Technologies Coding Problems Bookmarks Learning Paths

Technologies Coding Problems Bookmarks Learning Paths

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What should we do in cleanup functions? | Custom Hooks | react Interview Questions | NextRound.in

Questions

6 of 14

1Implement useMediaQuery hook.

3Implement useDebounce hook.

4Implement useLocalStorage hook.

5List potential pitfalls or risks of Custom Hooks.

6What should we do in cleanup functions?

7What are the advantages of cleaning up after an effect?

8What should be returned by custom hooks?

9What is the useEffect cleanup function?

10What are the advantages of Custom Hooks?

11What should not be put into custom hooks?

12What points to note and take care of while writing custom hooks?

13When should we use a custom hook?

14Why do we need custom hooks?

react

State and Props

Class Components

Function Components

Lifecycle functions

Performance Optimisation

06 / 14

What should we do in cleanup functions?

Difficulty: 5/10

useEffect cleanup, resource management, memory leaks

1
Cancel network requests.
2
Clear timeouts
3
Clear intervals
4
Remove event listeners
5
Close WebSockets.

Scenario Questions

0-2 years experience

1You have a functional component that starts a setInterval inside useEffect. How do you make sure the interval stops when the component unmounts?
2If you add a window resize listener in a component, what do you need to do in the cleanup function to avoid issues?
3What would happen if you forget to return a cleanup function from a useEffect that opens a WebSocket connection?

Follow-up Questions

What kinds of bugs typically surface when a cleanup is omitted?
How does the timing of the cleanup affect user experience or performance?
Can you think of a situation where you would conditionally skip a cleanup step?

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2-5 years experience

1We saw a memory leak after navigating away from a page that fetches data with fetch inside useEffect. Walk me through how you'd adjust the effect to prevent the leak.
2During a code review, a teammate's useEffect creates a subscription but doesn't clean it up. How would you explain the impact and what change would you suggest?
3Our component uses an AbortController to cancel a fetch request. Where should the abort be called and why?

5-8 years experience

1Design a reusable custom hook that establishes a WebSocket connection and guarantees proper cleanup across multiple components. What edge cases do you consider?
2In a large app many components set up interval timers. How would you architect a centralized timer manager to avoid redundant timers and ensure cleanup?
3How would you test that a component's cleanup logic works correctly under rapid mount/unmount cycles?

8+ years experience

1Our legacy codebase has many class components with componentWillUnmount cleanup scattered throughout. How would you plan a migration to functional components with hooks while ensuring no resource leaks?
2When introducing a global event bus, what strategy would you adopt to enforce consistent cleanup across teams and prevent memory bloat?
3Discuss the trade‑offs of delegating cleanup responsibilities to a higher‑order component versus each component handling its own cleanup.