Middleware Chain

You are a backend architect specializing in middleware patterns. The user wants to create and organize a middleware chain that handles sequential request processing with proper error handling and execution flow.

What to check first

• Verify your framework supports middleware (Express, Koa, FastAPI, Django) with npm list express or equivalent
• Confirm the order of middleware matters—check your routing setup with app.use() or middleware decorator order
• Review existing middleware in your app.js or main.py to avoid conflicts with global handlers

Steps

1. Define middleware functions that accept (req, res, next) signature for Express or equivalent for your framework
2. Create a middleware registry object or array to organize and name each middleware piece
3. Use app.use() to mount middleware in order: authentication → logging → body parsing → validation
4. Chain middleware by calling next() in each handler to pass control to the next function
5. Implement error-handling middleware as the last middleware with (err, req, res, next) signature
6. Test the chain by checking that middleware executes in expected order using console.log() at each step
7. Separate concerns by storing middleware in dedicated files (e.g., middleware/auth.js, middleware/validation.js)
8. Use conditional middleware with route-specific mounting: router.use(authMiddleware) for protected routes only

Code

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

// middleware/auth.js
const auth = (req, res, next) => {
  const token = req.headers.authorization;
  if (!token) return res.status(401).json({ error: 'Unauthorized' });
  req.user = { id: 1 }; // Simplified
  next();
};

// middleware/validation.js
const validateRequest = (req, res, next) => {
  if (!req.body || Object.keys(req.body).length === 0) {
    return res.status(400).json({ error: 'Empty request body' });
  }
  next();
};

// middleware/errorHandler.js
const errorHandler = (err, req, res, next) => {
  console.error('Error:', err.message);
  res.status(err.status || 500).json({ error: err.message });
};

// app.js
const express = require('express');
const app = express();

// Import middleware
const { logger } = require('./middleware/logger');
const { auth } = require('./middleware/auth');
const { validateRequest } = require('./middleware/validation');
const { errorHandler } = require('./middleware/errorHandler');

// Global middleware chain (executes for every request)
app.use(express.json());
app.use(logger);

// Protected route chain
app.post('/api/data', auth

Note: this example was truncated in the source. See the GitHub repo for the latest full version.

Common Pitfalls

• Treating this skill as a one-shot solution — most workflows need iteration and verification
• Skipping the verification steps — you don't know it worked until you measure
• Applying this skill without understanding the underlying problem — read the related docs first

When NOT to Use This Skill

• When a simpler manual approach would take less than 10 minutes
• On critical production systems without testing in staging first
• When you don't have permission or authorization to make these changes

How to Verify It Worked

• Run the verification steps documented above
• Compare the output against your expected baseline
• Check logs for any warnings or errors — silent failures are the worst kind

Production Considerations

• Test in staging before deploying to production
• Have a rollback plan — every change should be reversible
• Monitor the affected systems for at least 24 hours after the change