gistfile1.js

multiplexCallback = (function () {
  var callbacks = {};
  
  return function(callbackable, callback) {
    var callbackList = callbacks[callbackable] || [];
    callbackList.push(callback);
    
    if (!callbacks[callbackable]) {
      callbacks[callbackable] = callbackList;
      callbackable(function () {
        for (var i = 0, l = callbackList.length; i < l; i++) {
          callbackList[i]();
        }
        // delete processed callbacks
        delete callbacks[callbackable];
      });
    }
  };
})();


// same thing but different approach
// attaching callbacks directly to collbackable
function multiplexCallback (callbackable, callback) {
  if (!callbackable.list) {
    callbackable.list = [];
    callbackable.list.push(callback);
    callbackable(function () {
      for (var i = 0, l = callbackable.list.length; i < l; i++) {
        callbackable.list[i]();
      }
      // delete processed callbacks
      delete callbackable.list;
    });
  } else {
    callbackable.list.push(callback);
  }
}