React Guideline: Hooks — Custom hooks for reusable logic. Description: Extract shared stateful logic to custom hooks. Do: useCustomHook for reusable patterns. Don't: Duplicate hook logic across components. Good Example: const { data } = useFetch(url). Bad Example: Duplicate useEffect/useState in components. Severity: Medium. Docs: https://react.dev/learn/reusing-logic-with-custom-hooks.