Write a <UserCard userId={id} /> that fetches /api/users/:id and shows the name. Handle loading, error, and the id-changes case.

This counter logs the same number forever after the first click. Explain the stale-closure bug, then fix it.

When should data fetching live in a custom hook versus in the component? Name two concrete benefits of extracting it.

Why is it usually a bug to call setState from a fetch .then() without checking whether the component is still mounted? Show the safe pattern using AbortController.