React is a front end, JavaScript library for building user interfaces or UI 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. Its popularity stems from its flexibility and simplicity and this makes it the first choice in the development of mobile apps and web applications. More than 90,000 sites use React including tech giants such as Facebook, Netflix, Instagram, Airbnb, and Twitter to list a few.