// Every useEffect with subscriptions has cleanup
useEffect(() => {
  const subscription = api.subscribe();
  return () => subscription.unsubscribe(); // ✓
}, []);

// Event listeners are removed
useEffect(() => {
  window.addEventListener('resize', handler);
  return () => window.removeEventListener('resize', handler); // ✓
}, []);

// Timers are cleared
useEffect(() => {
  const timer = setInterval(poll, 1000);
  return () => clearInterval(timer); // ✓
}, []);

        __
        __