React.JS is a library for building user interfaces. It utilises JavaScript and a special syntax for describing a heirachy of components that make up the interface called JSX.
It was originally released in 2013 by Facebook as open-source software. It can be used to create user interfaces for web browsers and for mobile devices.
React components are JavaScript functions that return JSX elements. These elements are composed together to form more complicated features.
var myUiElement = function(name) {
return <div>Hello { name }.</div>;
}