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>