Prepare for your JavaScript interviews with these top questions and clear, concise answers. This guide covers JavaScript fundamentals, ES6 concepts, DOM manipulation, asynchronous programming, and debugging, ideal for freshers and experienced candidates.
JavaScript Interview Questions and Answers
1. What is JavaScript?
A high-level, interpreted programming language primarily used for web development.
2. What are the data types in JavaScript?
String, Number, Boolean, Undefined, Null, Symbol, Object.
3. What is the difference between == and === in JavaScript?
== checks value equality with type coercion, while === checks value and type equality.
4. What is hoisting in JavaScript?
JavaScript’s default behavior of moving declarations to the top of the scope.
5. What is the DOM?
Document Object Model, representing the structure of a web page.
6. What is an event in JavaScript?
An action that occurs in the browser, such as a click or keypress.
7. What is a callback function?
A function passed as an argument to another function, executed after the main function completes.
8. What is asynchronous programming in JavaScript?
Allows execution of non-blocking operations using callbacks, promises, or async/await.
9. What are promises in JavaScript?
Objects representing the eventual completion or failure of an asynchronous operation.
10. What is async/await?
Syntactic sugar over promises, allowing asynchronous code to be written in a synchronous style.
11. What is an arrow function in JavaScript?
A concise syntax for writing functions using the => syntax.
12. What is the difference between var, let, and const?
var is function-scoped, let and const are block-scoped; const cannot be reassigned.
13. What is closure in JavaScript?
A function that has access to its own scope, the outer function’s scope, and the global scope.
14. What is the event loop in JavaScript?
Handles asynchronous callbacks in the browser.
15. What is JSON?
JavaScript Object Notation, a lightweight data-interchange format.
16. What is localStorage?
A web storage API to store data locally within the user’s browser.
17. What is sessionStorage?
Similar to localStorage but data is cleared when the page session ends.
18. What is NaN in JavaScript?
Stands for Not-a-Number, a value representing an undefined or unrepresentable value in arithmetic operations.
19. What is typeof in JavaScript?
An operator used to determine the data type of a variable.
20. What is the difference between null and undefined?
null is an assignment value, undefined means a variable has been declared but not assigned.
21. What is a higher-order function?
A function that takes another function as an argument or returns a function.
22. What is a promise chain?
Linking multiple promises together to handle asynchronous operations sequentially.
23. What is fetch API?
Used to make network requests similar to XMLHttpRequest but with a simpler, cleaner syntax.
24. What is the spread operator?
(…) used to expand iterable elements like arrays or objects.
25. What is destructuring in JavaScript?
Extracting values from arrays or properties from objects into distinct variables.
26. What is the difference between synchronous and asynchronous code?
Synchronous code executes sequentially, while asynchronous code does not block the execution thread.
27. What is the use of the this keyword?
Refers to the object it belongs to.
28. What is strict mode in JavaScript?
A way to opt into a restricted variant of JavaScript for catching errors earlier.
29. What is a module in JavaScript?
Reusable pieces of code that can be imported or exported in other files.
30. What is a callback hell?
A situation where callbacks are nested within other callbacks, making code hard to read.
31. What is debounce in JavaScript?
A technique to limit the rate at which a function executes.
32. What is throttling in JavaScript?
Controlling the rate at which a function is executed repeatedly.
33. What is an IIFE?
Immediately Invoked Function Expression that runs as soon as it is defined.
34. What are template literals?
String literals allowing embedded expressions using backticks.
35. What is the difference between forEach and map?
forEach executes a function for each array element; map returns a new array with transformed elements.
36. What are JavaScript frameworks?
Libraries like React, Angular, and Vue that simplify web development.
37. What is event bubbling in JavaScript?
An event propagation method where the event bubbles up from the innermost element to the outermost.
38. What is event delegation?
Using a single event listener to handle events on multiple child elements.
39. What is memory leak in JavaScript?
When allocated memory is not released, causing increased memory usage.
40. What is the difference between call, apply, and bind?
They allow function borrowing; call and apply invoke functions immediately, while bind returns a new function.
41. What is a polyfill in JavaScript?
Code that implements a feature on web browsers that do not support it natively.
42. What is a generator function?
A function that can be paused and resumed using the yield keyword.
43. What is the Symbol type?
A primitive data type for creating unique identifiers.
44. What is a WeakMap in JavaScript?
A collection of key-value pairs where keys are objects and values can be collected if there is no other reference to the key.
45. What is a WeakSet?
A collection of objects where each object can occur only once and can be garbage-collected if no other reference exists.
46. What is event capturing?
The opposite of event bubbling; the event is captured from the outermost element to the innermost.
47. What is recursion in JavaScript?
A function calling itself to solve a smaller instance of the problem.
48. What is the eval function?
Evaluates JavaScript code represented as a string.
49. What is JSON.stringify and JSON.parse?
Used to convert objects to JSON strings and parse JSON strings back into objects.
50. Why choose a career in JavaScript development?
JavaScript is versatile, in-demand, and essential for modern web development, offering opportunities in frontend, backend, and full-stack roles.
Frequently Asked Questions
What skills are needed for JavaScript roles?
Strong JavaScript fundamentals, DOM manipulation, ES6+, debugging, and familiarity with frameworks like React or Angular.
How to prepare for a JavaScript interview?
Practice core concepts, coding problems, and build projects to strengthen hands-on skills.
Why is JavaScript important in web development?
It enables dynamic, interactive web applications and is supported by all modern browsers.