facebook/react

React is a JavaScript library for building user interfaces, both for web and for native (mobile) apps. The README describes its core ideas in three points. First, it is declarative: instead of writing step-by-step instructions for updating the screen, you describe what each "view" should look like for a given application state, and React figures out which parts to update efficiently when the underlying data changes. The README argues this makes code more predictable and easier to debug. Second, it is component-based: you build small, encapsulated components that manage their own internal state, and you compose them together into more complex interfaces. Component logic is written in JavaScript rather than in separate template files. Third, the slogan "Learn Once, Write Anywhere" means React does not assume the rest of your technology stack, can be added gradually to an existing project, and can be rendered on the server with Node or used to power mobile apps via React Native.

The way it works is that a developer writes components that return descriptions of UI, often using a syntax called JSX that looks like HTML embedded in JavaScript. The example in the README defines a HelloMessage component and uses createRoot from react-dom/client to render it into a container element on the page. JSX is not strictly required, but the README says it makes code more readable.

Someone would use React when they need to build interactive web or mobile interfaces, when they want to gradually adopt a UI library inside an existing project, or when they want one programming model for both web and native apps. The repository's primary language is JavaScript and the project is MIT licensed.