index.js

module.exports = function eventify(subject) {
  validateSubject(subject);

  var eventsStorage = createEventsStorage(subject);
  subject.on = eventsStorage.on;
  subject.off = eventsStorage.off;
  subject.fire = eventsStorage.fire;
  return subject;
};

function createEventsStorage(subject) {
  // Store all event listeners to this hash. Key is event name, value is array
  // of callback records.
  //
  // A callback record consists of callback function and its optional context:
  // { 'eventName' => [{callback: function, ctx: object}] }
  var registeredEvents = Object.create(null);

  return {
    on: function (eventName, callback, ctx) {
      if (typeof callback !== 'function') {
        throw new Error('callback is expected to be a function');
      }
      var handlers = registeredEvents[eventName];
      if (!handlers) {
        handlers = registeredEvents[eventName] = [];
      }
      handlers.push({callback: callback, ctx: ctx});

      return subject;
    },

    off: function (eventName, callback) {
      var wantToRemoveAll = (typeof eventName === 'undefined');
      if (wantToRemoveAll) {
        // Killing old events storage should be enough in this case:
        registeredEvents = Object.create(null);
        return subject;
      }

      if (registeredEvents[eventName]) {
        var deleteAllCallbacksForEvent = (typeof callback !== 'function');
        if (deleteAllCallbacksForEvent) {
          delete registeredEvents[eventName];
        } else {
          var callbacks = registeredEvents[eventName];
          for (var i = 0; i < callbacks.length; ++i) {
            if (callbacks[i].callback === callback) {
              callbacks.splice(i, 1);
            }
          }
        }
      }

      return subject;
    },

    fire: function (eventName) {
      var callbacks = registeredEvents[eventName];
      if (!callbacks) {
        return subject;
      }

      var fireArguments;
      if (arguments.length > 1) {
        fireArguments = Array.prototype.splice.call(arguments, 1);
      }
      for(var i = 0; i < callbacks.length; ++i) {
        var callbackInfo = callbacks[i];
        callbackInfo.callback.apply(callbackInfo.ctx, fireArguments);
      }

      return subject;
    }
  };
}

function validateSubject(subject) {
  if (!subject) {
    throw new Error('Eventify cannot use falsy object as events subject');
  }
  var reservedWords = ['on', 'fire', 'off'];
  for (var i = 0; i < reservedWords.length; ++i) {
    if (subject.hasOwnProperty(reservedWords[i])) {
      throw new Error("Subject cannot be eventified, since it already has property '" + reservedWords[i] + "'");
    }
  }
}