Most Asked JavaScript Interview Questions (With Answers)

In this article, we’ll cover the most asked JavaScript interview questions in 2025 — from beginner to advanced — with concise answers to help you prepare confidently.

Why JavaScript Interview Questions Matter

JavaScript is used extensively for building:

• Interactive front-end applications
   
• Real-time web apps with frameworks like React, Vue, and Angular
   
• Back-end services with Node.js
   

That’s why companies test JavaScript knowledge rigorously — not just the syntax, but how well you understand the language features, event handling, closures, async behavior, and design patterns.

Top JavaScript Interview Questions and Answers

Let’s break them down by fundamentals, advanced concepts, DOM manipulation, and practical coding problems.

🔹 1. What are the different data types in JavaScript?

JavaScript has the following data types:

• Primitive types: String, Number, Boolean, Null, Undefined, Symbol, BigInt
   
• Non-primitive type: Object (includes Arrays, Functions)
   

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

== compares values with type coercion


=== compares values without type coercion (strict equality)


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5 == '5'     // true

5 === '5'    // false

🔹 3. What is a closure in JavaScript?

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

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function outer() {

  let count = 0;

  return function inner() {

    count++;

    console.log(count);

  };

}

const counter = outer();

counter(); // 1

counter(); // 2

Closures are important for data privacy and stateful functions.

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

• var is function-scoped and hoisted.
   
• let and const are block-scoped.
   
• const cannot be reassigned.
   

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function test() {

  if (true) {

    var a = 10;

    let b = 20;

  }

  console.log(a); // 10

  console.log(b); // ReferenceError

}

🔹 5. Explain event bubbling and event capturing.

• Event bubbling: The event is first captured and handled by the innermost element and then propagates outward to ancestors.
   
• Event capturing (less used): The event starts from the outermost element and goes inward.
   

You can control propagation using stopPropagation().

🔹 6. What is hoisting in JavaScript?

Hoisting is JavaScript’s behavior of moving declarations to the top of their scope.

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console.log(x); // undefined

var x = 5;

Function declarations are fully hoisted; variables declared with let or const are not initialized until they are defined.

🔹 7. What is the difference between synchronous and asynchronous code?

• Synchronous code runs in sequence, blocking execution.
   
• Asynchronous code allows execution to continue without waiting, using callbacks, promises, or async/await.
   

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async function fetchData() {

  const res = await fetch('https://api.com');

  const data = await res.json();

}

🔹 8. What are Promises in JavaScript?

A Promise represents a value that may be available now, later, or never.

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

  setTimeout(() => resolve("Done"), 1000);

});

promise.then(console.log); // “Done”

🔹 9. Explain this in JavaScript.

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

• In a method, this refers to the object.
   
• In a function, this is undefined in strict mode (or window in non-strict).
   
• Arrow functions don’t have their own this.
   

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

• call() invokes a function with a given this value and arguments.
   
• apply() is similar but takes an array of arguments.
   
• bind() returns a new function with bound this.
   

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const obj = { name: 'Arpita' };

function greet(msg) {

  console.log(msg + ' ' + this.name);

}

greet.call(obj, 'Hello');  // Hello Arpita

greet.apply(obj, ['Hi']);  // Hi Arpita

const greetMe = greet.bind(obj);

greetMe('Hey');            // Hey Arpita

DOM & Web APIs

🔹 11. What is the DOM?

DOM (Document Object Model) is a tree-like structure that represents HTML elements as objects. JavaScript can interact with the DOM to manipulate content, styles, and structure dynamically.

🔹 12. How to select DOM elements in JavaScript?

• document.getElementById('id')
   
• document.querySelector('.class')
   
• document.querySelectorAll('div')
   
• document.getElementsByTagName('p')
   

🔹 13. How to handle events in JavaScript?

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

  alert('Clicked!');

});

🔹 14. What is event delegation?

Event delegation allows you to attach a single event listener to a parent element, and handle events triggered by its child elements using event bubbling.

Useful for optimizing performance in dynamic lists.

🔹 15. How to prevent default behavior in events?

Use event.preventDefault().

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document.querySelector('form').addEventListener('submit', function(e) {

  e.preventDefault();

});

Arrays, Objects, and Functions

🔹 16. What are higher-order functions?

A function that takes another function as an argument or returns a function is called a higher-order function.

Examples: map, filter, reduce.

🔹 17. Difference between map() and forEach()

• map() returns a new array with the transformed elements.
   
• forEach() performs an operation on each element but does not return anything.
   

🔹 18. How does reduce() work in JavaScript?

reduce() executes a reducer function on each array element to produce a single value.

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const sum = [1, 2, 3].reduce((acc, val) => acc + val, 0); // 6

🔹 19. Explain destructuring in JavaScript.

Destructuring allows extracting values from arrays or objects into variables.

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

const {name, age} = {name: 'John', age: 30};

🔹 20. What is the spread operator?

The spread operator (...) allows expanding arrays or objects.

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let arr1 = [1, 2];

let arr2 = [...arr1, 3, 4]; // [1, 2, 3, 4]

Bonus: Practical JavaScript Code Questions

🔹 21. Reverse a string

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function reverseStr(str) {

  return str.split('').reverse().join('');

}

🔹 22. Check if a number is prime

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function isPrime(n) {

  if (n <= 1) return false;

  for (let i = 2; i <= Math.sqrt(n); i++) {

    if (n % i === 0) return false;

  }

  return true;

}

🔹 23. Find the largest number in an array

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function max(arr) {

  return Math.max(...arr);

}

Prepare for Interviews with Uncodemy

If you're looking to master JavaScript for interviews, explore Uncodemy’s structured course:

👉 JavaScript Interview Preparation Course – Uncodemy

Course Highlights:

• Real-world coding interview questions
   
• Front-end + back-end JavaScript topics
   
• Practice projects with mock interviews
   

Final Thoughts

JavaScript is a deep and flexible language. Interviewers often mix theoretical concepts, code problems, and trick questions to test your understanding. The best way to prepare is to:

• Practice writing code without autocomplete
   
• Build small projects using core JS (without frameworks)
   
• Go through real interview questions like the ones above
   

In 2025, companies value logical clarity, real-world coding ability, and async handling — all of which can be developed with time, consistency, and the right learning path.