Are you preparing for a web development interview? One of the most common topics asked in technical rounds is JavaScript interview questions. JavaScript is the backbone of web development, used in front-end, back-end, and full-stack roles.
In this articles, we have listed 50 JavaScript interview questions with answers that cover basics, advanced concepts, and real-world coding scenarios. Whether you are a beginner or an experienced developer, these questions will help you understand the core concepts of JavaScript and boost your interview preparation.
🔹Basic JavaScript Interview Questions
1. What is JavaScript?
Answer: JavaScript is a lightweight, interpreted programming language used to make web pages interactive. It runs in browsers as well as on servers (Node.js).
2. Difference between Java and JavaScript?
Answer:
- Java → Compiled, object-oriented, runs on JVM.
- JavaScript → Interpreted, lightweight, runs on browsers & Node.js.
3. What are JavaScript Data Types?
Answer:
- Primitive Types: String, Number, Boolean, Null, Undefined, Symbol, BigInt.
- Non-Primitive: Objects, Arrays, Functions.
4. Difference between null and undefined?
Answer:
- null → Explicitly empty value.
- undefined → Variable declared but no value assigned.
5. What are var, let, and const?
Answer:
- var → Function scoped, redeclaration allowed.
- let → Block scoped, no redeclaration.
- const → Block scoped, no reassignment.
6. What is Hoisting?
Answer: JavaScript moves variable and function declarations to the top before execution.
console.log(a); // undefined
var a = 10;7. What are Template Literals?
Answer: Template literals allow string interpolation using backticks (`).
let name = "Rahul";
console.log(`Hello ${name}`);8. Difference between == and ===?
Answer:
- == → Compares values, type conversion happens.
- === → Compares values and types.
9. What are JavaScript Operators?
Answer: Operators are symbols used to perform operations. Examples: +, -, *, /, %, &&, ||, !, ===.
10. Explain Comments in JavaScript.
Answer:
- Single-line: // comment
- Multi-line: /* comment */
🔹Functions and Scope
11. What are Functions in JavaScript?
Answer: Functions are reusable blocks of code.
function add(a, b) { return a + b; }12. Difference between Function Declaration and Function Expression?
Answer:
- Declaration → Hoisted.
- Expression → Not hoisted.
13. What are Arrow Functions?
Answer: Short syntax, no own this.
const add = (a, b) => a + b;14. What are Higher-Order Functions?
Answer:
Functions that take other functions as arguments or return them. Example: map(), filter().
15. Explain Closures.
Answer: A closure is when an inner function remembers variables of outer scope.
function outer() {
let x = 10;
return function inner() { return x; }
}16. Difference between Global Scope and Block Scope?
Answer:
- Global → Accessible everywhere.
- Block → Accessible only within { }.
17. What is the ‘this’ keyword?
Answer: Refers to the current object context.
18. Difference between call(), apply(), and bind()?
Answer:
- call() → Pass arguments individually.
- apply() → Pass arguments as array.
- bind() → Returns a new function.
19. What is Function Currying?
Answer: Breaking a function with multiple arguments into a series of functions with single arguments.
20. What are Default Parameters?
Answer: Assign default values to function parameters.
function greet(name="Guest"){ console.log("Hello " + name); }🔹Objects and Arrays
21. What is an Object?
Answer: An object is a collection of key-value pairs.
22. How to create an Object?
Answer:
- Object literal → {}
- Constructor → new Object()
- Class → class Person {}
23. What is Object Destructuring?
Answer: Extracting values from objects into variables.
let {name, age} = {name:"Raj", age:25};24. What are Arrays?
Answer: Arrays store multiple values in one variable.
25. How do you loop through an Array?
Answer: for, for…of, forEach(), map().
26. Difference between forEach(), map(), filter(), reduce()?
Answer:
- forEach() → Iterates only.
- map() → Returns new array.
- filter() → Returns filtered array.
- reduce() → Returns single value.
27. Common Array Methods?
Answer: push(), pop(), shift(), unshift(), slice(), splice().
28. What is Spread Operator?
Answer: Expands elements.
let arr = [1,2,3];
console.log(...arr);29. What is Rest Operator?
Answer: Collects remaining values.
function sum(...nums){ return nums.reduce((a,b)=>a+b); }30. How do you merge Arrays?
Answer: Using concat() or spread operator.
🔹DOM and Events
31. What is DOM?
Answer: Document Object Model → representation of webpage in tree structure.
32. How to select elements?
Answer: getElementById(), querySelector(), querySelectorAll().
33. What is Event Bubbling and Capturing?
Answer:
- Bubbling → Event flows from child to parent.
- Capturing → Event flows from parent to child.
34. What are Event Listeners?
Answer: Functions executed when events occur.
35. Difference between inline event handling and addEventListener()?
Answer:
- Inline → In HTML attribute.
- addEventListener() → More flexible, allows multiple handlers.
36. What is Event Delegation?
Answer: Attaching event handler to parent instead of multiple child elements.
37. How to prevent default behavior?
Answer: Use event.preventDefault().
38. Difference between innerHTML and textContent?
Answer:
- innerHTML → Returns HTML + text.
- textContent → Returns only text.
39. How to create and append elements?
Answer: document.createElement() + appendChild().
40. Difference between localStorage and sessionStorage?
Answer:
- localStorage → Data persists even after closing browser.
- sessionStorage → Data cleared when session ends.
🔹Asynchronous JavaScript
41. What is Asynchronous JavaScript?
Answer: Execution where tasks run independently without blocking.
42. What is the Event Loop?
Answer: Mechanism that handles async tasks by moving them from queue to call stack.
43. What are Promises?
Answer: Object representing eventual completion/failure of async operation.
44. What is async/await?
Answer: Syntax for writing async code like synchronous.
45. What is Callback Function?
Answer: A function passed as an argument to another function.
46. Difference between Callbacks and Promises?
Answer:
- Callbacks → Nesting (callback hell).
- Promises → Cleaner, chainable.
47. What is setTimeout() and setInterval()?
Answer:
- setTimeout() → Executes after delay.
- setInterval() → Executes repeatedly.
48. What is Promise.all()?
Answer: Runs multiple promises in parallel and resolves when all are done.
49. What is Fetch API?
Answer: Used to make network requests.
50. Difference between synchronous and asynchronous code?
Answer:
- Synchronous → Blocks execution until complete.
- Asynchronous → Non-blocking, runs in background.
Conclusion
These JavaScript interview questions and answers are designed to help you crack your next web development interview with confidence. From simple topics like variables and data types to advanced concepts such as closures, promises, async, and ES6 features, this list covers everything a candidate must know.
If you practice these questions along with small coding projects, you will not only improve your problem-solving skills but also impress recruiters with your strong JavaScript foundation. Keep revising these JavaScript interview questions regularly to stay ahead in your career journey.