// Correct: specific routes before catch-all app.get('/api/users', getUsers); app.use('*', notFoundHandler);


### 2. Middleware Not Executing
**Symptoms:** Request hangs, next() not called, order issues
**Common Causes:**
- Forgetting to call `next()`
- Async middleware without proper error handling
- Wrong middleware order

```javascript
// Wrong: missing next()
app.use((req, res, next) => {
  console.log('Request received');
  // Hangs - next() never called
});

// Correct: always call next() or send response
app.use((req, res, next) => {
  console.log('Request received');
  next();
});

// Correct async middleware
app.use(async (req, res, next) => {
  try {
    await someAsyncOperation();
    next();
  } catch (err) {
    next(err); // Pass error to error handler
  }
});

3. CORS Errors

Symptoms: Browser blocks requests, preflight fails Common Causes:

CORS middleware placed after routes
Missing OPTIONS handler
Credentials not configured

const cors = require('cors');

// Wrong: CORS after routes
app.get('/api/data', handler);
app.use(cors()); // Too late

// Correct: CORS before routes
app.use(cors({
  origin: process.env.ALLOWED_ORIGINS?.split(',') || '*',
  credentials: true,
  methods: ['GET', 'POST', 'PUT', 'DELETE', 'OPTIONS'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));
app.get('/api/data', handler);

4. Async Error Handling

Symptoms: Unhandled promise rejections, app crashes Common Causes:

Missing try/catch in async handlers
Promises not caught
No global error handler

// Wrong: unhandled async error
app.get('/users', async (req, res) => {
  const users = await User.findAll(); // Throws, crashes app
  res.json(users);
});

// Correct: wrap async handlers
const asyncHandler = (fn) => (req, res, next) =>
  Promise.resolve(fn(req, res, next)).catch(next);

app.get('/users', asyncHandler(async (req, res) => {
  const users = await User.findAll();
  res.json(users);
}));

// Global error handler (must be last)
app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(err.status || 500).json({
    error: process.env.NODE_ENV === 'production'
      ? 'Internal server error'
      : err.message
  });
});

5. Memory Leaks

Symptoms: Heap growing, OOM errors, slow responses over time Common Causes:

Unclosed database connections
Event listeners not removed
Large objects in closures
Global caches without limits

// Wrong: unbounded cache
const cache = {};
app.get('/data/:id', (req, res) => {
  cache[req.params.id] = largeObject; // Memory leak
});

// Correct: use LRU cache with limits
const LRU = require('lru-cache');
const cache = new LRU({ max: 500, ttl: 1000 * 60 * 5 });

// Check for leaks
node --inspect --expose-gc app.js
// Use Chrome DevTools Memory tab

6. Unhandled Promise Rejections

Symptoms: Warnings in console, silent failures Setup global handlers:

// Add to app entry point
process.on('unhandledRejection', (reason, promise) => {
  console.error('Unhandled Rejection at:', promise, 'reason:', reason);
  // Log to error tracking service
});

process.on('uncaughtException', (err) => {
  console.error('Uncaught Exception:', err);
  // Graceful shutdown
  process.exit(1);
});

Debugging Tools

1. DEBUG Environment Variable

The most powerful built-in debugging tool for Express.

# See all Express internal logs
DEBUG=express:* node app.js

# Specific areas only
DEBUG=express:router node app.js
DEBUG=express:application,express:router node app.js

# Multiple packages
DEBUG=express:*,body-parser:* node app.js

# Your own debug statements
DEBUG=myapp:* node app.js

// In your code
const debug = require('debug')('myapp:server');
debug('Server starting on port %d', port);

2. Node Inspector (--inspect)

Start with Chrome DevTools support:

# Start with inspector
node --inspect app.js

# Break on first line
node --inspect-brk app.js

# Specific port
node --inspect=0.0.0.0:9229 app.js

Open chrome://inspect in Chrome, click "Open dedicated DevTools for Node".

3. VS Code Debugger

Create .vscode/launch.json:

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Debug Express",
      "program": "${workspaceFolder}/app.js",
      "env": {
        "DEBUG": "express:*",
        "NODE_ENV": "development"
      },
      "console": "integratedTerminal"
    },
    {
      "type": "node",
      "request": "attach",
      "name": "Attach to Process",
      "port": 9229
    }
  ]
}

4. Morgan Logger

HTTP request logging middleware:

const morgan = require('morgan');

// Development: colored, concise
app.use(morgan('dev'));

// Production: Apache combined format
app.use(morgan('combined'));

// Custom format with response time
app.use(morgan(':method :url :status :response-time ms - :res[content-length]'));

// Log to file
const fs = require('fs');
const accessLogStream = fs.createWriteStream('./access.log', { flags: 'a' });
app.use(morgan('combined', { stream: accessLogStream }));

5. ndb Debugger

Enhanced debugging experience:

npm install -g ndb
ndb node app.js

Features: Better UI, async stack traces, blackbox scripts, profile recording.

6. ESLint for Prevention

Catch errors before runtime:

npm install eslint eslint-plugin-node --save-dev
npx eslint --init

{
  "extends": ["eslint:recommended", "plugin:node/recommended"],
  "rules": {
    "no-unused-vars": "error",
    "no-undef": "error",
    "node/no-missing-require": "error"
  }
}

The Four Phases (Express-specific)

Phase 1: Reproduce and Isolate

Get exact error message - Check terminal, browser console, network tab
Identify the route - Which endpoint is failing?
Check request details - Method, headers, body, query params
Minimal reproduction - Can you trigger with curl/Postman?

# Test endpoint directly
curl -v http://localhost:3000/api/users
curl -X POST -H "Content-Type: application/json" \
  -d '{"name":"test"}' http://localhost:3000/api/users

Phase 2: Gather Information

Enable DEBUG logging
```
DEBUG=express:* node app.js
```

Add strategic logging

app.use((req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  console.log('Headers:', req.headers);
  console.log('Body:', req.body);
  next();
});

Check middleware order

app._router.stack.forEach((r, i) => {
  if (r.route) {
    console.log(`${i}: Route ${r.route.path}`);
  } else if (r.name) {
    console.log(`${i}: Middleware ${r.name}`);
  }
});

Inspect with breakpoints
- Set breakpoint at route handler entry
- Step through middleware chain
- Inspect req/res objects

Phase 3: Analyze and Hypothesize

Check the stack trace - Follow the call stack from error
Verify assumptions
- Is the route registered?
- Is middleware in correct order?
- Are environment variables set?
- Is database connected?
Common culprits checklist:
- Body parser before routes?
- CORS before routes?
- Auth middleware applied?
- Error handler at the end?
- Async errors caught?

Phase 4: Fix and Verify

Make one change at a time
Test the specific failing case
Run full test suite
Check for regressions

# Run tests
npm test

# Watch mode during fixes
npm test -- --watch

Quick Reference Commands

Start Debugging Session

# Full debug output
DEBUG=express:*,myapp:* node --inspect app.js

# Attach debugger and break immediately
node --inspect-brk app.js

# With nodemon for auto-reload
DEBUG=express:* nodemon --inspect app.js

Inspect Running Process

# List Node processes
ps aux | grep node

# Attach Chrome DevTools
# Open chrome://inspect in browser

# Memory usage
node --expose-gc -e "console.log(process.memoryUsage())"

Test Endpoints

# GET request with verbose output
curl -v http://localhost:3000/api/endpoint

# POST with JSON
curl -X POST http://localhost:3000/api/endpoint \
  -H "Content-Type: application/json" \
  -d '{"key": "value"}'

# With authorization
curl -H "Authorization: Bearer TOKEN" http://localhost:3000/api/protected

# Follow redirects
curl -L http://localhost:3000/redirect

# Show response headers
curl -I http://localhost:3000/api/endpoint

Check Middleware Stack

// Add to app.js temporarily
console.log('Middleware stack:');
app._router.stack.forEach((layer, index) => {
  if (layer.route) {
    console.log(`${index}: Route - ${Object.keys(layer.route.methods)} ${layer.route.path}`);
  } else if (layer.name === 'router') {
    console.log(`${index}: Router - ${layer.regexp}`);
  } else {
    console.log(`${index}: Middleware - ${layer.name}`);
  }
});

Memory Debugging

# Start with increased memory
node --max-old-space-size=4096 app.js

# Generate heap snapshot
node --inspect app.js
# In Chrome DevTools: Memory tab > Take heap snapshot

# Track memory over time
node -e "setInterval(() => console.log(process.memoryUsage()), 1000)"

Log Analysis

# Tail logs with filtering
tail -f app.log | grep ERROR

# Count error types
grep -o 'Error: [^,]*' app.log | sort | uniq -c | sort -rn

# Find slow requests (Morgan format)
grep -E '[0-9]{4,}ms' access.log

Diagnostic Middleware Template

Add this to quickly diagnose issues:

// debug-middleware.js
const debug = require('debug')('myapp:debug');

module.exports = function diagnosticMiddleware(req, res, next) {
  const start = Date.now();

  debug('Incoming request:');
  debug('  Method: %s', req.method);
  debug('  URL: %s', req.originalUrl);
  debug('  Headers: %O', req.headers);
  debug('  Body: %O', req.body);
  debug('  Query: %O', req.query);
  debug('  Params: %O', req.params);

  // Capture response
  const originalSend = res.send;
  res.send = function(body) {
    const duration = Date.now() - start;
    debug('Response:');
    debug('  Status: %d', res.statusCode);
    debug('  Duration: %dms', duration);
    debug('  Body length: %d', body?.length || 0);
    return originalSend.call(this, body);
  };

  next();
};

// Usage: app.use(require('./debug-middleware'));

Common Error Messages Reference

Error	Cause	Solution
`Cannot GET /path`	Route not found	Check route registration, order
`TypeError: Cannot read property 'x' of undefined`	Missing data in req	Validate req.body, req.params
`Error: Request timeout`	Slow operation, no response	Check DB, add timeout handling
`PayloadTooLargeError`	Body exceeds limit	Increase body-parser limit
`ECONNREFUSED`	Can't connect to service	Check DB/Redis is running
`EADDRINUSE`	Port already in use	Kill process or change port
`ERR_HTTP_HEADERS_SENT`	Response sent twice	Remove duplicate res.send()

debug:express

Express.js Debugging Guide

Common Error Patterns

1. Cannot GET /route (404 Errors)