Node.js & Express

Express Middleware

20 min Lesson 9 of 40

Understanding Middleware

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application's request-response cycle. Middleware is the heart of Express.js.

What is Middleware?

Think of middleware as a series of functions that a request passes through before reaching its final destination. Each middleware function can:

Execute any code
Make changes to the request and response objects
End the request-response cycle
Call the next middleware function in the stack

Analogy: Middleware is like a series of checkpoints. A request travels through each checkpoint (middleware), where it can be inspected, modified, or stopped before reaching the final handler.

Middleware Syntax

const express = require('express');
const app = express();

// Basic middleware structure
app.use((req, res, next) => {
  // Do something
  console.log('Middleware executed');

  // Pass control to the next middleware
  next();
});

Critical: Always call next() unless you're ending the response. Forgetting next() will cause your request to hang indefinitely.

Built-in Express Middleware

Express comes with several built-in middleware functions:

1. express.json() - Parse JSON Bodies

// Parse incoming JSON payloads
app.use(express.json());

app.post('/api/users', (req, res) => {
  // Now you can access req.body
  console.log(req.body.name);
  console.log(req.body.email);

  res.json({ message: 'User created', data: req.body });
});

2. express.urlencoded() - Parse URL-Encoded Bodies

// Parse form data (application/x-www-form-urlencoded)
app.use(express.urlencoded({ extended: true }));

app.post('/submit-form', (req, res) => {
  console.log(req.body.username);
  console.log(req.body.password);

  res.send('Form submitted');
});

extended: true vs false:

true: Uses qs library, supports nested objects
false: Uses querystring library, simple key-value pairs

3. express.static() - Serve Static Files

// Serve files from 'public' directory
app.use(express.static('public'));

// Multiple static directories
app.use(express.static('public'));
app.use(express.static('uploads'));

// With path prefix
app.use('/static', express.static('public'));
// Now: http://localhost:3000/static/style.css

Custom Middleware

You can create your own middleware functions for logging, authentication, validation, and more.

Simple Logger Middleware

// Custom logger
const logger = (req, res, next) => {
  const timestamp = new Date().toISOString();
  console.log(`[${timestamp}] ${req.method} ${req.url}`);
  next();
};

app.use(logger);

Exercise: Build a Middleware Chain

Create an Express app with custom middleware for logging, authentication, and validation
Install and configure CORS, Helmet, and Morgan
Create protected and public routes
Add proper error handling middleware
Test with Postman/Thunder Client