React

React is a JavaScript library for building user interfaces out of individual pieces called components. The project's own explanation is deliberately plain: you create components such as Thumbnail, LikeButton and Video, then combine them into entire screens, pages and applications.

The design decision that shapes everything else is that React components are ordinary JavaScript functions. React frames this as a learning advantage rather than a technical one, and states it directly: learning React is learning programming. To show content conditionally you use an if statement; to display a list you use array map. There is no separate template syntax to learn, because the language you already know is the template language.

That choice also produces React's compositional property at scale. Whether you work alone or alongside thousands of other developers, the project states that using React feels the same, because it is designed so components written by independent people, teams and organisations combine seamlessly. This is why the React ecosystem is as large as it is: a component from an unrelated library is just a function, so it drops in without a framework-level integration.

React is free and open source, maintained with backing from Meta and a large community of contributors. It underpins a substantial ecosystem of frameworks built on top of it, including Next.js, and the project runs React Conf as its community event. Because React is a library rather than a full framework, most production applications pair it with a router, a build setup and often a meta-framework, which is worth planning for at the outset.