1. // 指定日期的前n天的日期 年-月-日
    2. function transDatePreN(date = new Date(), n = 0) {
    3.   var beforeDay = new Date(date);
    4.   beforeDay.setDate(beforeDay.getDate() - n);
    5.   var supNum = function(n) {
    6.     return n < 10 ? '0' + n : n;
    7.   }
    8.   return `${
    9.     beforeDay.getFullYear()
    10.   }-${
    11.     supNum(beforeDay.getMonth()+1)
    12.   }-${
    13.     supNum(beforeDay.getDate())
    14.   }`;
    15. }