Top 25 JavaScript Interview Questions and Answers (2026) - Beginner to Advanced Guide

Introduction

JavaScript is one of the most important programming languages in the world. Whether you are preparing for a Frontend Developer, Backend Developer (Node.js), or Full Stack Developer interview, strong JavaScript fundamentals are absolutely essential.

Most companies - from startups to big tech firms - test your understanding of core JavaScript concepts like closures, hoisting, event loop, promises, prototypes, and ES6 features.

This article covers the Top 25 most asked JavaScript interview questions with detailed explanations, examples, and interview tips to help you crack technical interviews confidently.

If you master these questions, you can answer nearly 80-90% of JavaScript interview questions.

Why JavaScript is Important for Interviews?

• JavaScript is the foundation of modern frontend frameworks like React, Angular, and Vue.
• Node.js uses JavaScript for backend development.
• Understanding core concepts improves problem-solving skills.
• Interviewers test fundamentals more than framework knowledge.

Who Should Read This Guide?

• Beginner developers preparing for their first job
• Frontend developers preparing for React/Angular interviews
• Backend developers working with Node.js
• Full Stack developers refreshing core concepts
• Students preparing for coding rounds

Top 25 JavaScript Interview Questions List

1. Difference between == and ===
2. What is Hoisting?
3. Explain the Event Loop
4. What is a Closure?
5. var vs let vs const
6. JavaScript Data Types
7. Explain this keyword
8. Arrow Functions
9. call(), apply(), bind()
10. Prototypal Inheritance
11. Shallow Copy vs Deep Copy
12. What is Strict Mode?
13. What is a Promise?
14. async and await
15. null vs undefined
16. What is Callback Hell?
17. Event Bubbling
18. Event Delegation
19. Execution Context
20. Stack vs Heap Memory
21. Debouncing vs Throttling
22. Map vs Object
23. Set in JavaScript
24. Spread Operator
25. ES6 Modules

1. What is the difference between == and ===?

== (Abstract Equality) compares values after performing type conversion (type coercion).

=== (Strict Equality) compares both value and data type without conversion.

5 == "5"    // true
5 === "5"   // false

0 == false  // true
0 === false // false

2. What is Hoisting?

Hoisting is JavaScript's behavior of moving variable and function declarations to the top of their scope during compilation.

Example with var:

console.log(a);
var a = 10;

Output: undefined

Because internally JavaScript treats it as:

var a;
console.log(a);
a = 10;

let and const are hoisted but remain in the Temporal Dead Zone until initialized.

3. What is the Event Loop?

JavaScript is single-threaded but can handle asynchronous tasks using the Event Loop.

It works with:

• Call Stack
• Web APIs
• Callback Queue
• Event Loop

console.log("Start");

setTimeout(() => {
  console.log("Timeout");
}, 0);

console.log("End");

Output:

Start
End
Timeout

4. What is a Closure?

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

function outer(){
  let count = 0;
  return function(){
    count++;
    return count;
  }
}

Closures are used for data privacy and function factories.

5. Difference Between var, let, and const

Important: const objects can still be modified internally.

6. What are JavaScript Data Types?

JavaScript has two categories of data types:

Primitive: string, number, boolean, undefined, null, bigint, symbol

Reference (Non-Primitive): Object, Array, Function

Primitive values are stored in Stack memory, while reference types are stored in Heap memory.

7. What is the this keyword?

The value of this depends on how a function is called.

• Global scope → window (browser)
• Inside object method → the object itself
• Constructor → new instance
• Arrow function → inherits from parent scope

8. What are Arrow Functions?

Arrow functions provide shorter syntax and do not have their own this.

const add = (a,b) => a + b;

They cannot be used as constructors and do not have arguments object.

9. call(), apply(), and bind()

These methods control the value of this.

• call() – arguments passed individually
• apply() – arguments passed as array
• bind() – returns new function

fn.call(obj, arg1, arg2);
fn.apply(obj, [arg1, arg2]);
const newFn = fn.bind(obj);

10. What is Prototypal Inheritance?

JavaScript uses prototype-based inheritance. Every object has a hidden [[Prototype]] property.

If a property is not found, JS looks up the prototype chain.

11. Shallow Copy vs Deep Copy

Shallow Copy: Copies first level only.

Deep Copy: Copies nested objects fully.

const obj2 = {...obj1}; // shallow
const deep = JSON.parse(JSON.stringify(obj1));

12. What is "use strict"?

Strict mode helps write secure JS by preventing accidental globals and silent errors.

"use strict";
x = 10; // Error

13. What is a Promise?

A Promise represents future completion of async operation.

States: Pending → Fulfilled → Rejected

fetch(url)
 .then(res => res.json())
 .catch(err => console.log(err));

14. async and await

async makes a function return a Promise. await pauses execution until Promise resolves.

async function getData(){
 const res = await fetch(url);
}

15. null vs undefined

undefined: declared but not assigned.

null: intentional empty value.

16. What is Callback Hell?

Nested callbacks making code unreadable.

getData(function(a){
 getMoreData(a,function(b){
  getEvenMore(b,function(c){
  });
 });
});

Solution: Promises or async/await.

17. What is Event Bubbling?

Event starts from target element and bubbles up to parent elements.

18. What is Event Delegation?

Attach event listener to parent to manage child elements.

Improves performance and handles dynamic elements.

19. What is Execution Context?

Environment where JS runs.

Types:

• Global Execution Context
• Function Execution Context

Phases: Creation Phase and Execution Phase.

20. Stack vs Heap Memory

Stack: Stores primitive values.

Heap: Stores objects and reference types.

21. Debouncing vs Throttling

Debounce: Executes after delay.

Throttle: Executes at fixed intervals.

Common in search bars and scroll events.

22. Map vs Object

Map: Any key type, ordered, better performance.

Object: String/Symbol keys only.

23. What is Set?

Set is a collection of unique values.

const s = new Set([1,1,2,3]);
console.log(s); // 1,2,3

24. What is Spread Operator?

Spread operator (...) expands arrays or objects.

const arr2 = [...arr1];
const obj2 = {...obj1};

25. What are ES6 Modules?

Modern way to share code between files.

export default function(){}
import x from './file.js';

ES Modules replace CommonJS in modern JS development.