React Hook: useTimeout

useTimeout is the React version of cancellable timeout: schedule on mount/delay change, clear on unmount, and never call a stale callback. Naive move One effect that closes over callback. When the parent re-renders with a new callback, the timer still sees the old one — or you reset the timer too often. Where it breaks Stale closures and missing cleanup. delay === null should pause. Changing callback should not restart the timer; changing delay should. The fix Ref for the latest callback (effect #1). Effect #2 depends on delay, sets the timer, and clears on cleanup / null delay. Remember: Callback in a ref; timer effect keyed by delay only. Say this in the interview: I'd keep the latest callback in a ref and put setTimeout/clearTimeout in an effect that only re-runs when delay changes.