Top 50 JavaScript Interview Questions for Programmers and Web Developers

Hello guys, you may know that JavaScript is one of the most popular programming languages, having ranking #1 for a couple of years in the StackOverflow survey. There is no doubt that JavaScript is the most popular language or web development. The best thing is that you can use JavaScript to develop both frontend and backend using the same programming language and tech stack. You have so many popular frameworks like React.js, Angular, Node.js, Vue.js to implement sophisticated web applications for different domains. That's why the demand for JavaScript developers is very high, especially full-stack JavaScript developers who can create both frontend and backend of application on their own.

When it comes to JavaScript interview, it's not an easy nut to crack becuase JavaScript has evolved so much. With so many new features introduced in recent JavaScript releases, you need to spend a good deal of time to recap basics as well as discover new ways of doing things in JavaScript.

I have been collecting some JavaScript interview questions to practice and I decided to put together a list of 100+ JavaScript questions that I can take a look before going to an interview. So far, I have collected 50+ JavaScript interview questions which I am going to share with you today.

I have the plan to keep this post as a live document and keep adding new JavaScript interview questions as and when I find, if you want, you can also suggest and I can add those. I have not provided the answers of these JavaScript questions but you can easily find them by doing Google, Though I also plan to put short answers for a telephonic round on these questions

Btw, if you re new to JavaScript then I also suggest you first go through a comprehensive Java course like The Complete JavaScript Course 2020: Build Real Projects! to brush up your JavaScript fundamentals and learn some advanced JavaSCript features and concepts introduced in ES 6.



50+ JavaScript InterviweQuestiosn for Beginners

Without wasting any more of your time, here is my list of some of the most popular JavaScript interview questions. If you are going for a web developer interview, you can review these questions to gain some confidence.

Top 50 JavaScript Interview Questions for Programmers and Web Developers


1) What is difference between "==" and "===" in JavaScript? (answer)

2) What is NaN? What is its type? How can you reliably test if a value is equal to NaN?

3) How to reduce the loading time of the web page without concern too much refactoring?

4) If two div on the page have the same id ("mydiv") then getElementById("mydiv") will refer to which div?

5) How to handle large JSON on UI if rendering is slow?

6) What is the difference between creating a variable with var or without var like

var x = 3;
 
and 
 
x = 3;

7) Parent/Child communication in JavaScript?

8) How to make an Ajax call in JavaScript?

9) What is the setTimeout () in JavaScript?

10) What are closures in JavaScript?

11) What is the difference between JavaScript and TypeScript?

12) What are scopes in JavaScript?

13) How to define the class in JavaScript?

14) How to invalidate the session when the user clicks a back button?

15) What is # and ! in URL?

16) What are callbacks?

17) Suppose we have an error on production which is not producible on Local and we don’t have access to the production and we have only minified file the production then how you can track the error?

18) What is async and defer in JavaScript?

19) How to find the inner classes of a div by using JavaScript?

20) What is the difference between promise and callback?

21) What is the view engine?

22) What are bind filters and reduce map in js?

23) Difference between primitive and non-primitives data types?

24) How JavaScript interpret DOM that is rendering order or control flow?

25) What is the hoisting in JavaScript?

26) What is the difference between the following two snippets

console.log(abc());
function abc(){
 return “JavaScript”
}
 
and
 
console.log(abc());
var abc = function(){
  return “JavaScript”;
}

27) What is Inheritance in JavaScript?

28) Local scope and block-level scope in JavaScript?

29) What is the prototype in JavaScript?

30) What are some new features from JavaScript ES6?

31) Which Javascript framework and libraries you have used?

32) Is JavaScript an object-oriented programming language?

33)  What is event bubbling and capturing in JavaScript?

34)  How promises can be implemented in JavaScript?

35) How to prevent users from double submitting of form?

36) How do you declare an array in Javascript?

37) How do you convert String to number in Javascript?

38) What is the difference between null and undefined in Javascript?

39) How would you check if a variable is null/undefined?

40) How do you if a variable is an Object in Javascript?

41) What is the result of accessing the undeclared variable in JavaScript?

42) How do you print debug statements in JavaScript?

43) How to iterate an array in JavaScript

44) difference between == vs === in JavaScript? 

45)  What are some best practices in JavaScript?

46) Where do you place your JavaScript on the page?

47) What is an object in JavaScript?

48) What is the concept of Event bubbling?

49) What is "this" in JavaScript?

50) What is the difference between let, var, and const declaration in JavaScript?


That's all about some of the most popular JavaScript Interview Questions. You should review these questions before you go for any JavaScript and Web Developer interview. I haven't shared the answers but you can easily find them on the Internet and you may know many of them already. I also plan to put a short answer when I get some time.

Further Learning
JavaScript: Getting Started
Advanced JavaScript Concepts
The Complete JavaScript Course 2020: Build Real Projects!


Other JavaScript and Web Development Articles you may like:
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  • 10 Courses to Learn JavaScript in depth (courses)
  • Top 5 Books to Learn JavaScript for beginners (books)
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  • The Complete React Developer RoadMap (guide)
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  • 5 Free JavaScript Courses for Web Developers (free)
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  • Top 5 Courses to learn TypeScript in 2020 (courses)

Thanks for reading this article so far. If you find these JavaScript interview questions useful then please share with your friends and colleagues. If you have any doubts or questions or trouble finding the answer to any of the above JavaScript interview question then please drop a note.

P. S. - If you are completely new to JavaScript but want to be ready for an interview in a quick time like one week or one month then I suggest you start learning JavaScript from a hands-on course like The Complete JavaScript Course 2020: Build Real Projects! by Jonas Schemdtmann, It's one of the best courses to learn JavaScript. 

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