Khi nào useEffect chạy trong React 18 Strict Mode?

Trong React 18 Strict Mode development, mỗi component mount-unmount-mount lại để test cleanup.

• Effect chạy: mount (lần 1) → cleanup → mount (lần 2).
• Giúp phát hiện bugs do missing cleanup.
• Production không bị ảnh hưởng.
• Code đúng cần idempotent: connect/disconnect subscription, fetch với cancel, v.v.

In React 18 Strict Mode during development, each component goes through mount → unmount → remount to test cleanup correctness.

• The effect sequence is: run on mount (first) → cleanup → run on remount (second).
• This helps catch missing cleanup bugs.
• Production is not affected.
• Correct code must be idempotent: connect/disconnect subscriptions, fetch with cancellation, etc.