Addy Osmani - Essential JavaScript Design Patterns - Singleton issue

SingletonTester-issue.js

//This "SingletonTester" is not a valid Singleton because:
var InnerConstructor = SingletonTester.getInstance({
  pointX:  5
}).constructor;

console.log(new InnerConstructor() !== new InnerConstructor()); // true

SingletonTester.js

// From http://addyosmani.com/resources/essentialjsdesignpatterns/book/#singletonpatternjavascript

var SingletonTester = (function () {
 
  // options: an object containing configuration options for the singleton
  // e.g var options = { name: "test", pointX: 5};
  function Singleton( options )  {
    // set options to the options supplied
    // or an empty object if none are provided
    options = options || {};
    // set some properties for our singleton
    this.name = "SingletonTester";
    this.pointX = options.pointX || 6;
    this.pointY = options.pointY || 10;
  }
 
  // our instance holder
  var instance;
 
  // an emulation of static variables and methods
  var _static  = {
 
    name:  "SingletonTester",
 
    // Method for getting an instance. It returns
    // a singleton instance of a singleton object
    getInstance:  function( options ) {
      if( instance  ===  undefined )  {
        instance = new Singleton( options );
      }
 
      return  instance;
 
    }
  };
 
  return  _static;
})();
 
var singletonTest  =  SingletonTester.getInstance({
  pointX:  5
});
 
// Log the output of pointX just to verify it is correct
// Outputs: 5
console.log( singletonTest.pointX );