10 Most Asked JavaScript Interview Questions and Answers [2021]

10 Most Asked JavaScript Interview Questions and Answers [2021]
10 Most Asked JavaScript Interview Questions and Answers [2021]


Today Javascript is basically everywhere. If we didn’t have Javascript, we would also not have any websites (Imagine, no Google or no StackOverflow!!! ). I guess this basically answers everyone’s question: Is Javascript important? Should we learn it?

The next question generally is will learning this help us in our career? I will present you with the facts and you can be the judge of that.

Every action that you take on a website is handled by Javascript. But that is not all, Javascript is not just used for websites, but also for building mobile apps, video games and web servers. 

Javascript is a language that we can use to build both frontend and backend. Now that is killing two birds with one stone! Javascript also has a lot of cool frameworks – React, Angular, View, Meteor, Django, jQuery and so many more.

Knowing javascript along with HTML, CSS gives you an edge whenever you sit for an interview, be it campus placement or off-campus interviews, or even if you want to work as a freelancer.

You can actually start building your own projects just weeks into learning it with the help of these Javascript interview questions.

Demand of JavaScript and different job roles 

Javascript is one of the most in-demand skills in the IT industry in 2021. Since Javascript is used not just for the frontend but also for the backend, this puts Javascript right on top of every company’s must-have skill when hiring a developer. If you follow the stats, you would know that Javascript is not only the top used language on Github but it also reigns supreme on StackOverflow for the 8th year in a row! Come on now we cannot ignore the facts! 

Coming to frameworks, a large majority of Javascript developers use React, Jquery, Angular, Vue.js for the frontend. 

Read about Top 20 Javascript Libraries To Learn In 2021

Now since we have seen that such a majority of developers have been using Javascript, there must be a reason. Not all of them can do it for fun, right? So coming to Job stats, a lot of companies like Microsoft, Facebook, SAP, KPMG, Cognizant, Uber often ask Javascript in interviews.

The different job roles that you can aim in these companies are if you aim to prepare with the help of these Javascript interview questions: 

  • Front-end Developer
  • Back-end Developer
  • Full-Stack web Developer
  • WordPress Developer
  • Freelancer

In the current times, because of the pandemic situation, many people are working as freelancers on different projects. The benefit of that is you can start at any point, right after learning the basics you can take on small projects and start working on them.

Not only will this help you in gaining a better understanding of the language but help you perfect your skills. It would also be such a nice addition to your resume and you can start earning right away. Also, imagine being your own boss! There is no greater joy. 

Moving on in this article, I will be telling you the 10 most asked Javascript Interview questions and their answers. 

Basic Javascript Interview Questions & Answers

1. What are the different Data Types supported by Javascript?

Data types can be primitive as well as non-Primitive. Primitive data types are the ones that are predefined in Javascript whereas non-primitive data types are the ones that are created by the programmer. 

Primitive Data types : 

  • String
  • BigInt
  • Boolean
  • Number
  • Undefined
  • Null
  • Symbol

  Non Primitive Data types: 

  •  Objects 

2. What is the difference between ‘==’ and ‘===’ ?

Both the equals sign are used to test equality on the left and right-hand side of the operator but there is a difference between them. The ‘==’ is you can say a loose equality operator. This operator first changes the value on both sides of the operator to a common data type and then compares the value and returns true if they are equal and false if they are not. 

The ‘===’ is you can say a strict equality operator. This operator compares not just the values but also the type of values on both sides of the operator. 

3. Is Javascript a case sensitive language? 

Yes, Javascript is a case sensitive language. We have to be careful with capitalisation whenever we are using keywords or function name or variable name or any other identifier in Javascript.

4. What is the use/purpose of ‘this’ operator in Javascript? 

The ‘this’ keyword in Javascript is one of those features that confuses a lot of developers. The ‘this’ in Javascript is used differently than in any other language for example Java or Python. ’This’ references the object that is executing the current function. If that function is part of an object, we call that function a method. So if that function is a method in an object ‘this’ references that object itself. Otherwise, if that function is a regular function, that is if that function is not a part of an object, ‘this’ keyword references the global object. 

5. What is Callback in Javascript? 

A callback is basically a function that is to be executed after another function has finished executing. Hence giving it a name-callback. In javascript, functions are also objects because which functions can take functions as arguments and can be returned by another function.

Some of the callbacks are also events that are called to give the user a chance to react when a certain state is triggered, for example, an alarm getting triggered when the number of transactions to be executed per minute goes way beyond the threshold value.

6. What is a cookie in Javascript? 

A cookie is actually the amount of information that is in between the client-side and the server-side. This information is stored by the web browser at the time of browsing. That is the reason that before using some of the web sites, you get an option to accept all the cookies!  

Whenever a request is sent from the client-side to the server-side, each time it is treated as a new request sent by a new user. To recognise the old user, a cookie is added with the request. Due to this cookie, the server identifies the user on the client-side. 

7. What is NaN property in Javascript? 

NaN stands for Not a number. As it is pretty clear from the name, NaN property of Javascript returns true when a variable passed to it is not a number and otherwise returns false. The NaN property is the same as the Number.NaN property. We can use isNaN() – a global function to check if a value is NaN value. 

8. What is the difference between Java and Javascript? 

Most beginners often get confused between Java and Javascript. But the only thing in common between them is the word Java. It is an object-oriented programming language whereas Javascript is an object-oriented scripting language.      

 Java requires a pre-setup of JDK and JRE whereas javascript does not require them. In java, the code is first compiled but in Javascript, the codes are all in text and do not need to compile. It runs on a virtual machine whereas Javascript runs on a web browser. 

9. What is closure in Javascript?

In Javascript, a closure is always created with the creation of a function. The closure is a Javascript property in which the inner function has access to the outer function’s variable.

Due to this property, the closure has three scopes:

  • Access to variables of outer function variables as mentioned in the definition of closures.
  • Access to its own scope.
  • Access to all the global variables.

10. What is DOM in Javascript? 

 DOM stands for Document Object Model. DOM is used when we interact with web pages. Whenever a web page is loaded, the browser creates a Document Object Model of the page. Using DOM, we can manipulate and create different elements inside an HTML document. 

 It gives Javascript the ability to make any change in HTML elements and attributes on the page. Not only can we modify but also create, add or delete HTML elements and attributes. DOM also gives Javascript the power to change the styling, i.e, the CSS styles of the page. 

Frequently Asked Questions

Is Javascript important from an interview point of view?

Yes, Javascript is one of the most in demand skills in the IT industry today. Anyone who aims to become a developer in any tech firm should know Javascript. Companies like Microsoft, Uber, KPMG, and SAP frequently ask questions on Javascript.

Which language is best for coding?

Just because this article focuses on Javascript, don’t expect me to write that it is best for coding. There is no one language that one can say is best at coding. It is not the language, it is you who should be good at coding. Just focus on one language and learn all its concepts. Don’t focus on language, focus on developing your programming skills. No company demands you to learn a specific language for their interview.

Is Javascript good for coding interviews?

As mentioned before, no language is bad for coding interviews. So yes, if you like coding in
Javascript and want to give an interview in this language, it is as perfect as any other language.

Key Takeaways

These are just the basic set of Javascript interview questions that are generally asked every time. Some of the TopTech companies like IBM, eBay, Microsoft, SAP and Dassault have asked these questions in their interviews. These are some of the must-know questions and understanding them lays a strong foundation for us when we proceed to answer advanced level questions. 

Javascript is an interesting language and could be so much fun to learn from the start. This is one language that you can start applying to your college projects as soon as you learn the basics and see the results in the form of beautiful websites or apps or video games! 

By Yashvee Gupta

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