dog.js

/**
 * Private members in JavaScript
 *
 * @author Yuya Takeyama
 */
var Dog = (function () {
  var Dog,
    privateNameSpace = {};

  /**
   * Constructor
   * Create a new dog and name it.
   * Then private namespace is created also.
   *
   * @class A class represents Dogs.
   * @param {String} name The dog's name
   * @param {Number} age  The dog's age
   */
  Dog = function (name, age) {
    var d = new Date;

    /**
     * Identifier of the Dog object.
     * @type String
     */
    this.objectId = '' + d.getTime() + Math.random();

    // Create a private namespace
    privateNameSpace[this.objectId] = {};
    privateNameSpace[this.objectId].name = name;
    privateNameSpace[this.objectId].age  = age;
  };

  /**
   * Read-accessor of the dog's name.
   *
   * @return {String} The dog's name.
   */
  Dog.prototype.getName = function () {
    return privateNameSpace[this.objectId].name;
  };

  /**
   * Read-accessor of the dog's age.
   *
   * @return {Number} The dog's age.
   */
  Dog.prototype.getAge = function () {
    return privateNameSpace[this.objectId].age;
  };

  return Dog;
})();

sample.js

var pochi = new Dog('Pochi', 5);
var taro  = new Dog('Taro', 12);

window.alert(pochi.getName() + ' is ' + pochi.getAge() + ' years old.');
// => Pochi is 5 years old.

window.alert(taro.getName() + ' is ' + taro.getAge() + ' years old.');
// => Taro is 12 years old.

sample2.js

var pochi = new Dog('Pochi', 5);
var taro  = new Dog('Taro', 12);

window.alert(pochi.getName() + ' is ' + pochi.getAge() + ' years old.');
// => Pochi is 5 years old.

window.alert(taro.getName() + ' is ' + taro.getAge() + ' years old.');
// => Taro is 12 years old.

// ObjectId jacking.
pochi.objectId = taro.objectId;

window.alert(pochi.getName() + ' is ' + pochi.getAge() + ' years old.');
// => Taro is 12 years old.

window.alert(taro.getName() + ' is ' + taro.getAge() + ' years old.');
// => Taro is 12 years old.