EventHook: Simple library for modifiable EventEmitters

eventhook.js

/*
 *  ____ _  _ ____ _  _ ___ _  _ ____ ____ _  _ 
 *  |___ |  | |___ |\ |  |  |__| |  | |  | |_/  
 *  |___  \/  |___ | \|  |  |  | |__| |__| | \_ 
 *                                        
 *  Simple library for modifiable EventEmitters
 *
 *         Depends on the async module
 *       https://github.com/caolan/async
 *             npm install async
 *
 *  -------------------------------------------
 *  --  USAGE  --------------------------------
 *  -------------------------------------------
 *    var Events = require('events');
 *    var EventHook = require('./eventhook.js');
 *    var eventhook = new Events.EventEmitter();
 *
 *    eventhook.on('modifiable', function(modifyMe, callback){
 *      var error = false;
 *      modifyMe++;
 *      callback(error, modifyMe);
 *    });
 *    eventhook.on('modifiable', function(modifyMe, callback){
 *      var error = false;
 *      modifyMe++;
 *      callback(error, modifyMe);
 *    });
 *
 *    var toBeModified = 1;
 *    eventhook.hook('modifiable', toBeModified, function(error, result){
 *      if (error) throw new Error('Issue with EventHook listener: ' + result);
 *      // The value should end up as 3
 *      console.log(result);
 *      // Add more listeners for more
 *      // IE: 3 listeners should result
 *      // in the value being 4
 *    });
 *
 *
 *  -------------------------------------------
 *  --  ROADMAP  ------------------------------
 *  -------------------------------------------
 *    o Accept more than one argument
 *      (You can use an object for now)
 *
 */

'use strict'; /*jshint node: true*/

var EventEmitter = require('events').EventEmitter;
var async = require('async');

// Setup the hook prototype for EventEmitters
EventEmitter.prototype.hook = function(type, arg, callback) {
  // Check for missing requirements in development environment only
  if (process.env.NODE_ENV !== 'production') {
    if (!type) throw new Error('Type is required');
    if (!arg) throw new Error('Arg is required');
    if (!callback) throw new Error('Callback is required');
  }
  
  // No event listeners? Return the original argument
  if (!this._events) return callback(false, arg);
  var listeners = this._events[type];
  if (!listeners) return callback(false, arg);
    
  // This is because the first function in an
  // async.waterfall needs to pass the argument
  var passArg = function(cb) {
    cb(false, arg);
  };
  
  // Add passArg to the listeners array
  if (Array.isArray(listeners)) {
    listeners.unshift(passArg);
  } else {
    // async.waterfall requires an array
    // Listeners is not an array when
    // there is only 1 listener
    listeners = [passArg, listeners];
  }
  
  // Pass existing listeners to async.waterfall
  // Allows each listener to modify the argument
  async.waterfall(listeners, function(err, result){
    // Return the final result to the original callback
    return callback(err, result);
  });
};