How do hooks map to the old class lifecycle methods?

Quick answer

componentDidMount → useEffect(fn, []); componentDidUpdate → useEffect(fn, [deps]); componentWillUnmount → the cleanup returned from useEffect.

In detail

There are no named lifecycle methods in function components — useEffect expresses them all. An effect with [] runs on mount; with deps it runs on relevant updates; its returned cleanup runs on unmount (and before re-runs). One effect with setup + cleanup replaces the split between componentDidMount and componentWillUnmount, keeping related logic together.

useEffect(() => {
  const sub = subscribe();      // didMount
  return () => sub.unsubscribe(); // willUnmount
}, []);

Why interviewers ask this: Common when interviewers bridge from class to hooks knowledge.

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