index.js

/**
 * Module Dependencies
 */

var noop = function() {};

/**
 * Expose `Emitter`
 */

module.exports = Emitter;

/**
 * Initialize `Emitter`
 */

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

/**
 * Mixin the emitter properties.
 *
 * @param {Object} obj
 * @return {Object}
 * @api private
 */

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

/**
 * Listen on the given `event` with `fn`.
 *
 * @param {String} event
 * @param {Function} fn
 * @return {Emitter}
 * @api public
 */

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

/**
 * Adds an `event` listener that will be invoked a single
 * time then automatically removed.
 *
 * @param {String} event
 * @param {Function} fn
 * @return {Emitter}
 * @api public
 */

Emitter.prototype.once = function(event, fn){
  var self = this;
  this._callbacks = this._callbacks || {};

  function on() {
    self.off(event, on);
    fn.apply(this, arguments);
  }

  // need to forward on the fn length when we emit
  on.len = fn.length;
  fn._off = on;
  this.on(event, on);
  return this;
};

/**
 * Remove the given callback for `event` or all
 * registered callbacks.
 *
 * @param {String} event
 * @param {Function} fn
 * @return {Emitter}
 * @api public
 */

Emitter.prototype.off =
Emitter.prototype.removeListener =
Emitter.prototype.removeAllListeners = function(event, fn){
  this._callbacks = this._callbacks || {};
  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 i = callbacks.indexOf(fn._off || fn);
  if (~i) callbacks.splice(i, 1);
  return this;
};

/**
 * Emit `event` with the given args.
 *
 * @param {String} event
 * @param {Mixed} ...
 * @return {Emitter}
 */

Emitter.prototype.emit = function(event){
  this._callbacks = this._callbacks || {};
  var args = [].slice.call(arguments, 1),
      last = args[args.length-1],
      callbacks = this._callbacks[event],
      pending = 0,
      context = this;

  if (!callbacks) return this;

  var done = ('function' == typeof last) ? args.pop() : noop;

  function next(err) {
    if(err) return done(err);
    else if(!--pending) return done();
  }

  callbacks = callbacks.slice(0);
  callbacks.forEach(function(callback) {
    var len = callback.len || callback.length;
    callback.apply(context, args.concat(next));
    // If the callback length is greater than the argument length, assume async
    if(len > args.length) pending++;
  });

  if(!pending) return done();
};

/**
 * Return array of callbacks for `event`.
 *
 * @param {String} event
 * @return {Array}
 * @api public
 */

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

/**
 * Check if this emitter has `event` handlers.
 *
 * @param {String} event
 * @return {Boolean}
 * @api public
 */

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