Javascript String format(fmt)

String.prototype.format = function (fmt) {
    if (fmt==null) fmt = "yyyy-MM-dd";
    var myDate = new Date(this);
    var o = {/*from   ww  w  .ja v a2  s  . c om*/
        "M+": myDate.getMonth() + 1,
        "d+": myDate.getDate(),
        "h+": myDate.getHours(),
        "m+": myDate.getMinutes(),
        "s+": myDate.getSeconds(),
        "q+": Math.floor((myDate.getMonth() + 3) / 3),
        "S": myDate.getMilliseconds()
    };
    if (/(y+)/.test(fmt)) fmt = fmt.replace(RegExp.$1, (myDate.getFullYear() + "").substr(4 - RegExp.$1.length));
    for (var k in o)
        if (new RegExp("(" + k + ")").test(fmt)) fmt = fmt.replace(RegExp.$1, (RegExp.$1.length == 1) ? (o[k]) : (("00" + o[k]).substr(("" + o[k]).length)));
    return fmt;
};