Khi nào nên tách logic vào custom hook so với utility function?

Dùng custom hook khi logic cần: React hooks (useState, useEffect, v.v.), React lifecycle, hay stateful behavior.

• Dùng utility function (pure function) khi: chỉ xử lý data transformation, validation, formatting - không cần React context.
• Custom hook gọi được từ components, utility function gọi từ bất cứ đâu.

Use a custom hook when the logic requires React hooks (useState, useEffect, etc.), React lifecycle behavior, or stateful behavior.

• Use a plain utility function (pure function) when the logic only handles data transformation, validation, or formatting — no React context needed.
• Custom hooks can only be called from components; utility functions can be called from anywhere.