react

useState manages local state in functional components, returning a value and setter function. useEffect handles side effects (data fetching, subscriptions, DOM manipulation) and runs after render. The dependency array controls when effects re-run.

Context provides a way to pass data through the component tree without prop drilling. Use it for global data like themes, authentication, or locale. For frequently changing data or complex state logic, consider state management libraries instead.

Custom hooks are JavaScript functions starting with 'use' that encapsulate reusable stateful logic. They can use other hooks and share logic between components without duplicating code or adding components to the tree.

useRef creates a mutable reference that persists across renders without causing re-renders when changed. Use it for accessing DOM elements, storing mutable values that don't need to trigger updates, and keeping references to timers or previous values.

Controlled components have their form data managed by React state - the input value is always driven by state. Uncontrolled components let the DOM handle form data using refs to access values when needed. Controlled components are recommended for most cases.

React Router provides client-side routing for React applications, enabling navigation without page reloads. It uses components like BrowserRouter, Routes, Route, and Link, with hooks like useNavigate and useParams for programmatic navigation and accessing route parameters.

Ensure accessibility by using semantic HTML elements, proper ARIA attributes (aria-label, role), managing focus for dynamic content, supporting keyboard navigation, using htmlFor with labels, and testing with tools like eslint-plugin-jsx-a11y and screen readers.

UI libraries like Material-UI, Chakra UI, and Radix provide pre-built, accessible components. Integrate them by installing the package, wrapping your app with a ThemeProvider, and importing components. Use them for rapid development and consistent design; build custom when you need unique designs or minimal bundle size.