[JavaScript] Imitate String.Format() in c#

if (!String.prototype.format) {
    String.prototype.format = function () {
        var args = arguments;
        return this.replace(/{(\d+)}/g, function (match, number) {
            return typeof args[number] != ‘undefined‘
              ? args[number]
              : match;
        });
    };
}

"{0} {1} ".format("a","b");