Express Middleware Roadmap: Which to Learn First

Express Middleware Roadmap: Which to Learn First

A roadmap keeps your learning focused. Here is the path from Express middleware basics to production-ready.

Step 1: Understand (req, res, next)

The signature of every middleware. req is the request, res is the response, next passes control. Without next(), the request hangs. This is the foundation.

Step 2: Use Built-in Middlewares

Start with express.json and express.static. Learn how body parsing works. Understand that middlewares run in order.

Step 3: Write a Logger Middleware

Write a simple middleware that logs method and URL. This teaches you the pattern without complexity.

Step 4: Write an Auth Middleware

Verify a JWT, set req.user, call next or send 401. This is the most common custom middleware you will write in real apps.

Step 5: Write a Validation Middleware

Use Zod or express-validator. Write a factory that takes a schema and returns a middleware. Validate req.body, req.params, req.query separately.

Step 6: Learn Error-Handling Middleware

The four-parameter middleware. Write one. Use a custom ApiError class. Understand how next(err) jumps to it.

Step 7: Learn Async Error Handling

Wrap async handlers with asyncHandler. Understand why Express 4 does not catch promise rejections. Try Express 5 for automatic catching.

Step 8: Add Security Middlewares

helmet, cors, express-rate-limit, express-mongo-sanitize, xss-clean, hpp. Most are one-liners with big payoff.

Step 9: Add Logging Middlewares

morgan for dev, pino-http for production. Log method, path, status, response time. Ship logs centrally.

Step 10: Add Performance Middlewares

compression for gzip responses. express-rate-limit for abuse prevention. These are cheap and pay off.

Step 11: Apply Per-Router Middlewares

Use router.use(auth) for groups of routes. Keep public routes outside. Use role middlewares for admin.

Step 12: Compose Middlewares

Stack middlewares per route: router.post('/posts', auth, validate, createPost). Understand how they chain and short-circuit.

Step 13: Test Middlewares

Use Jest and supertest. Test that auth rejects unauthenticated requests. Test that validation rejects bad input. Test happy paths too.

The Takeaway

Express middleware roadmap: (req, res, next) signature, built-in middlewares, write a logger, write auth, write validation, learn error handling, learn async error handling, add security middlewares, add logging, add performance, apply per-router, compose per route, and test. Take it one step at a time.