ReactJS is an open source JavaScript library for the development of user interfaces. Developed by Facebook, it was used to create the news feed within the social network and to build the web version of Instagram. In addition to boasting a constantly growing community, React is appreciated and used by famous companies including AirBnB (AirBnB has also created Enzyme – a tool to test apps written in React), Netflix, Paypal and Uber. Starting from React, several libraries have been developed that use similar principles and syntax and that are partly or completely compatible with React.