useInterval with pause

Custom Hooks — Intermediate React challenge.

Dan Abramov's classic. Build a declarative `useInterval` hook that can be paused by passing `null` as the delay and always uses the latest callback.

Requirements

• Invoke `callback` every `delay` ms.
• If `delay` is null, pause the interval.
• Always call the latest callback — no stale closures.
• Clean up the timer on unmount.

Signature

function useInterval(callback: () => void, delay: number | null): void

Example

useInterval(() => setCount(c => c + 1), running ? 1000 : null);

Constraints

• No external dependencies.
• Must survive fast refresh.