HookTimer.jsx

function HookTimer() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    const interval = setInterval(() => setCount(count => count + 1), 1000);
    return () => clearInterval(interval);
  }, []);

  return (
    <div>
      <h1>Count: {count}</h1>
    </div>
  );
}