Encapsulation: Private Attribute

In OPP there are attributes called private attributes. Knowing an attribute is private or public, an API user can keep his focus to the big picture of the API. From the article function object context, random string generator, and class inheritance, we have all the information to create Javascript private attributes.
(function() {
  var private_id = guid();
  
  var init = function() {
    //private attribute holder
    this[private_id] = {};
    
    //create_time is a private attribute
    this[private_id].creation_time = (new Date()).getTime();
  };

  //this is the public method accessing the private attribute
  var get_creation_time = function() {
    return this[private_id].creation_time;
  };

  // Anything listed below are public methods
  My_Class = Class.extend({
    init: init,
    get_creation_time: get_creation_time
  });
})();