Express.js is a Node.js web application server framework, designed for building single-page, multi-page, and hybrid web applications. It is the de facto standard server framework for node.js. The original author, TJ Holowaychuck, described it as a Sinatra-inspired server, meaning that it is relatively minimal with many features available as plugins.Express is the backend part of the MEAN stack, together with MongoDB database and AngularJS frontend framework.