What is React?
A JavaScript library for building user interfaces from composable, state-driven components.
Definition
React, open-sourced by Facebook in 2013, models the UI as a pure function of state: you describe what the interface should look like for a given state, and React works out the DOM operations needed to get there. Components are functions returning JSX; hooks (`useState`, `useEffect`, `useMemo` and friends) give those functions state and lifecycle without classes. React 18 introduced concurrent rendering—interruptible renders, transitions, and Suspense for data—and React 19 added Server Components, which run on the server and ship no JavaScript to the client at all. React itself is deliberately unopinionated about routing, data fetching, and styling, which is why it is almost always used through a framework like Next.js or Remix.