Custom Hooks are the pinnacle of React code reuse. They allow you to extract complex stateful logic from your components, making them leaner, easier to test, and infinitely more readable.
1The Logic Extraction Protocol
A Custom Hook is simply a function that leverages other React hooks. By naming your function with the use prefix, you signal to React's linter that this function follows the Rules of Hooks. This allows you to pull useEffect and useState calls out of your UI components and into a separate logic layer, effectively decoupling your 'how' from your 'what'.
2The Isolation Principle
It's a common misconception that custom hooks share data. They do not. Every time a component calls a custom hook, it initializes a brand new set of the hooks inside it. This isolation is what makes them so powerful—you can use useToggle in 10 different components, and each one will maintain its own independent boolean state without any interference.
