js如何获取上个月第一天和最后一天

var nowdays = new Date();
            var year = nowdays.getFullYear();
            var month = nowdays.getMonth();
            if(month==0){
                month = 12;
                year = year-1;

            }
            if(month<10){
                month = '0'+month;
            }

            var myDate = new Date(year,month,0);

            var startDate = year+'-'+month+'-01 00:00:00'; //上个月第一天
            var endDate = year+'-'+month+'-'+myDate.getDate()+' 23:59:00';//上个月最后一天