Callback function is called only once.

[netkgk]

Callback function, when is set by emit, called only once, despite that many ack packages may appear. That's because it's deleted immediately after its first call in Socket.prototype.onack:

Socket.prototype.onack = function(packet){
  var ack = this.acks[packet.id];
  if ('function' == typeof ack) {
    debug('calling ack %s with %j', packet.id, packet.data);
    ack.apply(this, packet.data);
    delete this.acks[packet.id]; // <- Callback is deleted here for some reason.
  } else {
    debug('bad ack %s', packet.id);
  }
};