Top 50 JavaScript Interview Questions and Answers

Master the most frequently asked JavaScript interview questions covering Closures, Hoisting, Scope, Variables, and Core JavaScript Fundamentals.

JavaScript Fundamentals

1. What is JavaScript?

JavaScript is a high-level, single-threaded, interpreted programming language used to build dynamic and interactive web applications.

2. What are the primitive data types in JavaScript?

String
Number
Boolean
Undefined
Null
Symbol
BigInt

3. What is the difference between null and undefined?

undefined means a variable has been declared but not assigned a value.

null represents an intentional absence of value.

4. Difference between == and ===?

== performs type coercion before comparison.

=== compares both value and datatype.

"10" == 10   // true

"10" === 10  // false

5. What is NaN?

NaN stands for Not a Number and represents an invalid numerical result.

Number("Hello") // NaN

Hoisting Questions

6. What is Hoisting?

Hoisting is JavaScript's behavior of moving declarations to the top of their scope before execution.

7. What is hoisted in JavaScript?

Function Declarations
var Declarations
let and const (but not initialized)

8. Output?

console.log(a);

var a = 5;

Answer:

undefined

9. Output?

console.log(a);

let a = 5;

Answer:

ReferenceError

10. What is Temporal Dead Zone?

The period between variable hoisting and initialization of let or const variables.

11. Are functions hoisted?

Function declarations are fully hoisted.

greet();

function greet() {
 console.log("Hello");
}

12. Are function expressions hoisted?

No. Only the variable declaration is hoisted.

sayHi();

var sayHi = function() {
 console.log("Hi");
}

Scope Questions

13. What is Scope?

Scope determines where variables can be accessed in a program.

14. Types of Scope?

Global Scope
Function Scope
Block Scope

15. Difference between var, let, and const?

Feature	var	let	const
Block Scope	No	Yes	Yes
Redeclare	Yes	No	No
Reassign	Yes	Yes	No

Closures Questions

16. What is a Closure?

A closure is a function that remembers variables from its outer scope even after the outer function has finished executing.

17. Closure Example

function counter(){

 let count = 0;

 return function(){

  count++;

  return count;

 }

}

const increment = counter();

console.log(increment());
console.log(increment());

18. Why are Closures useful?

Private Variables
Data Hiding
Memoization
Event Handlers
Function Factories

19. What is Lexical Scope?

A child function can access variables from its parent scope.

20. Can Closures cause memory leaks?

Yes. Keeping unnecessary references alive can prevent garbage collection.

Promises Questions

21. What is a Promise in JavaScript?

A Promise is an object that represents the eventual completion or failure of an asynchronous operation.

22. What are the states of a Promise?

Pending
Fulfilled (Resolved)
Rejected

23. Create a Simple Promise

const promise = new Promise(
 (resolve, reject) => {

  resolve("Success");

 }
);

24. What is .then()?

.then() executes when a Promise is resolved successfully.

promise.then((data)=>{
 console.log(data);
});

25. What is .catch()?

.catch() handles Promise rejections and errors.

promise.catch((error)=>{
 console.log(error);
});

26. What is .finally()?

.finally() runs regardless of whether the Promise succeeds or fails.

27. Difference Between Promise.all() and Promise.allSettled()?

Method	Behavior
Promise.all()	Fails if any Promise fails
Promise.allSettled()	Waits for all Promises

28. What is Promise.race()?

Returns the first settled Promise (resolved or rejected).

29. What is Promise.any()?

Returns the first successfully resolved Promise and ignores rejections.

30. What is Callback Hell?

Callback Hell occurs when multiple nested callbacks make code difficult to read and maintain.

getUser(function(user){

 getPosts(user.id,function(posts){

  getComments(posts[0].id,function(comments){

   console.log(comments);

  });

 });

});

Async/Await Questions

31. What is async?

The async keyword makes a function automatically return a Promise.

async function hello(){

 return "Hello";

}

32. What is await?

await pauses execution until the Promise resolves.

const data =
await fetchData();

33. Why use async/await?

Cleaner Syntax
Better Readability
Easier Error Handling

34. How do you handle errors in async/await?

try{

 const data =
 await fetchData();

}catch(error){

 console.log(error);

}

35. Can await be used outside async?

No, except in modern ES modules supporting top-level await.

Event Loop Questions

36. What is the Event Loop?

The Event Loop allows JavaScript to handle asynchronous operations even though JavaScript is single-threaded.

37. Explain Event Loop Architecture

Call Stack

↓

Web APIs

↓

Callback Queue

↓

Event Loop

↓

Call Stack

38. What is the Call Stack?

The Call Stack stores function execution contexts and tracks currently executing functions.

39. What are Web APIs?

Browser-provided features such as:

setTimeout
DOM Events
Fetch API
Geolocation

40. What is the Callback Queue?

The Callback Queue stores asynchronous callbacks waiting to be executed.

41. Output?

console.log(1);

setTimeout(()=>{

 console.log(2);

},0);

console.log(3);

Answer:

1
3
2

42. What is the Microtask Queue?

A high-priority queue used for:

Promise callbacks
queueMicrotask()
MutationObserver

43. Output?

console.log("Start");

Promise.resolve()
.then(()=>{

 console.log("Promise");

});

console.log("End");

Answer:

Start
End
Promise

44. Which executes first: Promise or setTimeout?

Promise callbacks execute first because Microtasks have higher priority than Macrotasks.

this Keyword Questions

45. What is this in JavaScript?

this refers to the object that is executing the current function.

46. What is this in the Global Scope?

Browser:

window

Node.js:

module.exports

47. What is this inside an Object Method?

const user = {

 name:"John",

 greet(){

  console.log(
   this.name
  );

 }

};

this refers to the user object.

48. What is this inside a Regular Function?

Its value depends on how the function is invoked.

49. What is this inside an Arrow Function?

Arrow functions do not have their own this.

They inherit this from the surrounding lexical scope.

50. How can you change the value of this?

Using:

call()
apply()
bind()

function greet(){

 console.log(
  this.name
 );

}

greet.call({
 name:"John"
});

Prototype Questions

51. What is a Prototype?

Every JavaScript object has a hidden internal property called a prototype.

Objects inherit properties and methods from their prototype.

const obj = {};

console.log(
 obj.__proto__
);

52. What is Prototype Inheritance?

Prototype inheritance allows one object to access properties and methods from another object through the prototype chain.

const animal = {

 eat(){

  console.log("Eating");

 }

};

const dog =
Object.create(animal);

dog.eat();

53. What is the Prototype Chain?

When JavaScript cannot find a property on an object, it searches the object's prototype and continues up the chain until the property is found or null is reached.

54. Difference Between proto and prototype?

__proto__	prototype
Reference to parent object	Property of constructor function
Used by instances	Used for inheritance

call(), apply(), bind()

55. What is call()?

call() invokes a function immediately and allows you to set the value of this.

function greet(){

 console.log(this.name);

}

greet.call({
 name:"John"
});

56. What is apply()?

apply() works like call() but accepts arguments as an array.

function sum(a,b){

 console.log(a+b);

}

sum.apply(
 null,
 [10,20]
);

57. What is bind()?

bind() returns a new function with this permanently bound.

const user = {

 name:"John"

};

function greet(){

 console.log(this.name);

}

const fn =
greet.bind(user);

fn();

58. Difference Between call(), apply(), and bind()?

Method	Executes Immediately	Arguments
call()	Yes	Comma Separated
apply()	Yes	Array
bind()	No	Returns New Function

Tricky Output Questions

59. Output?

console.log(
 typeof null
);

Answer:

object

This is a historical JavaScript bug.

60. Output?

console.log(
 NaN === NaN
);

Answer:

false

61. Output?

console.log(
 [] + []
);

Answer:

""

62. Output?

console.log(
 [] + {}
);

Answer:

"[object Object]"

63. Output?

console.log(
 true + true
);

Answer:

64. Output?

console.log(
 "5" - 2
);

Answer:

65. Output?

console.log(
 "5" + 2
);

Answer:

"52"

66. Output?

for(var i=0;i<3 i="" settimeout="">{

  console.log(i);

 },0);

}

Answer:

3
3
3

67. Output?

for(let i=0;i<3 i="" settimeout="">{

  console.log(i);

 },0);

}

Answer:

0
1
2

Advanced JavaScript Questions

68. What is Event Delegation?

Event Delegation is a technique where a parent element handles events for its child elements using event bubbling.

69. What is Debouncing?

Debouncing delays function execution until a specified time has passed after the last event.

Common use:

Search Inputs
API Calls
Resize Events

70. What is Throttling?

Throttling limits how often a function can execute during a given time period.

71. What is Currying?

function add(a){

 return function(b){

  return a+b;

 }

}

console.log(
 add(2)(3)
);

72. What is Memoization?

Memoization stores previously calculated results to improve performance.

73. What is Shallow Copy?

A shallow copy copies only the first level of properties.

const copy = {
 ...obj
};

74. What is Deep Copy?

A deep copy recursively copies nested objects.

structuredClone(obj);

75. What is Garbage Collection?

JavaScript automatically frees memory that is no longer referenced.

JavaScript Interview Preparation Strategy

Must-Know Topics

Closures
Hoisting
Scope
Promises
Async/Await
Event Loop
this Keyword
Prototype Chain
call(), apply(), bind()
Debouncing & Throttling

2-Year Experience Interview Checklist

JavaScript Fundamentals

↓

Closures

↓

Hoisting

↓

Promises

↓

Async/Await

↓

Event Loop

↓

DOM Manipulation

↓

API Integration

↓

ES6+

↓

Interview Ready 🚀

Frequently Asked Questions

Which JavaScript topics are asked most?

Closures, Promises, Hoisting, Event Loop, Async/Await, and this keyword are the most common interview topics.

Are output questions important?

Yes. Many interviewers use output-based questions to test real understanding of JavaScript behavior.

Should I focus on theory or coding?

Both. Strong theoretical knowledge combined with practical coding examples gives the best interview performance.

What level of JavaScript is expected for 2 years experience?

You should be comfortable with asynchronous programming, closures, APIs, debugging, ES6 features, and common design patterns.

Conclusion

JavaScript interviews are less about memorizing syntax and more about understanding how the language works internally.

Mastering Closures, Promises, Hoisting, Event Loop, Async/Await, and the this keyword will help you answer a large percentage of interview questions confidently.

Practice these questions regularly, explain concepts aloud, and connect every concept to real projects you've built.

Strong JavaScript fundamentals remain one of the most valuable skills for frontend, backend, React, and Node.js developers.