useEffect cleanup function chạy khi nào? Tại sao quan trọng?

Cleanup chạy:

1. trước mỗi lần effect re-run khi dependencies thay đổi
2. khi component unmount. Quan trọng để tránh memory leaks: hủy subscriptions, abort fetch requests, clear timers

Ví dụ: return () => controller.abort() trong fetch effect.

The cleanup runs:

1. before each effect re-run when dependencies change, and
2. when the component unmounts. It is critical for preventing memory leaks: cancelling subscriptions, aborting fetch requests, and clearing timers

Example: return () => controller.abort() inside a fetch effect.