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Interview Questions on JavaScript

Here we have created an excellent list of frequently asked Interview Questions on Java Script. These are some basic which will mostly ask in interviews. Every fresher must know these questions on JavaScript.

Interview Questions on JavaScript

1. What are JavaScript Data Types?

Ans: Following are the JavaScript Data Types:

  • null
  • undefined
  • boolean
  • number
  • string
  • object
  • symbol- added in ES6!

JavaScript has seven built-in types: null, undefined, boolean, number, string, object, and symbol. They can be identified by the typeof operator.

2. What is an undefined value in JavaScript?

Ans: Undefined value means the

Variables used in the code don’t exist variable is not assigned to any valued property doesn’t exist.

3. What is negative infinity?

Ans: Negative infinity is a number in JavaScript that can be derived by dividing negative numbers by zero.

4. Which company developed JavaScript?

Ans: Netscape is the software company that developed JavaScript.

5. What are undeclared and undefined variables?

Ans: Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined Variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

6. What are all the types of Pop up boxes available in JavaScript?


  • Alert
  • Confirm
  • Prompt

These are some important interview questions on JavaScript with answers. If you have any doubt related to these JavaScript Questions and answers comment on the comment section.

7. What is the “this” keyword in JavaScript?

Ans: “This” keyword refers to the object from where it was called.

8. Explain the working of timers in JavaScript? Also elucidate the drawbacks of using the timer, if any?

Ans: Timers are used to execute a piece of code at a set time or also to repeat the code in a given interval of time. This is done by using the functions setTimeout, set intervals, and clearInterval.

The setTimeout(function, delay) function is used to start a timer that calls a particular function after the mentioned delay. The setInterval(function delayed) function is used to repeatedly execute the given function in the mentioned delay and only halts when canceled. The clearInterval(id) function instructs the timer to stop.

Timers are operated within a single thread, and thus events might queue up, waiting to be executed.

9. What is === operator?

===is called a strict equality operator which returns true when the two operands are having the same value without any type of conversion.

10. What are all the looping structures in JavaScript?

Ans: Following are looping structures in JavaScript:

  • For
  • While
  • do-while loops
Quiz Interview Questions on JavaScript

Quiz Interview Questions on JavaScript

11. Explain the difference between “==” and “===”?

Ans: “==” checks only for equality in value whereas “===” is a stricter equality test and returns false if either the value or the type of the two variables are different.

12. What do mean by NULL in JavaScript?

Ans: The NULL value is used to represent no value or no object. It implies no object or null string, no valid boolean value, no number, and no array object.


13. Is JavaScript a case-sensitive language?

Ans: Yes, JavaScript is a case-sensitive language. The language keywords, variables, function names, and any other identifiers must always be typed with a consistent capitalization of letters.

14. What are the features of JavaScript?

Ans: It is a lightweight, interpreted programming language. It is designed for creating network-centric applications. It is complementary to and integrated with Java. It is an open and cross-platform scripting language.

15. What is the advantage of JavaScript?

Ans: Less server interaction – you can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server. Immediate feedback to the visitors – They don’t have to wait for a page reload to see if they have forgotten to enter something. Increased interactivity – you can create interfaces that react when the user hovers over them with a mouse or activates them via keyboard. Richer interfaces – you can use JavaScript to include such items as drag and drop components and sliders to give a Rich Interface to your site visitors.

16. What is a closure? Give an example.

Ans: Closures are created whenever a variable is defined outside the current scope is accessed from within some inner scope. It gives you access to an outer function’s scope from an inner function. In JavaScript, closures are created every time a function is created. To use a closure, simply define a function inside another function and expose it.

17. What is a callback?

Ans: A callback is a plain JavaScript function passed to some method as an argument or option. It is a function that is to be executed after another function has finished executing, hence the name ‘callback’. In JavaScript, functions can take functions as arguments and can be returned by other functions.

18. Explain the window.onload and onDocumentReady?

Ans: The onload function is not run until all the information on the page is loaded. This leads to a substantial delay before any code is executed. onDocumentReady loads the code just after the DOM is loaded. This allows early manipulation of the code.

19. How to find an operating system in the client machine using JavaScript?

Ans: The ‘Navigator.appversion’ is used to find the name of the operating system in the client machine.

20. What is the use of blur function?

Ans: The blur function is used to remove the focus from the specified object.

Quiz Interview Questions on JavaScript

Quiz Interview Questions on JavaScript

Quiz Interview Questions on JavaScript

1. How to get the last element of the array with the built-in JavaScript method?

a) last()

b) get()

c) pop()

Ans: pop()   


2.  Which of the following function of the String object is used to match a regular expression against a string?

a) search()

b) match()

c) replace()

Ans: match() 


3. Which of the below is used in JavaScript to insert special characters?

a) %

b) &

c) \

Ans: \   


4. To insert JavaScript into an HTML page, which tag is used?

a) <javascript>

b) <script>

c) <js>

Ans: <script> 


5. What method returns the characters in a string beginning at the specified location?

a) substr()

b) pop()

c) getSubstring()

Ans: substr() 


6. Which of the following function of array object creates a new array with the results of calling a provided function on every element in this array?

a) Join()

b) pop()

c) map()

Ans: map()   


7. What attribute needs to be changed to make elements invisible?

a) visibility

b) invisibility

c) invisible

Ans: visibilty   


8. What is the alternate name for JavaScript?

a) ECMScript

b) ECMAScript

c) TypeScript

Ans: ECMAScript   


9. What function of a String object splits a String object into an array of strings by separating the string into substrings?

a) slice()

b) replace()

c) split()

Ans: split()   


10. Variable in JavaScript declared with which of the following keyword?

a) var

b) $

c) constant

Ans: var 


Source: Oleh Lazareko and Neha Kumari from Playstore…

These are some important Interview Questions on JavaScript. If you have any doubt related to JavaScript ask me on a Comment Section…

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