Express.js Deep Dive: Middleware Patterns

Middleware is the backbone of Express. Let's look at patterns that make your APIs more robust and maintainable.

What Is Middleware?

Middleware functions have access to the request, response, and next function. They can:

• Execute code
• Modify request/response
• End the request-response cycle
• Call the next middleware

const myMiddleware = (req, res, next) => {
  // Do something
  next(); // Pass to next middleware
};

Request Logging

const requestLogger = (req, res, next) => {
  const start = Date.now();
  
  res.on('finish', () => {
    const duration = Date.now() - start;
    console.log(`${req.method} ${req.url} ${res.statusCode} - ${duration}ms`);
  });
  
  next();
};

app.use(requestLogger);

Authentication

const authenticate = async (req, res, next) => {
  const token = req.headers.authorization?.split(' ')[1];
  
  if (!token) {
    return res.status(401).json({ error: 'No token provided' });
  }
  
  try {
    const user = await verifyToken(token);
    req.user = user;
    next();
  } catch (err) {
    res.status(401).json({ error: 'Invalid token' });
  }
};

// Apply to specific routes
app.get('/api/profile', authenticate, (req, res) => {
  res.json(req.user);
});

Error Handling

Error middleware has 4 parameters:

const errorHandler = (err, req, res, next) => {
  console.error(err.stack);
  
  if (err.name === 'ValidationError') {
    return res.status(400).json({ error: err.message });
  }
  
  if (err.name === 'UnauthorizedError') {
    return res.status(401).json({ error: 'Unauthorized' });
  }
  
  res.status(500).json({ error: 'Internal server error' });
};

// Must be registered last
app.use(errorHandler);

Throw errors from routes:

app.get('/api/users/:id', async (req, res, next) => {
  try {
    const user = await User.findById(req.params.id);
    if (!user) {
      const err = new Error('User not found');
      err.status = 404;
      throw err;
    }
    res.json(user);
  } catch (err) {
    next(err); // Passes to error handler
  }
});

Validation

const validate = (schema) => {
  return (req, res, next) => {
    const { error } = schema.validate(req.body);
    if (error) {
      return res.status(400).json({ error: error.details[0].message });
    }
    next();
  };
};

// Usage with Joi
const createUserSchema = Joi.object({
  email: Joi.string().email().required(),
  password: Joi.string().min(8).required()
});

app.post('/api/users', validate(createUserSchema), createUser);

Middleware Order Matters

// Order is important!
app.use(express.json());     // Parse JSON bodies
app.use(requestLogger);      // Log all requests
app.use('/api', apiRoutes); // API routes
app.use(notFoundHandler);    // 404 handler
app.use(errorHandler);       // Error handler (always last)

Middleware runs in the order it's registered. Keep that in mind when debugging.

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