Express Middleware & Request Pipeline

Middleware functions have access to request, response objects and the next middleware function. They execute in order and can modify req/res or end the request.

Basic Middleware

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

// Application-level middleware
app.use((req, res, next) => {
  console.log('Time:', Date.now());
  next();  // Pass to next middleware
});

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

// Parse JSON body
app.use(express.json());

// Parse URL-encoded body
app.use(express.urlencoded({ extended: true }));

// Static files
app.use(express.static('public'));

// Route-specific middleware
app.get('/api/users', authMiddleware, (req, res) => {
  res.json(users);
});

// Error-handling middleware (4 parameters)
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).json({ error: 'Internal Server Error' });
});