React is an open-source and free JavaScript library for building user interfaces that is based on components. It is maintained by Meta (formerly Facebook) along with a community of independent developers and companies. React allows for the creation of single-page applications, mobile, or server-side rendered applications using frameworks like Next.js. Its main advantage is that it re-renders only the parts of the page that have changed, avoiding unnecessary re-rendering of unchanged DOM elements.
React employs a declarative programming paradigm, meaning that developers design views for each state of the application, and React updates and renders components as data changes. Components in React can be declared as functions or classes, and since version 16.8, functional components can use state through Hooks, such as useState or useEffect.