implementation.js

YUI().use("singleton-class", function (Y) {
  var instance1 = new Y.SingletonClass(),
      instance2 = new Y.SingletonClass();
  
  Y.log(instance1 === instance2); // true!

  window.instance1 = instance1;

});
YUI().use("singleton-class", function (Y) {
  var instance3 = new Y.SingletonClass();
  
  Y.log(window.instance1 === instance3); // true!
});

singleton-class.js

YUI.add("singleton-class", function (Y) {
  
  var GlobalEnv, SingletonClass, instanceMethods, classMethods;
  
  // create your own namespace in the YUI global environment
  GlobalEnv = YUI.namespace("Env.SingletonClassNamespace");
  
  // define the singleton class
  SingletonClass = function (confg) {
    // if an instance doesn't exist yet
    // do the normal class initialization and set the global
    // instance to this instance
    if (Y.Lang.isUndefined(GlobalEnv.instance)) {
      SingletonClass.superclass.constructor.apply(this, arguments);
      GlobalEnv.instance = this;
    }
    // instead of returning this, return the global instance
    return GlobalEnv.instance;
  };

  // expose your singleton class to the world
  Y.SingletonClass = SingletonClass;
  
  // if an instance of the class already exists, you don't
  // even need to do anything else. This doesn't
  // work if your application relies on class methods, so
  // only uncomment this if adding instance and class methods
  // to your class is expensive (it's probably not).
  //
  // if (! Y.Lang.isUndefined(GlobalEnv.instance)) {
  //   return;
  // }
  
  // add your instance methods with at least these two defined
  instanceMethods = {
    initializer: function (config) {},
    destructor: function () {}
  };
  
  // add your class methods with the NAME and ATTRS properties
  classMethods = {
    NAME: "singleton-class",
    ATTRS: {
      foo: {
        value: "bar"
      }
    }
  };
  
  // add instance methods, class methods and attrs by extending Y.Base
  Y.extend(SingletonClass, Y.Base, instanceMethods, classMethods);
  
}, "3.3.0", { requires: ["base-base"] });