135+ JavaScript Interview Questions (With Answers) Every Frontend Developer Should Know in 2026

JavaScript has become an essential skill for frontend developers, and in 2026, it’s still one of the most sought-after competencies in the tech industry. Whether you’re applying for a junior position or aiming for a senior developer role, JavaScript knowledge is a cornerstone of most technical interviews. Recruiters and interviewers focus on everything from basic syntax to complex asynchronous concepts. Having a strong grasp of JavaScript fundamentals, DOM manipulation, ES6 features, and asynchronous patterns will set you apart in a competitive job market.

This article will guide you through over 135+ of the most common and challenging JavaScript interview questions that every frontend developer should know. These questions are carefully categorized by topics such as Basics, Advanced Concepts, Asynchronous JavaScript, Functions, DOM Manipulation, ES6 Features, and more. By preparing for these topics, you’ll be ready to tackle interviews for top companies, including FAANG, startups, or any enterprise looking for a proficient JavaScript developer.

A. JavaScript Basics

1. What Is JavaScript?

JavaScript is a high-level, dynamic, interpreted programming language used primarily for creating interactive effects within web browsers. It’s a multi-paradigm language, supporting procedural, functional, and object-oriented programming styles. JavaScript is the backbone of most web applications and can run both client-side (in the browser) and server-side (through Node.js).

2. What Are JavaScript’s Data Types?

JavaScript has two categories of data types:

• Primitive Types: These include strings, numbers, booleans, undefined, null, symbol, and bigint.
• Reference Types: These include objects, arrays, and functions.

Understanding the difference between primitive and reference types is crucial for managing variable assignments and avoiding bugs.

3. What Is Hoisting in JavaScript?

Hoisting refers to JavaScript’s behavior of moving all declarations (not initializations) to the top of their containing scope. For example, variables declared with var are hoisted to the top and initialized with undefined, while let and const are hoisted but remain in the Temporal Dead Zone (TDZ) until their declaration is reached.

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

B. Functions and Scope

4. What Is a JavaScript Function?

A function is a block of reusable code that performs a specific task. Functions can accept inputs (parameters), execute some logic, and return a value. Functions in JavaScript are first-class citizens, meaning they can be assigned to variables, passed as arguments, and returned from other functions.

function add(a, b) {
  return a + b;
}

5. What Is a Closure in JavaScript?

A closure is a function that retains access to its lexical scope, even after the outer function has returned. Closures allow for the creation of private variables and are frequently used in scenarios like event handling and callbacks.

function makeCounter() {
  let count = 0;
  return function() {
    return ++count;
  };
}
const counter = makeCounter();
console.log(counter()); // 1
console.log(counter()); // 2

C. Advanced JavaScript Concepts

6. What Is Prototypal Inheritance?

Prototypal inheritance is the mechanism by which an object can inherit properties and methods from another object. Every JavaScript object has a prototype, and when you access a property or method of an object, JavaScript first checks the object itself and then checks its prototype chain.

function Animal(name) {
  this.name = name;
}
Animal.prototype.sayHello = function() {
  return `Hello, my name is ${this.name}`;
};const dog = new Animal('Rex');
console.log(dog.sayHello()); // Hello, my name is Rex

7. What Is the Event Loop in JavaScript?

The event loop is responsible for executing code, collecting and processing events, and executing queued sub-tasks. It enables asynchronous programming by handling operations like I/O requests, timers, and user interactions without blocking the main thread.

D. Asynchronous JavaScript

8. What Is a Promise?

A Promise is an object that represents the eventual completion or failure of an asynchronous operation. Promises have three states: pending, fulfilled, or rejected. They allow you to chain asynchronous operations in a clean, readable way.

const myPromise = new Promise((resolve, reject) => {
  const success = true;
  if (success) {
    resolve('Task successful');
  } else {
    reject('Task failed');
  }
});myPromise.then(result => console.log(result)).catch(error => console.error(error));

9. What Is the Difference Between setTimeout() and setInterval()?

• setTimeout() executes a function once after a specified delay.
• setInterval() repeatedly executes a function with a fixed time delay between each call.

Both functions are used to handle asynchronous operations based on time intervals.

E. DOM Manipulation

10. What Is the DOM in JavaScript?

The Document Object Model (DOM) is a programming interface for web documents. It represents the document as a tree structure, where each node is an object representing part of the page. JavaScript interacts with the DOM to read or change the content, structure, and style of the webpage.

document.getElementById('myElement').textContent = 'Hello, World!';

F. ES6 Features

11. What Are Template Literals?

Template literals (introduced in ES6) allow you to embed expressions inside string literals, using ${expression} syntax. They also support multi-line strings and improve string interpolation.

const name = 'Alice';
const greeting = `Hello, ${name}!`;
console.log(greeting); // Hello, Alice!

12. What Is Arrow Function Syntax?

Arrow functions are a shorthand for writing functions in JavaScript. They are anonymous and do not have their own this context, which can simplify certain function expressions.

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

G. Frequently Asked Questions

1. What Are the Common Data Types in JavaScript?

JavaScript has several data types, including primitive types like string, number, boolean, undefined, null, symbol, and bigint, as well as reference types like object and array.

2. What Is a Callback in JavaScript?

A callback is a function passed into another function as an argument to be executed later. It is commonly used for asynchronous operations like handling data returned from an API request.

3. How Do Promises Help with Asynchronous JavaScript?

Promises provide a cleaner way to handle asynchronous code compared to callbacks, helping to avoid “callback hell.” They allow for better error handling and chaining of multiple asynchronous tasks.

4. How Do You Define Variables in JavaScript?

Variables in JavaScript can be declared using var, let, or const. let and const are block-scoped and more modern, while var is function-scoped.

5. What Is the Difference Between null and undefined?

• undefined means a variable has been declared but not assigned a value.
• null represents the intentional absence of a value, typically assigned by the developer.

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Conclusion:

By understanding these fundamental JavaScript concepts and practicing them, you’ll be well-prepared for any frontend developer interview. Whether you are applying for a junior position or aiming for a senior role, knowing the ins and outs of JavaScript will give you the confidence to tackle any interview. So, take the time to not only memorize the answers but also to understand the why behind the concepts. Happy coding!

More Questions You can find out there,
JavaScript Basics (50+ Questions)

1. What is JavaScript, and how does it differ from Java?
2. What are the different data types in JavaScript?
3. What is the difference between null and undefined in JavaScript?
4. What are primitive types and reference types in JavaScript?
5. What is hoisting in JavaScript?
6. What are NaN and how do you check for it?
7. What is the typeof operator used for in JavaScript?
8. What is the purpose of isNaN() function?
9. What are closures, and how are they used in JavaScript?
10. What is the difference between == and === in JavaScript?
11. What is the let keyword used for in JavaScript?
12. What is the const keyword used for in JavaScript?
13. What is the var keyword used for in JavaScript?
14. What is scope in JavaScript, and how does it work?
15. What is the difference between block scope and function scope in JavaScript?
16. How does the this keyword work in JavaScript?
17. What are arrow functions, and how do they differ from regular functions?
18. What is a function expression in JavaScript?
19. What is a function declaration in JavaScript?
20. What is a callback function in JavaScript?
21. How does JavaScript handle type coercion?
22. What is a prototype in JavaScript?
23. What is the prototype chain in JavaScript?
24. What are higher-order functions in JavaScript?
25. What is a memoization technique in JavaScript?
26. What is recursion, and how is it used in JavaScript?
27. What is the bind() method in JavaScript?
28. What is the call() method in JavaScript?
29. What is the apply() method in JavaScript?
30. What is a JavaScript module, and how do you export and import modules?
31. What is the default export in JavaScript?
32. What are named exports in JavaScript?
33. What is destructuring in JavaScript?
34. What is the difference between shallow and deep copy in JavaScript?
35. What are rest and spread operators in JavaScript?
36. What is an IIFE (Immediately Invoked Function Expression)?
37. What are closures, and why are they important in JavaScript?
38. What is event delegation in JavaScript?
39. What is the arguments object in JavaScript?
40. How can you define a class in JavaScript?
41. What are getter and setter methods in JavaScript?
42. What is the purpose of the super keyword in JavaScript?
43. What is inheritance in JavaScript?
44. What is the difference between Object.create() and a constructor function in JavaScript?
45. What is a singleton pattern in JavaScript?
46. What is event bubbling and capturing in JavaScript?
47. What is localStorage and sessionStorage in JavaScript?
48. What is the JSON object in JavaScript, and how do you parse and stringify JSON data?
49. What is eval() in JavaScript, and why is it considered dangerous?
50. How can you handle errors in JavaScript?
51. Advanced JavaScript Concepts (50+ Questions)

51. What is the event loop in JavaScript?
52. What is the call stack, and how does it work in JavaScript?
53. What are microtasks and macrotasks in the JavaScript event loop?
54. How does asynchronous JavaScript work in terms of promises and callbacks?
55. What is a Promise in JavaScript?
56. What are Promise.all() and Promise.race() in JavaScript?
57. What is the difference between async/await and promises?
58. What are generators in JavaScript, and how do they differ from regular functions?
59. What is the Symbol type in JavaScript?
60. What is the WeakMap and WeakSet in JavaScript?
61. What is Object.freeze() in JavaScript?
62. What is the Object.seal() method in JavaScript?
63. What is Proxy in JavaScript?
64. What is the Reflect API in JavaScript?
65. What is async/await in JavaScript, and how is it used for asynchronous operations?
66. What are Map and Set in JavaScript, and how do they differ from regular objects and arrays?
67. What is the difference between Map and Object in JavaScript?
68. What is Object.assign() in JavaScript?
69. How does JavaScript’s garbage collection work?
70. What are WeakRef and FinalizationRegistry in JavaScript?
71. What are Promise.allSettled() and Promise.any() in JavaScript?
72. What is Object.defineProperty() in JavaScript?
73. What is a proxy object in JavaScript?
74. What is the Iterator and Iterable protocol in JavaScript?
75. What is async iterator in JavaScript?
76. What is lazy loading in JavaScript?
77. What are service workers, and how do they work in JavaScript?
78. What is a web worker in JavaScript, and how is it used for background processing?
79. What are decorators in JavaScript?
80. What is memoization, and how is it implemented in JavaScript?
81. What are the differences between class and prototype inheritance in JavaScript?
82. What is destructuring assignment in JavaScript?
83. How does closure work in JavaScript?
84. What is deep cloning in JavaScript?
85. What are setTimeout() and setInterval() in JavaScript?
86. What are the benefits of functional programming in JavaScript?
87. What is currying in JavaScript?
88. What is debouncing in JavaScript?
89. What is throttling in JavaScript?
90. What is the callback hell issue in JavaScript?
91. How can promises solve the callback hell problem?
92. How do you define constants in JavaScript?
93. What are template literals in JavaScript?
94. How do you handle null and undefined in JavaScript?
95. How does event delegation help improve performance in JavaScript?
96. What is the purpose of destructuring in arrays and objects in JavaScript?
97. What is the typeof operator in JavaScript?
98. What is deep equality comparison in JavaScript?
99. How does JavaScript implement garbage collection?
100. What is immutable state in JavaScript?

ES6+ Features (50+ Questions)

What is flat() and flatMap() in JavaScript arrays?

What are arrow functions in ES6, and how do they differ from regular functions?

What is let in ES6, and how does it differ from var?

What are template literals in ES6?

What is spread syntax in JavaScript?

What is rest parameters in JavaScript?

What is object destructuring in JavaScript?

What is array destructuring in JavaScript?

What are default parameters in JavaScript functions?

What is Symbol in JavaScript?

What is the class syntax in JavaScript (ES6)?

What is Set and Map in JavaScript?

What are generator functions in JavaScript?

What is async/await syntax in JavaScript?

How does modules work in JavaScript with import/export?

What is the import/export syntax in JavaScript?

What is Object.assign() in ES6?

What are template strings in ES6?

What is the for...of loop in ES6?

How does for...in loop differ from for...of in JavaScript?

What is the Promise object in JavaScript (ES6)?

What is Promise.all() in JavaScript?

What is the Promise.race() in JavaScript?

How do async functions help in asynchronous programming?

What are class methods in ES6 JavaScript?

How does inheritance work in JavaScript with ES6 classes?

What is the super keyword in JavaScript ES6 classes?

What is the export default in JavaScript?

How do decorators work in JavaScript?

What are array methods introduced in ES6 (e.g., map, filter, reduce)?

What is Object.entries() in JavaScript?

What is Object.values() in JavaScript?

What are async iterators in JavaScript?

What is Array.from() in JavaScript?

What is the find() method in JavaScript arrays?