React is a free and open-source front-end JavaScript library for building user interfaces based on UI components.
It is maintained by Meta (formerly Facebook) and a community of individual developers and companies.
React can be used as a base in the development of single-page, mobile, or server-rendered applications with frameworks like Next.js.

React adheres to the declarative programming paradigm. Developers design views for each state of an application, and React updates and renders components when data changes.
React code is made of entities called components.
Components can be rendered to a particular element in the DOM using the React DOM library. When rendering a component, one can pass the values between components through "props".

React creates an in-memory data-structure cache, computes the resulting differences, and then updates the browser's displayed DOM efficiently.

The basic architecture of React applies beyond rendering HTML in the browser.
For example, Facebook has dynamic charts that render to <canvas> tags, and Netflix and PayPal use universal loading to render identical HTML on both the server and client.