ExpressJS is a fast, unopinionated, minimalist web framework for Node.js. Express.js, or simply Express, is a web application framework for Node.js, released as free and open-source software under the MIT License. It is designed for building web applications and APIs. It has been called the de facto standard server framework for Node.js.
The original author, TJ Holowaychuk, described it as a Sinatra-inspired server, meaning that it is relatively minimal with many features available as plugins. Express is the backend component of the MEAN stack, together with the MongoDB database software and AngularJS frontend framework.