JavaScript Coding Interview Questions and Answers (2025)

JavaScript has grown from a simple scripting language for browsers into one of the most in-demand technologies in the world. It powers everything from small websites to large-scale enterprise applications, which is why JavaScript is at the center of coding interview prep today.

If you’re preparing for a JavaScript interview, the right preparation will set you apart. Employers don’t just want to know if you can write a function or manipulate an array. They want to see if you understand language-specific quirks, how the event loop works, and whether you can use JavaScript to solve both frontend and backend problems. That’s what makes JavaScript coding interview questions unique: they test not just algorithms, but also your ability to reason about how JavaScript behaves under the hood.

This guide provides in-depth explanations, practical examples, and real-world insights into the most common JavaScript coding interview questions.

Why JavaScript Is a Popular Choice in Coding Interviews

When it comes to interviews, JavaScript is one of the most versatile languages you can be tested on. It runs in every browser and, thanks to Node.js, on servers too. That means your skills are immediately useful across the entire stack.

With JavaScript, you can write client-side code for interactive UIs and server-side code for APIs or backend systems. This dual capability makes it a favorite for interviewers who want to see how well you handle different environments. Nearly every company that builds web applications relies on JavaScript in some form. Whether it’s a startup using React or an enterprise running Node.js in production, JavaScript expertise is a must-have skill.

That’s why you’ll face many JavaScript coding interview questions designed to test your ability to think like both a frontend and backend developer. This makes mastering JavaScript one of the smartest moves you can make for interview prep.

Categories of JavaScript Coding Interview Questions

JavaScript interview prep becomes much easier once you understand the main categories of questions. Each category focuses on a specific skillset, from syntax basics to system-level thinking.

Here are the most common categories of JavaScript coding interview questions:

Basic syntax and operators
Covers data types, operators (== vs ===), and expressions.
Variables, scope, and hoisting
Tests your knowledge of var, let, const, and how variable lifetimes are managed.
Functions and closures
Focuses on lexical scope, closure behavior, and this binding.
Objects and prototypes
Includes inheritance, this, and prototype chains.
ES6+ features
Questions on arrow functions, destructuring, spread/rest operators, classes, and modules.
Asynchronous JavaScript
Tests knowledge of callbacks, promises, async/await, and the event loop.
DOM manipulation and events
Practical questions about interacting with HTML elements and handling user input.
Data structures and algorithms in JS
Classic coding problems solved using JavaScript arrays, objects, maps, and sets.
Advanced topics
Covers memory leaks, deep vs shallow copy, and performance optimization.
System design with JavaScript
For senior roles, designing scalable apps using JavaScript concepts.

By covering each category in depth, you’ll be ready for questions ranging from beginner-level syntax to advanced system design. In the next section, we’ll start with the basics.

Basic JavaScript Coding Interview Questions

Before tackling async problems or complex algorithms for coding interviews, interviewers want to see if you’ve mastered the fundamentals. Many JavaScript coding interview questions start with these basics before diving into closures, promises, or prototypes. Let’s explore some common ones.

1. What are the different data types in JavaScript?

Explanation: JavaScript has two main categories of data types.

Primitive types: string, number, boolean, null, undefined, symbol, bigint.
Non-primitive (reference) types: objects, arrays, functions.

Sample Answer:

“JavaScript has seven primitive data types, including string, number, and boolean. Non-primitives like arrays and objects are reference types, meaning they are stored by reference rather than value.”

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

Explanation:

== checks for equality after type coercion.
=== checks for strict equality without type conversion.

5 == ‘5’   // true

5 === ‘5’  // false

Sample Answer:

“== compares values with type conversion, while === checks both value and type. In interviews, it’s always safer to use === to avoid unexpected results.”

3. Explain var, let, and const.

Explanation:

var – function-scoped, hoisted, can be redeclared.
let – block-scoped, not hoisted the same way, can be reassigned but not redeclared.
const – block-scoped, must be initialized, cannot be reassigned.

var x = 1;

let y = 2;

const z = 3;

Sample Answer:

“var is function-scoped, while let and const are block-scoped. Use let for variables that change and const for those that don’t.”

4. What is hoisting?

Explanation:

Hoisting means variable and function declarations are moved to the top of their scope during compilation.

console.log(a); // undefined

var a = 5;

Sample Answer:

“Hoisting allows you to use variables or functions before declaring them. With var, the variable is hoisted but initialized as undefined. With let and const, you get a temporal dead zone error.”

5. Explain null vs undefined.

Explanation:

undefined means a variable has been declared but not assigned.
null is an assigned value that represents “no value.”

let a;

console.log(a); // undefined

let b = null;

console.log(b); // null

Sample Answer:

“undefined is the default state of an uninitialized variable, while null is explicitly set to represent nothing.”

Takeaway: These basic questions might feel easy, but they show whether you understand how JavaScript works at its core. Many JavaScript coding interview questions begin here to test your foundation before moving into more complex topics.

Functions, Scope, and Closures

Functions are at the heart of JavaScript. Interviewers love asking about them because they touch on scope, context, and one of the most powerful concepts in the language: closures.

1. Difference between function declaration and expression

Function declaration: Defined with the function keyword and hoisted to the top of the scope.
Function expression: Assigned to a variable. Not hoisted the same way.

// Declaration

function greet() {

  return “Hello!”;

}

// Expression

const greetExpr = function() {

  return “Hello!”;

};

Sample Answer:

“Function declarations are hoisted, meaning you can call them before defining them. Function expressions are not hoisted, so you must define them before calling.”

2. Arrow functions and their this binding

Arrow functions, introduced in ES6, are shorter and handle this differently.

const obj = {

  value: 42,

  arrowFn: () => console.log(this.value),

  normalFn: function() { console.log(this.value); }

};

obj.arrowFn();   // undefined

obj.normalFn();  // 42

Sample Answer:

“Arrow functions don’t bind their own this. Instead, they inherit it from the surrounding scope. Regular functions bind this dynamically based on how they are called.”

3. Explain closures with an example

A closure is when a function remembers variables from its lexical scope, even if called outside that scope.

function outer() {

  let count = 0;

  return function inner() {

    count++;

    return count;

  };

}

const counter = outer();

console.log(counter()); // 1

console.log(counter()); // 2

Sample Answer:

“A closure allows a function to access variables from its outer scope, even after that outer function has finished running. This is the foundation for data privacy and state management in JavaScript.”

4. What is lexical scope?

Lexical scope means a function’s scope is determined by where it is written, not where it is executed.

function outer() {

  let name = “JS”;

  function inner() {

    console.log(name);

  }

  inner();

}

outer(); // “JS”

5. Practical closure-based problem: Create a counter function

function createCounter() {

  let count = 0;

  return {

    increment: () => ++count,

    decrement: () => –count,

    reset: () => (count = 0)

  };

}

const counter = createCounter();

console.log(counter.increment()); // 1

console.log(counter.increment()); // 2

console.log(counter.reset());     // 0

This question tests if you can apply closures to real-world scenarios.

Takeaway: Closures and scope are fundamental. Many JavaScript coding interview questions revolve around them because they test your understanding of how functions and variables interact.

Objects, Prototypes, and Classes

Objects and inheritance are at the core of JavaScript’s design. You’ll often see interview questions about how prototypes and classes work.

1. Difference between objects and maps

Objects: Key-value pairs where keys are strings or symbols.
Maps: Introduced in ES6, allow any type as keys and preserve insertion order.

const obj = {a: 1};

const map = new Map();

map.set(1, “one”);

Sample Answer:

“Use objects when you need structured data with fixed keys. Use maps when keys can be dynamic or not strings.”

2. What is prototypal inheritance?

Every object in JavaScript has a hidden property called [[Prototype]]. If you try to access a property not found on the object, JavaScript looks up the prototype chain.

const parent = { greet: () => “Hello” };

const child = Object.create(parent);

console.log(child.greet()); // “Hello”

3. Explain how this works in JavaScript

this depends on how a function is called.
In a method, it refers to the object.
In global scope, it refers to the window (browser) or global object (Node).
In strict mode, global this is undefined.

const obj = { val: 100, get: function() { return this.val; } };

console.log(obj.get()); // 100

4. What are ES6 classes and how do they work behind the scenes?

ES6 classes are syntactic sugar over prototypes.

class Person {

  constructor(name) { this.name = name; }

  greet() { return `Hello ${this.name}`; }

}

Behind the scenes, methods are added to the prototype.

5. Example: Implement inheritance using both prototypes and ES6 classes

// Prototype-based

function Animal(name) { this.name = name; }

Animal.prototype.speak = function() { return `${this.name} makes noise`; };

function Dog(name) {

  Animal.call(this, name);

}

Dog.prototype = Object.create(Animal.prototype);

Dog.prototype.bark = function() { return `${this.name} barks`; };

// ES6 Class-based

class AnimalC {

  constructor(name) { this.name = name; }

  speak() { return `${this.name} makes noise`; }

}

class DogC extends AnimalC {

  bark() { return `${this.name} barks`; }

}

Takeaway: Objects and inheritance are essential. These JavaScript coding interview questions test whether you understand both the old prototype model and modern class syntax.

ES6+ Features in JavaScript Coding Interview Questions

Modern interviews often include ES6+ questions. They test whether you can use the features that make JavaScript cleaner and more powerful.

1. Template literals

const name = “Alice”;

console.log(`Hello, ${name}!`);

Sample Answer:

“Template literals let you embed variables and expressions inside backticks, making string handling easier.”

2. Destructuring assignment

const [a, b] = [1, 2];

const {x, y} = {x: 10, y: 20};

Makes it easier to unpack values from arrays and objects.

3. Spread and rest operators

const arr = [1,2,3];

const newArr = […arr, 4,5];

function sum(…nums) { return nums.reduce((a,b)=>a+b,0); }

Spread expands, rest gathers.

4. Modules (import/export)

// math.js

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

// main.js

import { add } from ‘./math.js’;

Encourages modular code.

5. Default parameters

function greet(name=”Guest”) { return `Hello, ${name}`; }

6. Example problem: Swap two variables using destructuring

let a=1, b=2;

[a, b] = [b, a];

console.log(a, b); // 2 1

Takeaway: Interviewers want to see if you can leverage modern syntax. Many JavaScript coding interview questions now focus on ES6+ because that’s what you’ll use daily in real projects.

Asynchronous JavaScript

Asynchronous code is one of the most tested areas because it separates average developers from those who understand how JavaScript really runs.

1. Explain callbacks and callback hell

function fetchData(callback) {

  setTimeout(() => callback(“data”), 1000);

}

fetchData(result => console.log(result));

Callback hell happens when callbacks are nested too deeply, making code unreadable.

2. Difference between promises and async/await

Promises: Represent future values.
async/await: Syntax sugar to write asynchronous code like synchronous code.

// Promise

fetch(“url”).then(res => res.json()).then(data => console.log(data));

// Async/await

async function getData() {

  const res = await fetch(“url”);

  const data = await res.json();

  console.log(data);

}

3. What is the event loop?

The event loop allows JavaScript to handle asynchronous tasks in a single-threaded environment.

Call stack executes synchronous code.
Web APIs handle async tasks.
Callback queue passes results back to the event loop.

4. Implement a function using setTimeout inside a loop

for (let i=1; i<=3; i++) {

  setTimeout(() => console.log(i), i*1000);

}

// prints 1, 2, 3 correctly with let

Using var would print 4 three times due to closure issues.

5. Promise.all vs Promise.race

const p1 = new Promise(res => setTimeout(()=>res(“one”), 1000));

const p2 = new Promise(res => setTimeout(()=>res(“two”), 2000));

Promise.all([p1,p2]).then(values => console.log(values)); // waits for all

Promise.race([p1,p2]).then(value => console.log(value)); // first resolved

Takeaway:

Understanding async code is a must. Many JavaScript coding interview questions revolve around promises, async/await, and the event loop because they directly impact performance and user experience.

DOM Manipulation and Events

The Document Object Model (DOM) is the interface between JavaScript and the browser. Many interviews test your ability to interact with it directly, since it reflects how you’ll handle real-world user interactions.

1. How to select and manipulate DOM elements

document.getElementById(“id”) → Select by ID.
document.querySelector(“.class”) → Select using CSS selectors.
element.innerText / element.innerHTML → Change text or HTML content.
element.style → Modify inline styles.

const title = document.getElementById(“title”);

title.innerText = “Hello, DOM!”;

title.style.color = “blue”;

2. Difference between event bubbling and capturing

Bubbling: Event starts from the target element and moves up the DOM tree.
Capturing: Event starts from the top of the DOM and moves down to the target.

element.addEventListener(“click”, handler, true);  // capturing

element.addEventListener(“click”, handler, false); // bubbling (default)

3. Explain event delegation

Instead of attaching an event listener to every child, you attach it to a parent and check the target. This improves performance.

document.getElementById(“list”).addEventListener(“click”, (e) => {

  if (e.target.tagName === “LI”) {

    console.log(“Clicked on”, e.target.innerText);

  }

});

4. Example problem: Create a button that increments a counter on click

<button id="btn">Click me</button>

<p id="count">0</p>

<script>

  let count = 0;

  document.getElementById("btn").addEventListener("click", () => {

    count++;

    document.getElementById("count").innerText = count;

  });

</script>

This tests whether you can connect user events to DOM updates.

Takeaway: DOM questions show up often because they test your ability to create interactive features. Many JavaScript coding interview questions focus on events, bubbling, and delegation.

Algorithmic JavaScript Coding Interview Questions

Algorithmic problems are the backbone of technical interviews. Solving them in JavaScript tests your ability to use arrays, objects, and built-in methods efficiently.

1. Two-sum problem (hashmap in JS)

Question: Find indices of two numbers that sum to a target.

Thought Process: Use a hashmap for \( O(n) \) lookup.

function twoSum(nums, target) {

  const map = {};

  for (let i=0; i<nums.length; i++) {

    let diff = target - nums[i];

    if (map[diff] !== undefined) return [map[diff], i];

    map[nums[i]] = i;

  }

}

Time complexity: \( O(n) \).

Space complexity: \( O(n) \).

2. Reverse a string without built-in methods

function reverseString(str) {

  let result = “”;

  for (let i = str.length-1; i>=0; i–) {

    result += str[i];

  }

  return result;

}

Time complexity: \( O(n) \).

3. Implement debounce and throttle

// Debounce

function debounce(fn, delay) {

  let timer;

  return function(…args) {

    clearTimeout(timer);

    timer = setTimeout(() => fn(…args), delay);

  };

}

// Throttle

function throttle(fn, limit) {

  let last = 0;

  return function(…args) {

    const now = Date.now();

    if (now – last >= limit) {

      fn(…args);

      last = now;

    }

  };

}

These test your understanding of performance optimization.

4. Binary search in JavaScript

function binarySearch(arr, target) {

  let left=0, right=arr.length-1;

  while(left<=right){

    let mid=Math.floor((left+right)/2);

    if(arr[mid]===target) return mid;

    if(arr[mid]<target) left=mid+1;

    else right=mid-1;

  }

  return -1;

}

Time complexity: \( O(log n) \).

5. DFS and BFS in JavaScript objects/graphs

// DFS

function dfs(graph, node, visited=new Set()) {

  if (visited.has(node)) return;

  console.log(node);

  visited.add(node);

  for (let neighbor of graph[node]) dfs(graph, neighbor, visited);

}

// BFS

function bfs(graph, start) {

  const queue=[start], visited=new Set([start]);

  while(queue.length){

    const node=queue.shift();

    console.log(node);

    for(let neighbor of graph[node]){

      if(!visited.has(neighbor)){

        visited.add(neighbor);

        queue.push(neighbor);

      }

    }

  }

}

6. Dynamic programming: Fibonacci with memoization

function fib(n, memo={}) {

  if(n<=1) return n;

  if(memo[n]) return memo[n];

  memo[n] = fib(n-1,memo) + fib(n-2,memo);

  return memo[n];

}

Time complexity: \( O(n) \).

Space complexity: \( O(n) \).

Takeaway: These classic problems are a staple. Many JavaScript coding interview questions revolve around them because they reveal how you solve problems efficiently.

Advanced JavaScript Coding Interview Questions

These advanced JavaScript coding interview questions are common in senior or full-stack interviews. They test your deep knowledge of the language.

1. What is the event loop and call stack?

Call stack: Executes synchronous code line by line.
Event loop: Handles async tasks by moving callbacks from the queue to the stack.

console.log(“A”);

setTimeout(()=>console.log(“B”),0);

console.log(“C”);

// Output: A, C, B

2. Explain memory leaks in JavaScript

Memory leaks happen when unused objects are not garbage collected. Common causes:

Global variables that aren’t cleared.
Forgotten timers or event listeners.
Closures holding unnecessary references.

3. Difference between deep copy and shallow copy

Shallow copy: Copies references (changes affect both).
Deep copy: Creates a full independent copy.

const obj = {a:1, b:{c:2}};

const shallow = {…obj}; // shallow

const deep = JSON.parse(JSON.stringify(obj)); // deep

4. How do you optimize performance in a large JavaScript app?

Use debounce/throttle for events.
Minimize DOM reflows.
Use lazy loading.
Split bundles using code-splitting.

Takeaway:

Answering these advanced JavaScript coding interview questions well shows you can handle not just syntax but also scalability and performance challenges.

Practice Section: Mock JavaScript Coding Interview Questions

Here are some full-length mock problems with solutions.

1. Design an LRU Cache in JavaScript

Thought Process: Use a Map which maintains insertion order.

class LRUCache {

  constructor(capacity){ this.cache=new Map(); this.capacity=capacity; }

  get(key){

    if(!this.cache.has(key)) return -1;

    let val=this.cache.get(key);

    this.cache.delete(key);

    this.cache.set(key,val);

    return val;

  }

  put(key,val){

    if(this.cache.has(key)) this.cache.delete(key);

    if(this.cache.size===this.capacity) this.cache.delete(this.cache.keys().next().value);

    this.cache.set(key,val);

  }

}

2. Implement currying

function curry(fn) {

  return function curried(…args) {

    if(args.length >= fn.length) return fn(…args);

    return (…next) => curried(…args,…next);

  }

}

3. Implement a custom Promise

class MyPromise {

  constructor(executor) {

    this.thenFn=null;

    executor((val)=>{ if(this.thenFn) this.thenFn(val); });

  }

  then(fn){ this.thenFn=fn; }

}

4. Build a simple event emitter

class EventEmitter {

  constructor(){ this.events={}; }

  on(event, listener){ (this.events[event]||(this.events[event]=[])).push(listener); }

  emit(event, …args){ (this.events[event]||[]).forEach(fn=>fn(…args)); }

}

5. Implement deep clone of an object

function deepClone(obj) {

  if(typeof obj!==”object” || obj===null) return obj;

  if(Array.isArray(obj)) return obj.map(deepClone);

  const copy={};

  for(let key in obj){ copy[key]=deepClone(obj[key]); }

  return copy;

}

Takeaway: Mock problems simulate real interviews. Practicing these JavaScript coding interview questions prepares you for coding challenges under pressure.

Tips for Solving JavaScript Coding Interview Questions

When solving interview problems, how you explain matters as much as the final answer.

Keep code clean and readable: Use indentation, avoid unnecessary tricks.
Always explain time and space complexity: Say it out loud.
Use meaningful variable names: sum is clearer than x.
Practice without browser consoles: Many interviews use plain editors or whiteboards.
Talk through your thought process: Explaining logic makes you more confident.
Avoid pitfalls:
- Implicit type coercion with ==.
- Overusing global variables.
- Forgetting to clean up event listeners → memory leaks.

These strategies will help you approach JavaScript coding interview questions with structure and clarity.

Wrapping Up

Preparing for interviews can feel overwhelming, but focusing on JavaScript gives you a huge advantage. It’s the one language that covers both frontend and backend.

By working through this guide, you’ve explored the most important categories of JavaScript coding interview questions, from syntax basics and closures to async programming, DOM, algorithms, and advanced performance challenges. You’ve also practiced with full-length problems that mirror real interviews.

Mastering JavaScript coding interview questions will give you confidence in both frontend and backend interviews. Keep practicing daily, revisit the fundamentals often, and challenge yourself with new problems.

Your preparation today is the path to acing your next interview.

JavaScript Coding Interview Questions​ and Answers (2025)

Why JavaScript Is a Popular Choice in Coding Interviews

Categories of JavaScript Coding Interview Questions

Basic JavaScript Coding Interview Questions

1. What are the different data types in JavaScript?

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

3. Explain var, let, and const.

4. What is hoisting?

5. Explain null vs undefined.

Functions, Scope, and Closures

1. Difference between function declaration and expression

2. Arrow functions and their this binding

3. Explain closures with an example

4. What is lexical scope?

5. Practical closure-based problem: Create a counter function

Objects, Prototypes, and Classes

1. Difference between objects and maps

2. What is prototypal inheritance?

3. Explain how this works in JavaScript

4. What are ES6 classes and how do they work behind the scenes?

5. Example: Implement inheritance using both prototypes and ES6 classes

ES6+ Features in JavaScript Coding Interview Questions

1. Template literals

2. Destructuring assignment

3. Spread and rest operators

4. Modules (import/export)

5. Default parameters

6. Example problem: Swap two variables using destructuring

Asynchronous JavaScript

1. Explain callbacks and callback hell

2. Difference between promises and async/await

3. What is the event loop?

4. Implement a function using setTimeout inside a loop

5. Promise.all vs Promise.race

DOM Manipulation and Events

1. How to select and manipulate DOM elements

2. Difference between event bubbling and capturing

3. Explain event delegation

4. Example problem: Create a button that increments a counter on click

Algorithmic JavaScript Coding Interview Questions

1. Two-sum problem (hashmap in JS)

2. Reverse a string without built-in methods

3. Implement debounce and throttle

4. Binary search in JavaScript

5. DFS and BFS in JavaScript objects/graphs

6. Dynamic programming: Fibonacci with memoization

Advanced JavaScript Coding Interview Questions

1. What is the event loop and call stack?

2. Explain memory leaks in JavaScript

3. Difference between deep copy and shallow copy

4. How do you optimize performance in a large JavaScript app?

Practice Section: Mock JavaScript Coding Interview Questions

1. Design an LRU Cache in JavaScript

2. Implement currying

3. Implement a custom Promise

4. Build a simple event emitter

5. Implement deep clone of an object

Tips for Solving JavaScript Coding Interview Questions

Wrapping Up