Middleware ordering and its importance in Express

Introduction

Middleware runs in the order you add it. This order decides how requests are handled step-by-step.

When you want to log every request before processing it

When you need to check if a user is logged in before showing a page

When you want to handle errors after all other middleware runs

When you want to serve static files before other routes

When you want to parse incoming data before using it

Syntax

Express

app.use(middlewareFunction);

// Middleware runs in the order added

Middleware added first runs first on each request.

Order matters because some middleware depends on previous ones.

Examples

Logger runs first, then auth checks user, then router handles routes.

Express

app.use(logger);
app.use(auth);
app.use(router);

Static files are served before auth runs, so public files load fast.

Express

app.use(express.static('public'));
app.use(auth);

Sample Program

This example shows middleware running in order: logger first, then auth, then route handler.

Express

import express from 'express';

const app = express();

// Logger middleware
app.use((req, res, next) => {
  console.log('Logger: Request received');
  next();
});

// Auth middleware
app.use((req, res, next) => {
  console.log('Auth: Checking user');
  next();
});

// Route handler
app.get('/', (req, res) => {
  res.send('Hello World');
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Important Notes

If middleware order is wrong, some features may not work as expected.

Always put error-handling middleware last.

Use next() to pass control to the next middleware.

Summary

Middleware runs in the order you add it.

Ordering controls how requests flow through your app.

Put middleware that parses or logs requests early, and error handlers last.