## TL;DR

Express.js is a minimal, unopinionated web framework for Node.js, created by TJ Holowaychuk (2010). It provides routing, middleware, request/response handling, and template engines. Express is the most popular Node.js framework and the foundation of MERN/MEAN stacks.

## Core Explanation

Middleware: functions with (req, res, next) — can modify request, end response, or pass control. `app.use(middleware)` for global, `app.get(path, handler)` for routes. Built on Node's http module. `express.Router()` for modular route handling. Error handling: four-argument middleware (err, req, res, next). Express 5.0 (2024) added Promise-based error handling and path regex changes.

## Further Reading

- [Express.js Documentation](https://expressjs.com/)