Skip to main content

Top 50 JavaScript Interview Questions & Answers (Beginner to Advanced)

Whether you're preparing for a web development job or improving your JS skills, these 50 JavaScript interview questions will help you master the important concepts.

1. What is JavaScript?

JavaScript is a high-level programming language used to make web pages interactive.

2. How do you print output in JavaScript?

document.write("Hello World!");
console.log("Printed successfully");

3. What are variables?

Variables store data. JS has var, let, and const. 

let name = "Alex";
const age = 21;
document.write(name + " - " + age);

4. Difference between let and var?

  • var → function scoped
  • let → block scoped (recommended)

5. What are data types in JavaScript?

  • String
  • Number
  • Boolean
  • Undefined
  • Null
  • Object
  • Array

6. What is typeof?

document.write(typeof "Hello");

7. What is a function?

A function is a reusable block of code.

function greet(){
  document.write("Welcome!");
}
greet();

8. What is an arrow function?

const add = (a, b) => a + b;
document.write(add(2,3));

9. What is an object?

let user = { name: "John", age: 22 };
document.write(user.name);

10. What is an array?

let skills = ["HTML", "CSS", "JS"];
document.write(skills.join(", "));

11. What is DOM?

DOM (Document Object Model) allows JavaScript to control and modify HTML.

12. Get element by ID example

document.write("<h2 id='title'>Old Text</h2>");
document.getElementById("title").innerHTML = "Changed using JS!";

13. What is an event?

Event = something user does (click, hover, keypress, etc.).

14. Example of click event

document.write('<button id="btn">Click Me</button>');
document.getElementById("btn").onclick = () => {
  alert("Button Clicked!");
};

15. What is localStorage?

It stores small data in the browser, even after refresh.

16. Save to localStorage

localStorage.setItem("user", "Alex");
document.write("Saved!");

17. Get from localStorage

document.write(localStorage.getItem("user"));

18. What is sessionStorage?

Similar to localStorage, but removed when the tab is closed.

19. What is JSON?

JSON = JavaScript Object Notation (data format).

20. Convert object to JSON

let user = { name: "Alex" };
document.write(JSON.stringify(user));

21. What is a promise?

Promise handles async operations.

22. Example Promise

let p = new Promise(res => res("Done!"));
p.then(msg => document.write(msg));

23. What is async/await?

Cleaner way to handle promises.

24. Example async/await

async function test(){
  return "Hello Async!";
}
test().then(msg => document.write(msg));

25. What is NaN?

Not-a-Number (invalid number result).

26. What is isNaN()?

``` isNaN("ABC") // true ```

27. What is hoisting?

JS moves variable and function declarations to the top before execution.

28. What are template literals?

let name = "Alex";
document.write(`Hello, ${name}`);

29. What is == vs ===?

  • == compares value
  • === compares value + type

30. What is a callback?

A function passed as argument to another function.

31. Example callback

function greet(cb){
  cb();
}
greet(() => document.write("Callback called!"));

32. What is setTimeout?

setTimeout(() => document.write("Hello after 1 second"), 1000);

33. What is setInterval?

let i = 1;
setInterval(() => document.write(i++ + " "), 500);

34. What is a class?

class User{
  constructor(name){
    this.name = name;
  }
}
let u = new User("Alex");
document.write(u.name);

35. What is inheritance?

``` class Child extends Parent {} ```

36. What is spread operator?

``` let arr = [1,2,3]; let newArr = [...arr]; ```

37. What is rest operator?

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

38. What is map()?

``` [1,2,3].map(x => x * 2) ```

39. What is filter()?

``` [1,2,3,4].filter(x => x % 2 === 0) ```

40. What is reduce()?

``` [1,2,3].reduce((a,b) => a + b) ```

41. What is try/catch?

``` try { ... } catch(err){ ... } ```

42. What is strict mode?

``` "use strict"; ```

43. What is AJAX?

AJAX loads data from server without page refresh.

44. What is fetch()?

``` fetch(url).then(...) ```

45. What is a closure?

Function remembering its outer scope.

46. Example closure

``` function outer(){ let x = 5; return function(){ return x; } } ```

47. What is event bubbling?

Event travels from child → parent DOM nodes.

48. What is event capturing?

Event travels from parent → child.

49. What is debounce?

Function runs after user stops triggering event.

50. What is throttle?

Limits how often function executes over time.

Final Words

These 50 JavaScript questions will help you prepare for real frontend/web developer interviews, create better JS logic, and improve coding confidence.

Comments

Post a Comment

Popular posts from this blog

HTML Forms Tutorial HTML Forms Tutorial HTML Forms allow users to enter and submit data. They are used everywhere—login forms, signup forms, search boxes, feedback forms, etc. In this complete tutorial, you will learn all form elements with examples and demos. 1. What is an HTML Form? A form collects user input using different form elements like text fields, checkboxes, radio buttons, buttons, and more. <form> Form elements go here... </form> 2. Basic Form Structure <form action="#" method="post"> <input type="text" placeholder="Enter name"> <button>Submit</button> </form> Submit Attributes: action → URL where form data is sent method="POST" → Secure data sending method="GET" → Data shows in URL 3. Text Input Field <i...
Post a Comment
Read more
📸 HTML Image Gallery Tutorial for Beginners Image galleries make your website more beautiful and professional. In this tutorial, you will learn how to create different types of image galleries using HTML and CSS. 📌 1. Simple HTML Image Gallery This is the most basic gallery layout. <div> <img src="image1.jpg" width="200"> <img src="image2.jpg" width="200"> <img src="image3.jpg" width="200"> </div> ✔ Explanation All images are inside a <div> Each image has the same width They appear in a row 📌 2. Responsive Image Gallery (CSS Grid) This gallery automatically adjusts on mobile and PC. <style> .gallery { display: grid; grid-template-columns: repeat(auto-fit, minmax(200px, 1fr)); gap: 15px; } .gallery img { width: 100%; border-radius: 10px; } </style> <div class="galle...
Post a Comment
Read more
📝 HTML Notes App Tutorial for Beginners (Full Project) In this tutorial, you will learn how to build a fully functional Notes App using HTML, CSS, and JavaScript. This app works on all devices and saves notes even after refreshing the page! ✨ Features of This Notes App Add new notes Edit notes Delete notes Dark Mode Search notes Auto-save using LocalStorage No backend needed 📌 Step 1 — HTML Structure Copy Code Run Demo <div class="container"> <h1>Notes App</h1> <button id="toggleDark">🌙 Dark Mode</button> <input type="text" id="searchBox" placeholder="Search notes..."> <textarea id="noteInput" placeholder="Write your note here..."></textarea> <button id="addNote">Add Note</button> <div id="notesList"></div> ...
Post a Comment
Read more
HTML Tags Tutorial – A Complete Beginner-Friendly Guide HTML tags are the basic building blocks of every webpage. Whether you want to create a simple webpage or a full website, understanding HTML tags is the first and most important step. In this post, we’ll explore what HTML tags are, how they work, why they matter, and the most commonly used tags with examples. Table of Contents What Are HTML Tags? How HTML Tags Work Types of HTML Tags Most Common HTML Tags Self-Closing Tags Nesting Tags Practical Examples Best Practices What Are HTML Tags? HTML tags are keywords enclosed in angle brackets < > that tell the browser how to display content. Tags create elements such as headings, paragraphs, images, links, lists, buttons, forms, and much more. Example of a simple tag: <p>This is a paragraph.</p> How HTML Tags Work Most HTML tags come in pairs: <opening-tag>...
Post a Comment
Read more
HTML Images Tutorial HTML Images Tutorial The <img> tag is used to display images on a webpage. In this tutorial, you'll learn how to add images, resize them, add borders, captions, and more. 1. Basic Image Tag The simplest way to add an image is by using the src and alt attributes. <img src="image.jpg" alt="My Image"> Attributes: src → Image URL alt → Text shown if image fails to load 2. Image Size (width & height) You can resize an image using width or height . <img src="photo.jpg" width="300" height="200"> 3. Responsive Images Responsive images automatically adjust to device size. <img src="photo.jpg" style="width:100%;max-width:400px;"> 4. Add Border to Images <img src="file.jpg" style="border:3px solid #333;...
Post a Comment
Read more