of an undefined variable. 

Some of the errors that can occur in a JavaScript program are as follows


This command is used to execute a particular piece of code when a browser has completely loaded the DOM tree along with all the resources such as objects, images and more.

This command allows the developer to execute a particular piece of code when the browser has loaded only the DOM tree and does not wait for any images of objects to load. ,

es, JavaScript is made to support Boolean operators which can generally return the value of true or false. The Boolean () function is defined to find if the provided expression is true or false. 

When a web application is hosted over multiple web servers in order to balance the load of it comet is called a web farm and when there are several worker processes of an application, it is called a web garden. 

There are two functional components in JavaScript. These are:

First-class functions: Normal functions are recognized as first-class functions in JavaScript. They can be stored easily in data structures, assigned to variables, can be passed as arguments to other functions, and can also be retrieved as a result of other functions.
Nested functions: These are the functions that are defined inside another function in a nested format and are called every time the main is executed.

Yes, JavaScript provides support for regular expressions as well

Basically, a JavaScript Image Map is a simple image on a web page which has different links that navigate the user to different web pages for different sections of that particular web page. 

There are two different methods that can be used to redirect user from one web page to another web page:

javascript interview questions and answers

If the program is stuck in an infinite loop running based on javascript, it will result in crashing the browser. 

Yes, the tolSOString() method is specially designed to convert the JavaScript date to ISO standard. Basically what it does is it converts the JavaScript date object into a string with the help of the ISO standard. 

There is a separate function called setInterval() pictures used to call a function repetitively in the defined time of interval. The syntax is 

setInterval(function, milliseconds, param1, param2, … )

the and [&&] operator or [ || ]operator and the not [! ] operators are allowed to be used in JavaScript. 

you can simply clone the object in JavaScript by using the code below

var obj = {a : 1, b: 2}

var objclone = Object.assign({}, obj);

The document represents the web page loaded in the browser and it is an entry point to the page content. This provides a powerful API to interact with the DOM like getElementsById, getElementsByClassName, getElementsByTagName, and querySelector. Through this, we can select one or group of elements and manipulate them.

These are commonly asked JavaScript Interview Questions and Answers for Experienced candidates in an interview

Server applications
Web applications
Smartwatch apps
Mobile apps
Flying robots

Nowadays, JavaScript is supported by all the modern web browsers that are available and have an inbuilt interpreter.

No, JavaScript is a single-threaded language and because of this, some developers who are majorly working with multithreaded languages think that JavaScript is slow.

Yes, hackers can try to hack JavaScript in order to extract sensitive information from the victim’s website. The CSRF attack, also known as JavaScript Hijacking is done by the hackers to damage a page of the victim’s website in order to extract sensitive information like JSON and JavaScript.

Math.max() > Math.Min


It sounds quite weird but the fact is that, in JavaScript, when no argument is passed in min() and max() function they return the values Infinity and -Infinity respectively. And hence, min is proved greater than max.

Yes, the parenthesis position is very important in JavaScript. It needs every syntax to be perfect and everything to be in its right place in order to run without any errors. 

Currying function is a process that takes a function as an input along with multiple arguments and then returns it by turning it into a sequence of functions in which each function has only one single argument. This function type is named after Haskell Curry, a mathematician. With the help of this function we can create unary functions from n-ary functions. 

The ‘let’ keyword statement of JavaScript is used to declare block scope of a local variable. The statement or a variable which is defined with the keyword ‘let’ has its scope limited to the block or statement or expression where it is used. It is contrary to the var keyword. 

The Temporal dead zone which occurs when the developer declares a variable using the const and let keywords, but not with var keyword. Reference-Error is cost in ECMAScript 6 when we try to access a let or const variable before it is declared. And the time duration when this happens is known as the temporal dead zone. 

IIFE is an acronym for immediately invoked function expression and it is a JavaScript function with the property that it runs as soon as this function is defined. The main reason for using this function is for data privacy. 

Here is the syntax of this function:

(function () 

// Logic here



The main three benefits of using modules for sprawling are


 A different method is used to mitigate any SQL Injection.

Here are some of the server sent events:

Onopen – Used when connection to the server is being opened
Onmessage – Used whenever a message is received
Onerror – Used when an error occurs

Here are some of the main rules of promise:

Its an object that is used to supply .then() method
The promise in the pending state might change its status to either rejected or fulfilled.
One the promise reaches the rejected or fulfilled stage, it should not change its state again.
Once a promise is settled, the value must not change

JavaScript is embedded in the <script> part of the HTML coding. 

We can use the ‘Navigator.appName’ command to get information about the browser of the Client. 

In JavaScript, we can use the (.) dot operator to read as well as write properties of an object. 


Whenever a negative number is divided by zero in JavaScript, the negative infinity occurs. 

Yes, to import all the exports of a file as objects, the following syntax is used:

It’s a rarely used method in which the process is started with the outermost element to capture the event and then mitigating it to the innermost element. It is completely contrary to Event Bubbling.


The above are the commonly asked JavaScript Interview Questions and Answers in an Interview for Freshers and Experienced candidates. In the coming days, we will also add more Interview Questions and Answers to this blog that are prevalent in the Industry.We provide the best-in-class JavaScript Training in Chennai or JavaScript Training in Bangalore under expert guidance with certification. 

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