React Hook

A special React function (starting with 'use') that lets you tap into React features like state or side effects inside a function component.

Functions like `useState`, `useEffect`, `useContext`, `useRef`, and `useReducer`. Must be called at the top level of a component or custom hook — not inside conditions or loops. Custom hooks are plain functions that call other hooks.

Hooks store their state in a linked list on the fiber. The call order determines which fiber slot each hook reads from — hence the no-conditional-calls rule. The React DevTools highlight stale closures and missing dependencies.