Singleton Object in JavaScript

Singleton.js

var Singleton = (function () {
  var
    obj = {},
    id,
    init,
    get_instance,
    get_id;

  init = function () {
    id = Math.random();
  };

  obj.get_instance = function () {
    if (typeof id === 'undefined') {
      init();
    }
    return obj;
  };

  obj.get_id = function () {
    return id;
  };

  return obj;
})();