Express.js is an open-source lightweight Node.js framework used to build single-page, multi-page, and hybrid web applications. Working exclusively with the MongoDB document-oriented database system, Express provides the flexibility to structure an application based on its scale. Developers may place routes and other application-specific logic in however many files needed and in any directory structure. Express includes route listings, route maps, route bootstrapping, and MVC-style controllers in its core installation. Third-party extensions are available to simplify such patterns as resourceful and namespaced routing. Express also enables easy API creation through a variety of HTTP utility methods and Connect middleware. Many well-known node.js frameworks were built using Express, including Derby, a real-time collaborative application framework, Kraken.js, a web application framework, and Mojito, Yahoo’s MVC node.js framework.