useing mikeal's request when piping something,how to listen events to show progress with a progress-bar

I did a quick search on the NPM registry and found progress, a module that shows a progress bar in the command line. It looks nice and it even has an example showing how to use it to show a download progress. I'm reproducing it here:

var ProgressBar = require('progress')
  , https = require('https');

var req = https.request({
    host: 'download.github.com'
  , port: 443
  , path: '/visionmedia-node-jscoverage-0d4608a.zip'
});

req.on('response', function(res){
  var len = parseInt(res.headers['content-length'], 10);

  console.log();
  var bar = new ProgressBar('  downloading [:bar] :percent :etas', {
      complete: '='
    , incomplete: ' '
    , width: 20
    , total: len
  });

  res.on('data', function(chunk){
    bar.tick(chunk.length);
  });

  res.on('end', function(){
    console.log('\n');
  });
});

req.end();

The code above will generate a progress bar that looks like this:

downloading [=====             ] 29% 3.7s​

Notice how he checks the content-length header to get the length of the file being downloaded and uses that to calculate the completion percentage. It then passes the progress bar the length of each chunk.