Express Middleware in Express.js

Express Middleware is the third chapter of the free Express.js tutorial. Middleware is the heart of Express: small functions that every request passes through.

What each lesson covers:

  • Middleware Fundamentals: The (req, res, next) pipeline, the three ways to finish, mounting and order, custom middleware, role checks, factories and async errors.
  • Body Parsers and Third-Party Middleware: Reading JSON, forms, text and raw bodies, handling bad and oversized bodies, and choosing npm middleware.

Middleware is the heart of Express. Every request passes through a chain of functions, and each one can read the request, change it, answer it, or pass it on with next(). The first lesson walks through that pipeline: the three ways a middleware can finish, application, router and error middleware, mounting on a path, why order matters, and writing your own, including access checks and async middleware. The second lesson reads request bodies with express.json() and express.urlencoded(), handles invalid JSON and size limits, and adds trusted third-party middleware.

Every lab runs a real Express 5 server in your browser. Watch the terminal to see the order middleware runs in.

Lessons in this section

  1. Middleware Fundamentals

    What middleware is, the (req, res, next) pipeline, the three ways to finish, middleware types, global, path-based and route-specific mounting, registration order, custom middleware, access-control middleware, factories and async middleware in Express 5.

    • Explain how the middleware pipeline runs
    • Finish middleware with next, a response or next(err)
    • Mount middleware globally, on a path or on a route
    • Write custom and configurable middleware
  2. Body Parsers and Third-Party Middleware

    express.json and its options, handling invalid JSON, express.urlencoded for forms, express.text and express.raw for webhooks, body size limits, and how to choose, install, configure and place third-party middleware.

    • Parse JSON, form, text and raw bodies
    • Answer clearly for invalid and oversized bodies
    • Read nested form data with extended
    • Use a raw body for webhook checks