Routing and Middleware

Routing determines how your application responds to client requests. Middleware functions are the backbone of Express — they process requests at every stage of the pipeline.

Route Methods

Express provides methods for every HTTP verb:

HTTP Methods at a Glance

Route Parameters

Capture dynamic values from the URL path:

Query Parameters

Query parameters come after ? in the URL:

Route Chaining with route()

Group different methods for the same path:

What is Middleware?

Middleware functions have access to the request object, response object, and the next function. They can execute code, modify the request/response, end the request-response cycle, or call next() to pass control to the next middleware.

The Middleware Pipeline

Requests flow through middleware in the order they are registered:

Request → Logger → Auth → Route Handler → Error Handler → Response

Building Custom Middleware

Request Logger

Request Validator

Rate Limiter

CORS Middleware

Cross-Origin Resource Sharing allows your API to be called from different domains:

npm install cors

Serving Static Files

Files in the public directory become accessible at the root URL (or under /assets with the prefix).

Error Handling Middleware

Error middleware has four parameters — Express identifies it by the (err, req, res, next) signature:

Practical Exercise

Build a middleware stack for a production API:

Key Takeaways

• Express routes match HTTP methods and URL patterns to handler functions.
• Middleware functions form a pipeline — each can modify the request/response or pass control with next().
• Register middleware in order: parsers first, then loggers, auth, routes, and error handlers last.
• Custom middleware like validators and rate limiters keep your route handlers clean.
• Error-handling middleware must have exactly four parameters: (err, req, res, next).