.is() strawman

gistfile1.js

/*global IMVU:true*/
var IMVU = IMVU || {};
(function() {
    IMVU.extendError = function extendError(baseErrorType, errorName) {
        var errorClass = IMVU.createNamedFunction(errorName, function(message) {
            this.name = errorName;
            this.message = message;
            this.__isa = Object.create(baseErrorType.prototype.__isa || null);
            this.__isa[errorName] = true;

            var stack = (new Error(message)).stack;
            if (stack !== undefined) {
                this.stack = this.toString() + '\n' +
                    stack.replace(/^Error(:[^\n]*)?\n/, '');
            }
        });
        errorClass.prototype = Object.create(baseErrorType.prototype);
        errorClass.prototype.constructor = errorClass;
        errorClass.prototype.toString = function() {
            if (this.message === undefined) {
                return this.name;
            } else {
                return this.name + ': ' + this.message;
            }
        };
        errorClass.prototype.is = function (type) {
            return !!this.__isa[type];
        };

        return errorClass;
    };
})();