js获取当前指定的前几天的日期（如当前时间的前七天的日期）

<script>
  function getBeforeDate(n) {
    var n = n;
    var d = new Date();
    var year = d.getFullYear();
    var mon = d.getMonth() + 1;
    var day = d.getDate();
    if(day <= n) {
      if(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();
    s = year+"-"+(mon<10?('0'+mon):mon)+"-"+(day<10?('0'+day):day);
    return s;
  }
  console.log(getBeforeDate(1));//昨天的日期
  console.log(getBeforeDate(7));//前七天的日期
</script>