React is a declarative, efficient, and flexible JavaScript library for building user interfaces. It lets you compose complex UIs from small and isolated pieces of code called “components”. It is maintained by Facebook and a community of individual developers and companies. React can be used as a base in the development of single-page or mobile applications. It is maintained by Facebook and a community of individual developers and companies. React can be used as a base in the development of single-page or mobile applications.