Creating our own NODE_DEBUG flags

Core modules tend to use the util module's debuglog method to generate a logging function that defaults to a no-op (an empty function) but writes log messages to STDOUT when the relevant flag appears in the NODE_DEBUG environment variable.

We can use util.debuglog to create our own core like log messages.

Let's take the app folder we created in the main recipe and copy it to instrumented-app:

$ cp -fr app instrumented-app

Now let's make index.js look like this:

const util = require('util') 
const express = require('express') 
const debug = util.debuglog('my-app') 
const app = express() 
 
app.get('/', (req, res) => { 
  debug('incoming request on /', req.route) 
  res.send('hey') 
}) 
 
setTimeout(function myTimeout() {  
   debug('timeout complete') 
   console.log('I waited for you.') 
}, 100) 
 
app.listen(3000) 

Now we can turn on our custom debug logs like so:

$  NODE_DEBUG=my-app node index.js

If we make a request to http://localhost:3000, the output of our process should look something like this:

MY-APP 30843: timeout complete 
I waited for you. 
MY-APP 30843: incoming request on / Route { 
  path: '/', 
  stack: 
   [ Layer { 
       handle: [Function], 
       name: '<anonymous>', 
       params: undefined, 
       path: undefined, 
       keys: [], 
       regexp: /^\/?$/i, 
       method: 'get' } ], 
  methods: { get: true } } 

Prefer the debug module
In many cases, using the third-party debug module instead of util.debuglog is preferable. The debug modules supports wildcards, and the output is time stamped and color-coded, while the production cost of using it is negligible. See the Enabling debug logs recipe in this chapter for more.