title: In Node.js, what's "on"?
body: <p>In <a href="http://nodejs.org/docs/v0.6.3/api/http.html#http.ClientResponse">official doc</a>, there is some sample code:</p>
  
  <pre><code>var req = http.request(options, function(res) {
    console.log('STATUS: ' + res.statusCode);
    console.log('HEADERS: ' + JSON.stringify(res.headers));
    res.setEncoding('utf8');
    res.on('data', function (chunk) {
      console.log('BODY: ' + chunk);
    });
  });
  </code></pre>
  
  <p>I can understand it except one part: what's <code>on</code> in <code>res.on</code>? What's the difference between it and <code>addListener</code>?</p>