title: How to send acknowledgment from Socket.IO server?
body: <p>I'm having trouble sending acknowledgements from my socket.IO server. I've followed the documentation and added a function "fn" to my message handler function. Unfortunately fn is undefined. What's wrong with my code?</p>
  
  <pre><code>socket.on('search', function (searchParamsFromClient, fn) {
  </code></pre>