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Express uses the debug module internally to log information about route matches, middleware functions, application mode, and the request-response cycle.

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Using the Debug Module

The debug module allows you to selectively enable debug output by setting the DEBUG environment variable.

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View All Internal Logs

To see all internal logs used in Express, set the DEBUG environment variable to express:*: 
DEBUG=express:* node app.js

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Filter Debug Output

You can filter debug output to specific modules: 
# Only router logs
DEBUG=express:router node app.js

# Router and application logs
DEBUG=express:router,express:application node app.js

# All express logs plus your app logs
DEBUG=express:*,myapp:* node app.js

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Adding Debug to Your App

1

Install debug module

The debug module comes with Express, but you can also install it separately:
npm install debug
2

Create debug instances

Create debug instances for different parts of your application:
const debug = require('debug')('myapp:server');
const dbDebug = require('debug')('myapp:database');

debug('Server starting on port 3000');
dbDebug('Connected to database');
3

Run with debug enabled

Enable your debug output:
DEBUG=myapp:* node app.js

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Example Debug Integration

const express = require('express');
const debug = require('debug')('myapp:server');
const dbDebug = require('debug')('myapp:db');

const app = express();

app.use((req, res, next) => {
  debug('%s %s', req.method, req.url);
  next();
});

app.get('/users', async (req, res) => {
  dbDebug('Fetching users from database');
  // ... database query
  res.json(users);
});

const port = 3000;
app.listen(port, () => {
  debug('Server listening on port %d', port);
});

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Node.js Inspector

Use the built-in Node.js debugger for advanced debugging with breakpoints and step-through execution.

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Start with Inspector

node --inspect app.js
Then open Chrome and navigate to chrome://inspect to connect to your app.

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Debugging with VS Code

Create a .vscode/launch.json file: 
{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "Debug Express App",
      "program": "${workspaceFolder}/app.js",
      "env": {
        "DEBUG": "express:*,myapp:*"
      }
    }
  ]
}

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Logging Middleware

Use Morgan for HTTP request logging: 
const morgan = require('morgan');

// Use 'dev' format in development
if (app.get('env') === 'development') {
  app.use(morgan('dev'));
}

// Use 'combined' format in production
if (app.get('env') === 'production') {
  app.use(morgan('combined'));
}
Avoid logging sensitive data like passwords, tokens, or personal information.

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Debug Patterns

const debug = require('debug')('myapp:middleware');

app.use((req, res, next) => {
  debug('Middleware executed for %s', req.url);
  next();
});

const debug = require('debug')('myapp:routes');

app.get('/api/data', (req, res) => {
  debug('Fetching data for user %s', req.user?.id);
  // ... handler code
});

const debug = require('debug')('myapp:error');

app.use((err, req, res, next) => {
  debug('Error occurred: %s', err.message);
  debug('Stack: %s', err.stack);
  // ... error handling
});

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Best Practices

  • Use descriptive namespace patterns like app:module:submodule
  • Disable debug output in production unless troubleshooting
  • Use different debug instances for different components
  • Combine debug with proper error handling
  • Use Morgan for HTTP request logging

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