event_handler.js

/**
 * EventHandler.js
 *
 * @description Extends object with .on(), .off() and .emit() methods
 * @author: francois@n-dream.com
 * @constructor
 * @param {Object} source - The object to extend with events
 */
var EventHandler = function() {

  var EventHandler = function(source) {
    this.source = source;
    this.events = {};
    this.bindHandler(source);
  };

  /**
   * Resets the entire EventHandler
   */
  EventHandler.prototype.reset = function() {
    this.events = {};
    this.source.on = null;
    this.source.off = null;
    this.source.emit = null;
  };

  /**
   * Extends the source object with event handler functions
   */
  EventHandler.prototype.bindHandler = function(source) {
    this.source.on = this.on.bind(this);
    this.source.off = this.off.bind(this);
    this.source.emit = this.emit.bind(this);
  };

  /**
   * @param {String} event_name - Name of the event to trigger
   * @param {Function} callback - The callback function to call when emits is called
   * @return {String} - An id. Store it to be able to call .off(id)
   */
  EventHandler.prototype.on = function(event_name, callback) {
    if (!event_name) throw "Missing param event_name.";
    if (typeof callback !== 'function') throw "Event callback is not a function";
    if(!this.events[event_name]) {
      this.events[event_name] = {};
    }
    var id = this.getRandomId();
    this.events[event_name][id] = callback;
    return id;
  };

  /**
   * Removes a previously bounded function from the events
   * @param {String} event_name - Name of the event to trigger
   */
  EventHandler.prototype.off = function(id) {
    if (!id) throw "Missing param id";
    var events = this.events;
    for (var evt_name in events) {
      for (var evt_id in events[evt_name]) {
        if (evt_id === id) {
          this.events[evt_name][evt_id] = null;
          delete this.events[evt_name][evt_id];
        }
      }
    }
  };

  /**
   * Dispatches the event by event name
   * @param {String} event_name - Name of the event to trigger
   * @param {Mixed} params - The params to pass to the callback function
   */
  EventHandler.prototype.emit = function(event_name, params) {
    if (!event_name) throw "Missing param event_name.";
    var evts = this.events[event_name];
    if (evts) {
      for (var id in evts) {
        var fn = evts[id];
        if (typeof fn === 'function') {
          fn.call(this.source, this.source, params);
        } else {
          console.warn("Failed to emit function:", event_name);
        }
      }
    }
  };

  /**
   * Returns random generated Id
   * @return {String}
   */
  EventHandler.prototype.getRandomId = function(chars) {
    chars = Math.max(2, Math.min(chars || 15, 36));
    return (Math.random() + 1).toString(36).substring(2, chars);
  };

  return EventHandler;
}();

example.js

// Some custom object:
var MyCustomObj = function() {
 this.event_handler = new EventHandler(this);
};

MyCustomObj.prototype = {

  // Emit event
  startParty: function() {
    this.emit("ON_PARTY", { "beer" : true });
  }
  
};

var my_obj = new MyCustomObj();

// Listen for the event
var event_id = my_obj.on("ON_PARTY", function(source_object, params) {
  if (params && params.beer) {
    console.log("Yay beer!");
  }
};
                         
// Trigger event
my_obj.startParty();

// Unsubscribe event
my_obj.off(event_id);