Middleware Intro

Middleware is the chain of functions every request flows through. Master it and you've mastered Express.

4 min read Level 1/5 #express#middleware#pipeline

What you'll learn

Understand the (req, res, next) signature
Recognize the four kinds of middleware
See the pipeline as data flow

A middleware is a function (req, res, next) => { ... }. Express calls them in order. Each one either:

Responds — sends a response, ending the chain
Passes — calls next() to invoke the next middleware
Errors — calls next(err) to skip to error middleware

That’s the whole model.

Visualizing the Pipeline

Request → logger → bodyParser → auth → route handler → Response
            ↓ next   ↓ next     ↓ next      ↓ res.send

Every Express feature — JSON parsing, sessions, auth, CORS, compression — is implemented as middleware. So is your business logic.

Four Kinds

Kind	Mount	Use For
Application	`app.use(fn)`	Site-wide (logging, body parsing)
Router	`router.use(fn)`	Per-router (auth on `/api/*`)
Route	`app.get(path, fn1, fn2, handler)`	Specific to a route
Error	`app.use((err, req, res, next) => ...)`	4-arg signature

A Tiny Example

import express from "express";

const app = express();

// 1. application middleware
app.use((req, res, next) => {
  console.log(`${req.method} ${req.url}`);
  next();
});

// 2. parses JSON bodies
app.use(express.json());

// 3. route — also a middleware
app.get("/", (req, res) => {
  res.send("hello");
});

app.listen(3000);

When a request hits /, the logger logs, JSON parser does nothing (no body on GET), then the route sends the response.

Why Order Matters

Middleware runs in the order you mount it. Mount the JSON parser before routes that need req.body. Mount error middleware last. Mount static middleware before catch-all 404s.

The Express Mental Model

Once you internalize “request flows down the chain, response flows back up,” everything in Express makes sense. Auth? Add middleware before routes. Logging? Add middleware first. CORS? Middleware. Compression? Middleware. Sessions? Middleware. Rate limiting? Middleware.

Frameworks like Koa and Hono use the same model, with slightly different signatures. Once you get Express middleware, you get them all.

Authoring Middleware →