emitter.js

function Emitter(obj) {
  if (obj) return mixin(obj);
};

function mixin(obj) {
  for (var key in Emitter.prototype) {
    obj[key] = Emitter.prototype[key];
  }
  return obj;
}

Emitter.prototype.on =
Emitter.prototype.addEventListener = function(event, fn){
  this._callbacks = this._callbacks || {};
  (this._callbacks['$' + event] = this._callbacks['$' + event] || [])
    .push(fn);
  return this;
};

Emitter.prototype.once = function(event, fn){
  function on() {
    this.off(event, on);
    fn.apply(this, arguments);
  }

  on.fn = fn;
  this.on(event, on);
  return this;
};

Emitter.prototype.off =
Emitter.prototype.removeListener =
Emitter.prototype.removeAllListeners =
Emitter.prototype.removeEventListener = function(event, fn){
  this._callbacks = this._callbacks || {};

  // all
  if (0 == arguments.length) {
    this._callbacks = {};
    return this;
  }

  // specific event
  var callbacks = this._callbacks['$' + event];
  if (!callbacks) return this;

  // remove all handlers
  if (1 == arguments.length) {
    delete this._callbacks['$' + event];
    return this;
  }

  // remove specific handler
  var cb;
  for (var i = 0; i < callbacks.length; i++) {
    cb = callbacks[i];
    if (cb === fn || cb.fn === fn) {
      callbacks.splice(i, 1);
      break;
    }
  }

  // Remove event specific arrays for event types that no
  // one is subscribed for to avoid memory leak.
  if (callbacks.length === 0) {
    delete this._callbacks['$' + event];
  }

  return this;
};

Emitter.prototype.emit = function(event){
  this._callbacks = this._callbacks || {};
  var args = [].slice.call(arguments, 1)
    , callbacks = this._callbacks['$' + event];

  if (callbacks) {
    callbacks = callbacks.slice(0);
    for (var i = 0, len = callbacks.length; i < len; ++i) {
      callbacks[i].apply(this, args);
    }
  }

  return this;
};

Emitter.prototype.listeners = function(event){
  this._callbacks = this._callbacks || {};
  return this._callbacks['$' + event] || [];
};

Emitter.prototype.hasListeners = function(event){
  return !! this.listeners(event).length;
};

if (typeof module !== 'undefined') {
  module.exports = Emitter;
}