JavaScript interview questions

All interview topics

JavaScript interview questions from fundamentals to the tricky parts: types and coercion, closures and this, prototypes, async code and the event loop, browser and security topics, and coding exercises, with examples you can run.

37 questions
14 easy
17 medium
6 hard
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37 of 37 questions

JavaScript interview questions

1What are the differences between var, let and const?EasyBasics

2What are JavaScript's data types, and what are the quirks of typeof?EasyBasics

3What is the difference between == and ===?EasyBasics

4What are truthy and falsy values?EasyBasics

5What is hoisting?MediumBasicsScope

6What is the difference between null and undefined?EasyBasics

7Explain type coercion with some tricky examples.MediumBasics

8How do you check for NaN, and why is 0.1 + 0.2 not 0.3?MediumBasics

9What is a closure? Give a practical example.MediumFunctionsScope

10How does this work in JavaScript?HardFunctions

11What are call, apply and bind?MediumFunctions

12What are the differences between arrow functions and regular functions?EasyFunctions

13What are debounce and throttle? Implement debounce.MediumFunctionsPerformance

14What is memoization? Implement a memoize function.MediumFunctionsPerformance

15What is currying?MediumFunctions

16What are higher-order functions and pure functions?EasyFunctions

17How does prototypal inheritance work, and how do classes relate to it?MediumObjects

18How do you copy an object, and what is the difference between a shallow and a deep copy?MediumObjects

19When would you use Map, Set or WeakMap instead of plain objects and arrays?MediumObjects

20Explain map, filter and reduce, and which array methods mutate the original.EasyArrays

21Coding: remove duplicates from an array, and flatten a nested array.EasyArraysCoding

22Explain the event loop. What does this code print?HardAsync

23What problems do promises solve compared with callbacks?EasyAsync

24What is the difference between Promise.all, allSettled, race and any?MediumAsync

25Coding: add a timeout to a promise, and retry a failing async function.HardAsyncCoding

26Coding: run many async tasks with at most N running at the same time.HardAsyncCoding

27What are event bubbling, capturing and delegation?MediumBrowser

28What is the difference between localStorage, sessionStorage and cookies?EasyBrowser

29What is CORS and why do you get CORS errors?MediumBrowserSecurity

30What is the difference between script, script async and script defer?EasyBrowserPerformance

31What is XSS, and how do you prevent it in front-end code?MediumBrowserSecurity

32What is the difference between ES modules and CommonJS?MediumModern JS

33Explain optional chaining (?.) and nullish coalescing (??).EasyModern JS

34Explain destructuring, spread and rest.EasyModern JS

35What are iterators and generators?HardModern JS

36What causes memory leaks in JavaScript applications?MediumPerformance

37Predict the output: a few classic JavaScript puzzles.HardBasicsCoding