Express is a web application framework for Node.js: a library that a JavaScript program on a server uses to receive HTTP requests and send responses. Its own description is "fast, unopinionated, minimalist": it gives a thin layer of basic web features and leaves the rest to the developer. It is a project of the OpenJS Foundation and the Express.js contributors.
A developer creates an app object, defines routes (a URL path and an HTTP method such as GET, linked to a handler function), and starts the app on a port; each handler reads the request and sends the response. Most other features come from middleware: functions that run in sequence on each request, for example to parse the request body, set CORS headers, read cookies, keep sessions, compress responses or serve static files. Express also works with template engines that generate HTML pages, and it serves both web applications and APIs. The current major version is Express 5.
Express is free and open source under the MIT license and is installed from npm.