Make any function singleton

singletonify.js

function singletonify(_Class) {
  if (typeof singletonify.prototype._singletonifyInstances === 'undefined') {
    singletonify.prototype._singletonifyInstances = [];
  }
  var _instances = singletonify.prototype._singletonifyInstances;
  var _index = _instances.push(void(0)) - 1;
  function genSingleton() {
    if (typeof _instances[_index] !== 'undefined') {
      return _instances[_index];
    } else {
       _Class.apply(_instances[_index] = Object.create(_Class.prototype), arguments);
      return _instances[_index];
    }
  }
  genSingleton.prototype = _Class.prototype;
  return genSingleton;
}



// test

function Foo(a, b) { this.a = a; this.b = b; }
var Bar = singletonify(Foo);

var f1 = new Bar('a', 'b');
var f2 = new Bar('c', 'd');
console.log(f1, f2);
console.assert(f1 instanceof Bar, 'instanceof Bar');
console.assert(f1 instanceof Foo, 'instanceof Foo');


function Foo0(a, b) { this.a = a; this.b = b; }
Foo0 = singletonify(Foo0);

function Foo1(a, b) { this.a = a; this.b = b; }
Foo1 = singletonify(Foo1);

function Foo2(a, b) { this.a = a; this.b = b; }
Foo2 = singletonify(Foo2);

function Foo3(a, b) { this.a = a; this.b = b; }
Foo3 = singletonify(Foo3);

console.assert(new Foo0('a', 'b') === new Foo0('c', 'd'), '00');
console.assert(new Foo0('e', 'f') === new Foo0('g', 'h'), '01');
console.assert(    Foo0('i', 'j') ===     Foo0('k', 'l'), '02');
console.assert(new Foo1('a', 'b') ===     Foo1('c', 'd'), '1');
console.assert(    Foo2('a', 'b') === new Foo2('c', 'd'), '2');
console.assert(    Foo3('a', 'b') ===     Foo3('c', 'd'), '3');