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A popular framework for building web servers in Node.js — like a toolkit that gives you the basic plumbing so you can focus on building rooms.
A minimal, unopinionated web framework for Node.js that provides routing, middleware composition, and HTTP utility methods. Express handles the request/response cycle and delegates business logic to middleware functions.
Express implements a layered middleware stack where each layer is a (req, res, next) function executed in registration order. The Router is a self-contained middleware with its own stack, enabling modular route composition. Express 4's generator-free design relies on the middleware pattern for all concerns (parsing, auth, error handling). Error-handling middleware uses a (err, req, res, next) signature and is matched when next(err) is called.