unitEmitter.js

// for units that are owned by a particular player

function clientEmitter (game, unitId, clientId) {
  
  var ev = new EventEmitter()

  var cachedEmit = ev.emit

  // we need to override the event emitters emit function to put our own juice in there
  ev.emit = function (msg, data) {

      // first lets emit this stuff to the client, including whatever data
      // is being sent. (we can always not send the data too).
      // the controller on the client will get 'unitMessage' events
      // and will have to check the args to do the right things
      game.emit(clientId, 'unit-message', unitId, msg, data)

      // now we need to call the underlying emitters emit method, so we 
      // call the cached function (since we overwrote ev.emit) but we also
      // have to rebind this floating method to the ev object. So we use
      // the function.call(this, argument1, argument2 [, ...]) pattern.
      cachedEmit.call(ev, msg, data)
  }

  // so now we have replaced the emitters emit function with our own
  // get our business done, then hook back into the real event emitters
  // emit machinery so it's 'on' method will get called

  return ev

}