Simple JS Multiton/Factory

gistfile1.js

var Multiton = (function() {
    var _instances = {},
        _products, _default;

    function create_instance(name) {
        var prefix = 'Product_',
            product_name;

        product_name = prefix + name.slice(0, 1).toUpperCase() + name.substring(1);
        if (typeof _products[product_name] === 'function') {
            return new _products[product_name]();
        }

        // warn user of invalid instantiation attempt
        return null;
    }

    _products = {
        Product_Concrete: function Product_Concrete() {
            // product code
        }
    };
    _default = 'concrete';

    return {
        instance: function(name) {
            name = typeof name === 'string' || _default;

            if (typeof _instances[name] === 'undefined') {
                _instances[name] = create_instance(name);
            }

            return _instances[name];
        }
    };
})();​