It is an open-source JavaScript library that enables users to build reusable UI components. In simple words, it is a library for building composable user interfaces. It is used mainly in building single-page applications. It is one of the most popular front-end JavaScript libraries having a strong foundation and a large community. It was developed and maintained by Facebook.

ReactJS is all about components, you need to think of everything as a component. It is a declarable, efficient and flexible JavaScript library. Users choose ReactJS in order to develop user interfaces that improve the speed of the applications. It moreover uses virtual DOM that further helps to improve the performance of the application. Apart from these, ReactJS also uses component and data patterns that significantly improve readability and helps to maintain larger applications. ReactJS can also be used on the client and server-side as well as with other frameworks too. ReactJS also implements one-way data flow, this reduces the boilerplate and is easier to reason about than traditional data binding.