React is a popular JavaScript library that lets you compose user interfaces by combining small, reusable components. It is declarative, efficient, and ensures you only re-render what actually changes.

Official documentation and source code are well maintained on the project site and GitHub, and there is an active community with many resources and extensions.

React has grown from an internal Facebook tool to a favorite among many developers. It focuses exclusively on the UI layer, allowing you to choose other libraries freely for routing, state management, and server rendering.

With the introduction of Hooks in 2019, functional components became the standard, making code more modular and easier to test. The virtual DOM and an efficient diffing algorithm keep renders fast, and support for server- and native-rendering makes React versatile for both web and mobile platforms.