node/lib/events.js

var EventEmitter = exports.EventEmitter = process.EventEmitter;

var isArray = Array.isArray;

EventEmitter.prototype.emit = function(type) {
  // If there is no 'error' event listener then throw.
  if (type === 'error') {
    if (!this._events || !this._events.error ||
        (isArray(this._events.error) && !this._events.error.length))
    {
      if (arguments[1] instanceof Error) {
        throw arguments[1]; // Unhandled 'error' event
      } else {
        throw new Error("Uncaught, unspecified 'error' event.");
      }
      return false;
    }
  }

  if (!this._events) return false;
  var handler = this._events[type];
  if (!handler) return false;

  if (typeof handler == 'function') {
    switch (arguments.length) {
      // fast cases
      case 1:
        handler.call(this);
        break;
      case 2:
        handler.call(this, arguments[1]);
        break;
      case 3:
        handler.call(this, arguments[1], arguments[2]);
        break;
      // slower
      default:
        var args = Array.prototype.slice.call(arguments, 1);
        handler.apply(this, args);
    }
    return true;

  } else if (isArray(handler)) {
    var args = Array.prototype.slice.call(arguments, 1);

    var listeners = handler.slice();
    for (var i = 0, l = listeners.length; i < l; i++) {
      listeners[i].apply(this, args);
    }
    return true;

  } else {
    return false;
  }
};

// EventEmitter is defined in src/node_events.cc
// EventEmitter.prototype.emit() is also defined there.
EventEmitter.prototype.addListener = function(type, listener) {
  if ('function' !== typeof listener) {
    throw new Error('addListener only takes instances of Function');
  }

  if (!this._events) this._events = {};

  // To avoid recursion in the case that type == "newListeners"! Before
  // adding it to the listeners, first emit "newListeners".
  this.emit('newListener', type, listener);

  if (!this._events[type]) {
    // Optimize the case of one listener. Don't need the extra array object.
    this._events[type] = listener;
  } else if (isArray(this._events[type])) {
    // If we've already got an array, just append.
    this._events[type].push(listener);
  } else {
    // Adding the second element, need to change to array.
    this._events[type] = [this._events[type], listener];
  }

  return this;
};

EventEmitter.prototype.on = EventEmitter.prototype.addListener;

EventEmitter.prototype.once = function(type, listener) {
  var self = this;
  self.on(type, function g() {
    self.removeListener(type, g);
    listener.apply(this, arguments);
  });
};

EventEmitter.prototype.removeListener = function(type, listener) {
  if ('function' !== typeof listener) {
    throw new Error('removeListener only takes instances of Function');
  }

  // does not use listeners(), so no side effect of creating _events[type]
  if (!this._events || !this._events[type]) return this;

  var list = this._events[type];

  if (isArray(list)) {
    var i = list.indexOf(listener);
    if (i < 0) return this;
    list.splice(i, 1);
    if (list.length == 0)
      delete this._events[type];
  } else if (this._events[type] === listener) {
    delete this._events[type];
  }

  return this;
};

EventEmitter.prototype.removeAllListeners = function(type) {
  // does not use listeners(), so no side effect of creating _events[type]
  if (type && this._events && this._events[type]) this._events[type] = null;
  return this;
};

EventEmitter.prototype.listeners = function(type) {
  if (!this._events) this._events = {};
  if (!this._events[type]) this._events[type] = [];
  if (!isArray(this._events[type])) {
    this._events[type] = [this._events[type]];
  }
  return this._events[type];
};