title: About app.listen() callback
body: <p>I'm new in javascript and now i'm learn about express.js, but i get some code that makes me confused about how they work. I was tring to figure out how this code work but i still don't get it:</p>
  
  <pre><code>var server = app.listen(3000, function (){
    var host = server.address().address;
    var port = server.address().port;
    console.log('Example app listening at http://%s:%s', host, port);
  });
  </code></pre>
  
  <p>My question is how this anonymous function can using the server variable, when the server variable getting return value from <code>app.listen()</code>.</p>