js获取前几天日期

<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>