Top 51 JavaScript Interview Questions for 1 to 2 Years Experienced 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 front-end and back-end applications independently.

When it comes to JavaScript interviews, it's not an easy nut to crack because 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 the basics and discover new ways of doing things in JavaScript.

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

I plan to keep this post as a live document and keep adding new JavaScript interview questions as and when I find them; if you want, you can also suggest, and I can add those. I have not provided the answers to 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.

If you're new to JavaScript, then I also suggest you first go through these comprehensive best JavaScript online courses to brush up on your JavaScript fundamentals and learn some advanced JavaScript features and concepts introduced in ES 6.

50+ JavaScript Interview Questions Answers for Web Developers

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.

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

Both == and === are used to compare values of variables in JavaScript but the == only compare values while === compares both value and type. This means === is used for for strict comparision and also known as strict equality operator. 

For example if you have two variables

var a = 5;
var b = "5"

console.log( a == b) // will print true
console.log( a===b) // will print false

because even though both variables a and b has same value 5, a is a numeric variable and b is a string variable. 

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 for 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) Can you explain 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 that 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 does 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 have you used?

32) Is JavaScript an object-oriented programming language?

33)  What is event bubbling and capturing in JavaScript?

34)  How can promises 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 difference between subStr() and subString() method in JavaScript? (answer)

While both these functions are used to get the subsequence or substring from a given String but their difference lies in what parameter they take and what does the return. The substr(to, length) and substring(to, from) both take two parameters, but substr takes the length of the substring to be returned, while substring takes end index (excluding) for substring.

Here is a nice diagram which highlight the difference between subStr() and subString() functions in JavaScript:

51. What is the difference between let, var, and const declaration in JavaScript?

All three keywords are used to declare variables in JavaScript but var provides function level scope and let and const provide block level scope. This means  if a variable is declared using var keyword outside a function then it will act as a global variable and accessible everywhere but let variable are only visible on block like if they are declared in loop then they will not be accessible outside the loop.

Also let and const are only available from ES7 onwards while var is available in all JavaScript version. You can also reassign values to var and let variable but const variable cannot be reassigned as it is used to declare constant, much like final modifier in Java programming language. 

If you want to learn more about these three keywords you can also checkout my article var vs let vs const variables in JavaScript where I have explained them in details with code examples. 

Here is also a nice diagram which highlights the key difference between var, let, and const variables in JavaScript. 

That's all about popular JavaScript Interview Questions and answers for practice 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.


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• Top 5 Books to Learn JavaScript for beginners (books)
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• 5 Free JavaScript Courses for Web Developers (free)
• 10 JavaScript frameworks Web developers can learn (framework)
• Top 5 Courses to learn TypeScript for Beginners (courses)

Thanks for reading this article so far. If you find these JavaScript interview questions useful, then please share them with your friends and colleagues. If you have any doubts or questions or trouble finding the answer to any of the above JavaScript interview questions, 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 these best JavaScript online courses; it's one of the best courses to learn JavaScript.