javascript, String, formats

String.formats = function (str) {
    if (arguments.length == 0) return null;
    var args = Array.prototype.slice.call(arguments, 1);
    return str.replace(/\{(\d+)\}/g, function (m, i) {
        return args[i];
    });
};
alert(String.formats("I am {0} Hi {1}, {0}", "wangxl", "looloo"));