emitter.once(event, listener)

server.once('connection', function (stream) {
  console.log('Ah, we have our first user!');
});

EventEmitter.prototype.once = function(type, listener) {
  if (!util.isFunction(listener))
    throw TypeError('listener must be a function');
  function g() {
    this.removeListener(type, g);
    listener.apply(this, arguments);
  }
  g.listener = listener;
  this.on(type, g);
  return this;
};