Middleware — Logging, Auth, Error Handling

Error handling middleware is a special type of middleware that catches errors thrown in your route handlers or other middleware. It always has four parameters: err, req, res, next.

Think of it like Swiggy's customer support — when something goes wrong (food spilled, wrong order), the customer support team handles it gracefully instead of just ghosting you.

Error handling middleware pattern:

import express from 'express';
const app = express();

// Standard middleware
app.use(express.json());

// Route that throws an error
app.get('/biryani/:id', (req, res) => {
  const id = Number(req.params.id);

  if (isNaN(id)) {
    // Throw an error — caught by error middleware!
    throw new Error('Invalid ID! Sirf numbers daalo bhai!');
  }

  res.json({ id, name: 'Chicken Biryani' });
});

// Route that calls next() with an error
app.post('/orders', (req, res, next) => {
  if (!req.body.item) {
    // Pass error to error middleware
    return next(new Error('Item is required!'));
  }
  res.json({ status: 'ordered' });
});

// ⚡ ERROR HANDLING MIDDLEWARE — always LAST, always 4 params!
app.use((err, req, res, next) => {
  console.error('💥 Error:', err.message);
  console.error(err.stack);  // Full stack trace for debugging

  res.status(500).json({
    success: false,
    message: 'Kuch to gadbad hai bhai!',
    error: process.env.NODE_ENV === 'production'
      ? 'Internal server error'
      : err.message  // Show details only in development
  });
});

app.listen(3000);

Complete middleware stack in order:

// 1. Third-party middleware (first!)
app.use(morgan('dev'));
app.use(express.json());
app.use(cors());

// 2. Custom middleware
app.use((req, res, next) => {
  console.log('Request:', req.method, req.url);
  next();
});

// 3. Auth middleware (selective)
app.use('/admin', authMiddleware);

// 4. Routes
app.get('/biryani', ...);
app.post('/orders', ...);

// 5. 404 handler (catch-all)
app.use((req, res) => {
  res.status(404).json({ error: 'Not found' });
});

// 6. Error handler (LAST!)
app.use((err, req, res, next) => {
  console.error(err);
  res.status(500).json({ error: 'Server error' });
});

Rules for error middleware:

• Must have exactly 4 parameters — Express identifies it by the arity (number of params)
• Must be placed LAST in the middleware stack
• Catches errors from throw and from next(error)
• Don't show stack traces in production!