JavaScript Interview Questions and Answers (Beginner to Advanced)

This section covers commonly asked JavaScript interview questions from beginner to professional level. These questions test understanding of core concepts, practical usage, and advanced JavaScript behavior.

What is JavaScript? How do you install it? What are the most popular JavaScript frameworks? Learn all this and more in our blog: “What is JavaScript? Complete Guide for Beginners to Professionals.”

Beginner JavaScript Interview Questions

1. What is JavaScript?

Answer:

JavaScript is a high level, interpreted programming language used to create interactive and dynamic web pages. It runs in web browsers and can also run on servers using Node.js.

It is mainly used for:

• Website interactivity

• Web applications

• Backend APIs

• Mobile apps

• Desktop applications

Example:

console.log("Hello World");

2. What are the different data types in JavaScript?

Answer:

JavaScript has two main types of data types.

Primitive Data Types

• String

• Number

• Boolean

• Undefined

• Null

• Symbol

• BigInt

Example:

let name = "John";
let age = 25;
let isActive = true;

Non Primitive Data Types

• Objects

• Arrays

• Functions

Example:

let user = {
  name: "John",
  age: 25
};

3. What is the difference between var, let, and const?

Answer:

Example:

let age = 25;
age = 30; // allowed

const name = "John";
// name = "Mike"; // error

Best practice: use const by default, then let, and avoid var.

4. What is a function in JavaScript?

Answer:

A function is a reusable block of code designed to perform a specific task.

Example:

function greet(name) {
  return "Hello " + name;
}

console.log(greet("John"));

Functions help in:

• Code reuse

• Modularity

• Better maintainability

5. What is an array?

Answer:

An array is a data structure used to store multiple values in a single variable.

Example:

let fruits = ["Apple", "Banana", "Mango"];
console.log(fruits[0]);

Common array methods:

• push()

• pop()

• map()

• filter()

6. What is the DOM?

Answer:

DOM stands for Document Object Model.

It represents the HTML structure of a webpage as objects, allowing JavaScript to manipulate elements dynamically.

Example:

document.getElementById("title").innerText = "New Title";

This allows JavaScript to:

• Change content

• Modify styles

• Handle events

Intermediate JavaScript Interview Questions

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

Answer:

Example:

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

Best practice: always use ===.

8. What is a closure in JavaScript?

Answer:

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

Example:

function outer() {
  let count = 0;

  return function inner() {
    count++;
    return count;
  }
}

const counter = outer();
console.log(counter());

Closures are commonly used for:

• Data privacy

• Callbacks

• Functional programming

9. What is hoisting?

Answer:

Hoisting is JavaScript’s behavior of moving variable and function declarations to the top of their scope before execution.

Example:

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

JavaScript interprets it as:

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

Output:

undefined

10. What are Promises in JavaScript?

Answer:

A Promise represents the result of an asynchronous operation.

A promise has three states:

• Pending

• Resolved

• Rejected

Example:

let promise = new Promise((resolve, reject) => {
  resolve("Success");
});

promise.then(result => console.log(result));

11. What is async/await?

Answer:

async/await is a modern way to handle asynchronous code in JavaScript.

Example:

async function getData() {
  let response = await fetch("https://api.example.com");
  let data = await response.json();
  console.log(data);
}

Benefits:

• Cleaner code

• Easier error handling

• Better readability

12. What is event delegation?

Answer:

Event delegation is a technique where a parent element handles events for its child elements.

Example:

document.getElementById("list").addEventListener("click", function(e) {
  if (e.target.tagName === "LI") {
    console.log("List item clicked");
  }
});

Benefits:

• Better performance

• Fewer event listeners

Advanced JavaScript Interview Questions

13. What is the JavaScript Event Loop?

Answer:

The event loop allows JavaScript to perform non blocking asynchronous operations even though it is single threaded.

It works with:

• Call Stack

• Callback Queue

• Event Loop

Process:

1. Code executes in the call stack.

2. Async tasks go to Web APIs.

3. Results go to the callback queue.

4. Event loop pushes them back to the stack.

14. What is prototype inheritance?

Answer:

JavaScript uses prototype based inheritance, meaning objects inherit properties from other objects.

Example:

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

Person.prototype.greet = function() {
  console.log("Hello " + this.name);
}

let user = new Person("John");
user.greet();

15. What is debouncing?

Answer:

Debouncing limits how often a function executes.

Commonly used for:

• Search inputs

• Resize events

• Scroll events

Example:

function debounce(fn, delay) {
  let timer;

  return function() {
    clearTimeout(timer);
    timer = setTimeout(fn, delay);
  }
}

16. What is throttling?

Answer:

Throttling ensures a function runs only once within a specified time interval.

Used for:

• Scroll events

• Button clicks

• API calls

17. What is garbage collection in JavaScript?

Answer:

Garbage collection is the automatic process of freeing memory that is no longer used by the program.

JavaScript engines automatically manage memory using techniques like:

• Mark and sweep algorithm

Developers do not need to manually free memory.

18. What are JavaScript design patterns?

Answer:

Design patterns are reusable solutions to common programming problems.

Common JavaScript patterns include:

• Module Pattern

• Singleton Pattern

• Observer Pattern

• Factory Pattern

Example (Module Pattern):

const counter = (function() {
  let count = 0;

  return {
    increment: function() {
      count++;
      return count;
    }
  }
})();

Pro Level JavaScript Interview Questions

19. What is the difference between synchronous and asynchronous code?

Answer:

Synchronous

Code executes line by line.

Asynchronous

Tasks execute in the background without blocking the main thread.

Example:

setTimeout(() => {
  console.log("Async task");
}, 1000);

20. What is the difference between call, apply, and bind?

Answer:

These methods control the value of this.

Example:

function greet() {
  console.log(this.name);
}

const user = { name: "John" };

greet.call(user);