JavaScript interview questions

Last update: 30 Mar 2020, 30 QuestionsAsk Question
A Quick Overview of JavaScript
JavaScript interview 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.

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.

Best JavaScript interview questions And Answers

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.

  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

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.

In JavaScript, arrays is used to store multiple values within a single variable at a time.

Syntax for creating Arrays in JavaScript:

var arrayName = [value1, value2, value3];


var name = [‘best’,’interview’,’questions’];

We can access these values with:

var name1 = name[0]; console.log(name1); // it with display best.

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");

The difference between "==" and "===" is as following

S.no "==" "==="
1. The operator "==" tests the abstract equality, which means before making equality comparison, it does the necessary type conversions. The operator "===" tests only strict equality, i.e., this will not perform type conversion; hence if values compared here are not the same type, it will return false.

Both are used for function declaration in JavaScript but the difference are given below :-

  let var
1. It can be only available inside the scope it's declared, like in for loop. It can be accessed outside the loop.
2. It is block scoped. It is function scoped

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

This Javascript Question and Answer can be a game changer for you in your Interview.

In JavaScript, Null represents the absence of any object value. If a user tries to reference a null variable, he/she will get an error. It implies to no string, no number, no valid Boolean value, and no array object.

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

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 succesfully!’);
     // do something

There are two types of comments we can write in JavaScript; single line comment and multiline comment. To create a single line comment, the users have to place two slashes “//” in front of text or code which they wish to have the JavaScript interpreter ignore. For multiline comments, use /* from the beginning of the comment and end it with */.

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.

This fundamental difference can is asked in many JavaScript Interview Questions and Answers from both freshers and experienced.

It is a client side technology, it is mainly used for gives client side validation, but it have lot of features which are given below;

  • It is an object-based scripting language.
  • It helps the user more control over the browser.
  • JavaScript Handling dates and time also.
  • It also helps to Detect the client's browser and Operating System.
  • JavaScript is light weighted & case sensitive.
  • All most JavaScript control statements syntax is same as syntax of control statements in C programming language.


These JavaScript Interview Questions can be beneficial in your interviews.

S.no unescape() escape()
1. The unescape() function uses a string as a parameter, and used to decode that string encoded by the escape() function. The escape() function in JavaScript is used to encode a string. It makes a string portable so that it can be transmitted across any network to any computer with ASCII character support.
S.no decodeURI() encodeURI()
1. The decodeURI() function works reverse to the encodeURI(), taking an encoded string and then replacing the tokens with the regular characters. encodeURI() is used to pass the string value that the user requires to convert. This function returns the URL encoded value.

The 'use strict' is a new ECMAScript version 5 directive. It’s a literal expression, not a statement, ignored by previous JavaScript versions. It is purposed to indicate that the code should be executed in the "strict mode."

Not-a-Number or NaN is a weirdo Global Object present in JavaScript and frequently returned when failed mathematical operation happened. We can use global function isNaN() to find out if the variable is a NaN.

S.no Arrow function Normal function
1. Introduced in ES6, it provides a concise way to write functions in the JavaScript. The context present inside arrow functions is statically or lexically defined. It’s a procedure to produce a set of statements that calculate a value or performs a task. We have to define a function in the scope from which we wish to call it, to use it.

To count the number of elements in an array using JavaScript, use the following



var count = 0;

for(var i = 0; i

if(array[i] == 2)



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.

S.no Export Import
1. In JavaScript, the export statement is used when creating modules for JavaScript to export functions, primitive values or objects from the blades so they can be used by the programs with import statement which works opposite to it. The import statement is used when we import buildings which are exported by another module. All imported modules are in strict mode. This statement can’t be used in embedded scripts.

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

The javascript function is a block of code which can be designed according to the user to perform a particular task. A basic JavaScript function syntax is mentioned below:



function myFunction(p1, p2) {

return p1 * p2; // The function returns the product of p1 and p2


To perform a character count in JavaScript, use the following syntax:

var str = "Hello World!";

var n = str.length;

There are three types of cloning available in JavaScript.
  • Deep copy using iteration
  • Converting to JSON and back
  • Using Object.assign

To change the title of a web page using JavaScript, we have to use the following syntax:

document.title = 'Your desired title';

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.

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.

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