React

Building complex UIs through imperative DOM manipulation does not scale. React solves this with a declarative component model: describe what the interface should look like for a given state, and React computes the minimal DOM changes. Meta (originally Facebook) open-sourced it in May 2013, and it has grown into the most widely used JavaScript UI library.

Version 18 introduced concurrent rendering — the ability to prepare multiple UI states simultaneously and interrupt low-priority work when user input arrives. React 19, the current major release, stabilized Server Components, Actions, and the use hook. Server Components move rendering and data fetching to the server, sending only the result to the client, which reduces bundle size and simplifies data access patterns.

The react.dev documentation covers fundamentals and advanced patterns in a single, well-organized reference. The source code is on GitHub under the MIT license.