Make an object eventable.

makeeventable.js

function makeEventable(obj) {

  var getEventMap = function () {
      this.__eventMap = this.__eventMap || {};
      return this.__eventMap;
    },

    validateEventExistence = function (name) {
      var eventMap = getEventMap.call(this);
      if (typeof eventMap[name] === 'undefined') {
        throw new Error('The event "' + name + '" is not defined.');
      }
    },

    shallowcopy = function (dest, source) {
      if ( ! source) { return dest; }

      for (var i in source) {
        if (source.hasOwnProperty(i)) {
          dest[i] = source[i];
        }
      }

      return dest;
    },

    register = function (name, handler, opts) {
      validateEventExistence.call(this, name);

      if (typeof handler !== 'function') {
        throw new Error('The handler of "' + name + '" should be a function.');
      }

      var eventMap = getEventMap.call(this);

      var info = shallowcopy({
        func: handler
      }, opts);

      eventMap[name].push(info);
    };

  obj.defineEvent = function (name) {
    var eventMap = getEventMap.call(this);
    eventMap[name] = [];
    return this;
  };

  obj.on = function (name, handler, opts) {
    register.call(this, name, handler);
    return this;
  };

  obj.once = function (name, handler) {
    register.call(this, name, handler, {
      once: true
    });
    return this;
  };

  obj.trigger = function (name/*, args ... */) {
    validateEventExistence.call(this, name);

    var eventMap = getEventMap.call(this);

    var handlerInfos = eventMap[name].slice(), // use copied
      args = Array.prototype.slice.call(arguments, 1),
      i = 0,
      len = handlerInfos.length,
      info;

    for (; i < len; i++) {
      info = handlerInfos[i];
      info.func.apply(this, args);

      if (info.once === true) {
        // splice from original array
        eventMap[name].splice(i, 1);
      }
    }

    return this;
  };

}