js 获取指定天数前日期

getBeforeDate(n) {//传入天数
 var n = n;
 var d = new Date();
 var year = d.getFullYear();
 var mon = d.getMonth() + 1;
 var day = d.getDate();
   if (Number(mon) > 1) {
     mon = mon - 1;
   } else {
     year = year - 1;
     mon = 12;
   }
   d.setDate(d.getDate() - n);
   year = d.getFullYear();
   mon = d.getMonth() + 1;
   day = d.getDate();
   var s = year + "-" + (mon < 10 ? ('0' + mon) : mon) +"-"+(day < 10 ? ('0' + day):day);
   return s;
 }