Instrumenting code with debug

We can use the debug module in our own code to create logs that relate to the context of our application or module.

Let's copy our app folder from the main recipe, call it instrumented-app, and install the debug module:

$ cp -fr app instrumented-app
$ cd instrumented-app
$ npm install --save debug

Next, we'll make index.js look like so:

const express = require('express') 
const app = express() 
const stylus = require('stylus') 
const debug = require('debug')('my-app') 
 
app.get('/some.css', (req, res) => { 
  debug('css requested') 
  const css = stylus(` 
    body 
      color:black 
  `).render() 
  res.send(css) 
}) 
 
app.listen(3000) 

We've required debug, created a logging function (called debug) with the my-app namespace and then used it in our route handler.

Now let's start our app and just turn on logs for the my-app namespace:

$ DEBUG=my-app node index.js

Now let's make a request to http://localhost:3000/some.css, either in the browser, or with curl we could do the following:

$ curl http://localhost:3000/some.css

This should create the following log message:

my-app css requested +0ms