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2. React.js

You write components: plain JavaScript functions that take an object of inputs and return markup. React calls them, compares the result with what it rendered last time, and touches only the DOM nodes that differ. You never again have to remember which of seven event handlers also has to hide the error message. State changes; the screen follows.The version here is React 19.3.0, released on 9 September 2026, which has absorbed much of the boilerplate older tutorials still teach: refs are ordinary props, use reads a promise inside render, form actions handle pending and error states, and the React Compiler can insert memoization for you. Class components, propTypes and ReactDOM.render appear only where you need to recognize them in old code. Volume 1 covers the JavaScript, TypeScript, DOM and CSS underneath; Chapter 3 supplies the Express API these examples call, and Chapter 5 takes server components from concept to practice.

  • How JSX compiles, and why components must be pure functions of props and state.

  • How to hold state with useState and useReducer, and when to lift it up.

  • How effects synchronize a component with the network, the DOM and subscriptions.

  • How context, refs, memoization and the React Compiler change re-render behavior.

  • How concurrent rendering, Suspense and transitions keep an interface responsive.

  • How to build forms, custom hooks and accessible, typed components, and test them.

  • How to route with React Router, hold shared state in Redux Toolkit, and render on the server.