javascript 延伸date formatstring 寫法

 Date.prototype.addDays = function (days) {
            this.setDate(this.getDate() + parseInt(days));
            return this;
        };
        Date.prototype.yyyymmdd = function() {
            var yyyy = this.getFullYear().toString();
            var mm = (this.getMonth()+1).toString(); // getMonth() is zero-based
            var dd  = this.getDate().toString();
            return yyyy + (mm[1]?mm:"0"+mm[0]) + (dd[1]?dd:"0"+dd[0]); // padding
        };