react-19

Next.js 15 App Router: page/layout/loading/error files, nested layouts, dynamic and catch-all segments, route groups, parallel routes, and the Metadata API for SEO — with the file tree as a diagram.

From hand-rolled useEffect fetches to TanStack Query v5 and SWR: race conditions, caching, mutations, optimistic updates, error boundaries, and when to push fetching to the server.

useState, useEffect, useReducer, and useContext in React 19: the rules, snapshot semantics, dependency arrays, when NOT to useEffect, and the dependency-array footgun that bites everyone.

Master React 19's core primitive: function components, JSX, props, children, composition, conditional rendering, and stable keys — built around a small TODO list you can ship.