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interview-questions / react / hooks
Explain how useEffect cleanup works and when the cleanup function runs.
Short answer
The cleanup runs before the next effect and on unmount. It receives the previous render's closure — useful for unsubscribing or aborting in-flight work.
component.tsx
12msuseEffect(() => {
const ctl = new AbortController();
fetchUser(id, ctl.signal);
return () => ctl.abort();
}, [id]);AI Feedback
8.4 / 10
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Follow-up: how would StrictMode change what you just described?
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sandbox · node24 · 256MB
function twoSum(nums, target) {
const seen = new Map();
for (let i = 0; i < nums.length; i++) {
const need = target - nums[i];
if (seen.has(need)) return [seen.get(need), i];
seen.set(nums[i], i);
}
}Run output
- basic case2ms
- duplicates3ms
- hidden #11ms
- hidden #21ms
- hidden #3 · empty input1ms
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This week
+320XP
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3
Tracks
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