Top 20 JavaScript Interview Questions and Answers (2023)

4 min readJul 11, 2023

JavaScript is one of the most popular programming languages in the world, and it is in high demand by employers. If you are looking for a job as a JavaScript developer, you will need to be prepared to answer some tough interview questions.

Here is a list of the top 20 JavaScript interview questions and answers, along with some explanations. This list is designed to help you prepare for your next JavaScript interview.

1. What is JavaScript?

JavaScript is a high-level, interpreted programming language that is used to create interactive web pages and dynamic user interfaces. It is a client-side scripting language, which means that it is executed on the user’s browser.

2. Who developed JavaScript?

JavaScript was developed by Brendan Eich in 1995. He was working at Netscape at the time, and he was tasked with creating a scripting language that would allow web pages to be more interactive.

3. What are the differences between Java and JavaScript?

Java and JavaScript are two very different programming languages. Java is a compiled language, while JavaScript is an interpreted language. This means that Java code is converted into machine code before it is executed, while JavaScript code is interpreted line by line by the browser.

Java is also a statically typed language, while JavaScript is a dynamically typed language. This means that the data types of variables in Java must be declared, while the data types of variables in JavaScript are inferred from the values that are assigned to them.

4. What are the JavaScript Data Types?

The JavaScript Data Types are:

Number
String
Boolean
Null
Undefined
Object
Array
Function
RegExp

5. Why Should We Study JavaScript?

There are many reasons why you should study JavaScript. Here are a few of the most important reasons:

JavaScript is one of the most popular programming languages in the world.
JavaScript is used to create interactive web pages and dynamic user interfaces.
JavaScript is a client-side scripting language, which means that it is executed on the user’s browser.
JavaScript is a relatively easy language to learn.

6. What is the basic use of the isNaN function in JavaScript?

The isNaN function in JavaScript is used to check if a value is NaN (Not a Number). NaN is a special value that is used to represent a number that is not a number.

7. Who is faster among JavaScript and ASP Script?

JavaScript is generally faster than ASP Script. This is because JavaScript is a compiled language, while ASP Script is an interpreted language.

8. What do you mean by negative infinity?

Negative infinity is a special value that is used to represent a number that is less than any other number.

9. Can we break JavaScript Code into several lines? If yes, then How?

Yes, JavaScript Code can be broken into several lines. This can be done by using the \ (backslash) character. For example:

Code snippet

var myVar = 10;

// This is a single line of code.var myOtherVar = 20;

Use code with caution. Learn more

10. What are undeclared and undefined variables?

An undeclared variable is a variable that has not been declared in the JavaScript code. An undefined variable is a variable that has been declared, but has not been assigned a value.

11. What do you understand by event bubbling?

Event bubbling is a mechanism in JavaScript that allows events to propagate up the DOM tree. This means that an event that is triggered on an element will also be triggered on its parent elements.

12. Is JavaScript case-sensitive?

Yes, JavaScript is case-sensitive. This means that the names of variables, functions, and other objects must be spelled exactly the same way, regardless of case.

13. What is the difference between a web garden and a web frame?

A web garden is a group of web servers that are used to serve web pages. A web frame is a single web page that contains multiple HTML documents.

14. Describe the role of deferred scripts in javascript.

Deferred scripts are scripts that are executed after the rest of the page has loaded. This is useful for scripts that do not need to be executed immediately, such as scripts that load images or videos.

15. What are the different functional components of JavaScript?

The different functional components of JavaScript are:

Functions
Methods
Events
Objects
Arrays

16. What do you mean by screen objects? Explain.

Screen objects are objects that represent the user’s screen. These objects can be used to get information about the screen, such as its size, resolution, and color depth.

17) Define BOM.

BOM (Browser Object Model) is the core of Javascript on the web that helps to interact with the browser.

It means that objects are exposed by the web browser for modifying properties and objects referenced to them.

18) What is the use of “Use Strict”?

The main purpose of “use strict” is to indicate that the code should be executed in “strict mode”.

19) Explain WeakMap in JavaScript.

A weak map is a compilation of key-value pairs where the keys are poorly referred to. These keys must be objects with values of any arbitrary JavaScript type with weak references to its keys.

20) Define Prototype Chaining.

When the prototype of an object accesses the same property from the constructor function, the object inclines to extract the property from the constructor function.

This type of linking to another is called Prototype chaining or Prototype chain in JavaScript.