This is in-regard to a comment someone made about having a triple nested return for "getInstance".

MyClass.js

define([ 'utils/Singleton' ],
function(Singleton) {

  'use strict';
  
  
  var MyClass;
  MyClass = (function() {
    
    function MyClass() {}
    
    MyClass.prototype.methodA = function() {};
    MyClass.prototype.methodB = function() {};
    MyClass.prototype.methodC = function() {};
    
    return MyClass;
  })();


  return Singleton(MyClass);
});

Singleton.js

function Singleton(classInstance) {
  var __instance = null;

  return {
    getInstance: function() {
      if (__instance === null) {
        __instance = new classInstance();
      }
      return __instance;
    }
  };
}

Additionally, if you're not using this within a module, the return for the self-invoking anonymous function would then be return Singleton(MyClass); instead of simply return MyClass;

So, it would look something like:

var MyClass;
MyClass = (function() {
...
return Singleton(MyClass);
})();