title: Using Socket.IO, how can I find out the session ID of a disconnected user?
body: <p>When a user disconnects from the server, how can I find out the session ID?</p>
  
  <p>At the moment I’ve got an ugly method of asking all existing clients to send a message back.</p>
  
  <p>e.g. on the server:</p>
  
  <pre><code>socket.on('disconnect', function() {
      // What’s the sessionid?
  });
  </code></pre>