React is a front-end JavaScript library which is used to build the user interface or the UI components. It is free and open source. It is maintained by Facebook and a community of individual developers and companies. React allows you to interface with other libraries and frameworks. React is a declarative, efficient, and flexible JavaScript library for building interfaces. It lets you compose complex UIs from small and isolated pieces of code called “components”. Instead of manipulating the browser’s DOM directly, React creates a virtual DOM in memory, where it does all the necessary manipulating, before making the changes in the browser DOM. React finds out what changes have been made, and changes only what needs to be changed.