js，昨天的日期

var mydate = new Date(); ???????var yester = mydate-24*60*60*1000; ???????var yesterday = new Date(); ???????yesterday.setTime(yester); ??????????????????var str = "" + yesterday.getFullYear() + "-"; ??????????????????????if (yesterday.getMonth()>=9) { ??????????????????????????str+=(yesterday.getMonth()+1) + "-"; ??????????????????????} else{
 ??????????????????????????str +="0"+(yesterday.getMonth()+1)+"-"; ??????????????????????} ?????????????????????if ((yesterday.getDate())<10) { ?????????????????????????str+="0"+(yesterday.getDate()); ?????????????????????} else{ ?????????????????????????str += (yesterday.getDate()); ?????????????????????} ??????????????????????$("#mydata").val(str);