React.js is a JavaScript library for building user interfaces. It’s commonly used on the front end of web applications for its fast performance and ease of use.. It was created by Facebook and Instagram developers who needed a library to help them build large web applications  in 2011 and it has been open-sourced since 2013 under the BSD license., React is used in the Facebook website, which has billions of users worldwide. It allows programmers to create reusable components that are easy to maintain and can be reused in different applications. This helps programmers save time and effort when they want to update the application because they only need to make changes in one place. It has since become one of the most popular frameworks in the industry.