Meta JavaScript Interview Questions 2026
Published July 22, 2026 · Updated July 22, 2026
5 real JavaScript interview questions asked at Meta. Upvoted by engineers who cleared the loop. Covers Technical rounds.
5 questions
220 engineers asked
308 upvotes
Company: Meta
Technology: JavaScript
All JavaScript Questions Asked at Meta
Meta
Hard
Technical round
ReactJavaScript
Explain how React's virtual DOM diffing algorithm (reconciliation) works. Why is it O(n) instead of O(n3)? What are the assumptions it makes and when do those break down (causing performance issues)?
↑ 56 upvotes · 34 engineers asked this · SDE2
Meta
Medium
Technical round
PythonJavaJavaScript
Given an array of N integers, find the maximum product subarray. Explain why you need to track both the maximum and minimum product ending at each position, and handle zeros and negative numbers.
↑ 52 upvotes · 33 engineers asked this · SDE1
Meta
Medium
Technical round
4–6 YearsReactJavaScript
What is the difference between useMemo and useCallback? Give concrete examples of when each actually helps vs. when it hurts.
↑ 107 upvotes · 84 engineers asked this · Full Stack Developer
Meta
Hard
Technical round
3-7 yearsReactJavaScript
A news feed component re-fetches and re-renders every item whenever a single like count changes. Walk through how you would restructure state so only the affected item re-renders, without introducing a global state library.
↑ 52 upvotes · 31 engineers asked this · Senior SDE
Meta
Medium
Technical round
1-4 yearsJavaScript
Explain the output order of this code and why: console.log(1); setTimeout(()=>console.log(2),0); Promise.resolve().then(()=>console.log(3)); console.log(4);
↑ 41 upvotes · 38 engineers asked this · Senior SDE
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Frequently asked questions
Explain how React's virtual DOM diffing algorithm (reconciliation) works. Why is it O(n) instead of O(n3)? What are the assumptions it makes and when do those break down (causing performance issues)?
Explain how React's virtual DOM diffing algorithm (reconciliation) works. Why is it O(n) instead of O(n3)? What are the assumptions it makes and when do those break down (causing performance issues)?
Given an array of N integers, find the maximum product subarray. Explain why you need to track both the maximum and minimum product ending at each position, and handle zeros and negative numbers.
Given an array of N integers, find the maximum product subarray. Explain why you need to track both the maximum and minimum product ending at each position, and handle zeros and negative numbers.
What is the difference between useMemo and useCallback? Give concrete examples of when each actually helps vs. when it hurts.
What is the difference between useMemo and useCallback? Give concrete examples of when each actually helps vs. when it hurts.
A news feed component re-fetches and re-renders every item whenever a single like count changes. Walk through how you would restructure state so only the affected item re-renders, without introducing a global state library.
A news feed component re-fetches and re-renders every item whenever a single like count changes. Walk through how you would restructure state so only the affected item re-renders, without introducing a global state library.
Explain the output order of this code and why: console.log(1); setTimeout(()=>console.log(2),0); Promise.resolve().then(()=>console.log(3)); console.log(4);
Explain the output order of this code and why: console.log(1); setTimeout(()=>console.log(2),0); Promise.resolve().then(()=>console.log(3)); console.log(4);