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