JavaScript 获取指定日期（今天、昨天、前后N天）的方法

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
 <head>
 <title>www.jb51.net 日期计算</title>
 <meta name="Generator" content="EditPlus">
 <meta name="Author" content="">
 <meta name="Keywords" content="">
 <meta name="Description" content="">
 </head>
 <body>
 <select id="num" name="num" style="width:150px" onchange="ff()">
        <option value="-1">昨天</option>
        <option value="0">今天</option>
        <option value="1">明天</option>
 </select><br>
 <textarea id="ar" rows="3" cols="20">
 haha
</textarea>
<br/>
<input type="text" value="" name="bday" id="bday"/>天之前
<input type="button" onclick="bday()" value="计算"><br/>
<input type="text" value="" name="aday" id="aday"/>天之后
<input type="button" onclick="aday()" value="计算"><br/>
 </body>
 <script>
 ff();
function ff(){
 var obj=document.getElementById("num");
 var v1=obj.options[obj.selectedIndex].text;
 var v2=obj.options[obj.selectedIndex].value;
 var tvv=giveDateStr(getTargetday(v2));
 document.getElementById('ar').innerText=tvv;
}
function isInteger(obj) {
 var r = /^\+?[1-9][0-9]*$/;　　//正整数 
 return r.test(obj); 
}
function bday(){
 var bef=document.getElementById("bday").value;
 if(isInteger(bef)){
   var bvv=giveDateStr(getTargetday(-bef));
   document.getElementById('ar').innerText=bvv;
 }else{
   alert("请输入正整数！");
   document.getElementById("bday").value='';
   document.getElementById("bday").focus();
 }
 
}
function aday(){
 var atf=document.getElementById("aday").value;
 if(isInteger(atf)){
   var avv=giveDateStr(getTargetday(atf));
   document.getElementById('ar').innerText=avv;
 }else{
   alert("请输入正整数！");
   document.getElementById("aday").value='';
   document.getElementById("aday").focus();
 }
}
function getTargetday(num){
  var number=null;
  if(num==null||num==0){
   number=0;
  }else{
    number=num;
  }
  var  today=new  Date();
  var  targetday_milliseconds=today.getTime()+1000*60*60*24*number;
  var  targetday=new  Date();
  targetday.setTime(targetday_milliseconds); //注意，这行是关键代码
  return targetday;
}
function giveDateStr(dd){
  var riqiyue=dd.getMonth()+1;
  var riqihao=dd.getDate();
  var riqixingqi=null;
  if(dd.getDay()=='0'){
   xingqi="星期日";
  }else if(dd.getDay()=='1'){
   xingqi='星期一';
  }else if(dd.getDay()=='2'){
   xingqi='星期二';
  }else if(dd.getDay()=='3'){
   xingqi='星期三';
  }else if(dd.getDay()=='4'){
   xingqi='星期四';
  }else if(dd.getDay()=='5'){
   xingqi='星期五';
  }else if(dd.getDay()=='6'){
   xingqi='星期六';
  }
  var riqi=riqiyue+"月"+riqihao+"日"+"("+xingqi+")";
  return riqi;
}
</script>
</html>

  function getDay(day){
    var today = new Date()
    // 获取时间戳(毫秒级)
    /*
      day为1，则是，明天的时间戳
      day为-1，则是，昨天的时间戳
      day为-2，则是，前天的时间戳
    */
    var targetday_milliseconds = today.getTime() + 1000 * 60 * 60 * 24 * day
    // Date.setTime(时间戳)：设置当前日期的时间
    today.setTime(targetday_milliseconds)
    console.log('today=', today) // today= Sun Mar 05 2023 16:14:56 GMT+0800 (中国标准时间)
    var tYear = today.getFullYear() // 年
    var tMonth = today.getMonth() // 月
    var tDate = today.getDate() // 日
    tMonth = this.doHandleMonth(tMonth + 1)
    tDate = this.doHandleMonth(tDate)
    console.log('返回年月日=', tYear + '-' + tMonth + '-' + tDate)
    return tYear + '-' + tMonth + '-' + tDate
  }
  function doHandleMonth(month) {
    var m = month
    if (month.toString().length == 1) {
      m = '0' + month
    }
    return m
  }

/* 
  * @params date 日期
  * @params type 日期 prev/current/next 昨天/今天/明天
  * @params fmt 日期拼接符
*/
function getDays(date, type, fmt) {
    let currentDate = new Date(date)
    let y = currentDate.getFullYear()
    let m = currentDate.getMonth() + 1
    let d = currentDate.getDate()
    function dateFormat(date, fmt) {
        let y = new Date(date).getFullYear()
        let m = new Date(date).getMonth() + 1
        let d = new Date(date).getDate()
        return `${y}${fmt}${m}${fmt}${d}`
    }
    switch (type) {
        case "prev":
            if (d - 1 < 1) {
                if (m - 1 < 1) {
                    y = y - 1
                    m = 12
                } else {
                    m = m - 1
                }
                d = new Date(y, m, 0).getDate()
            } else {
                d = d - 1
            }
            break
        case "current":
            break
        case "next":
            if (d + 1 > new Date(y, m, 0).getDate()) {
                if (m + 1 > 12) {
                    y = y + 1
                    m = 1
                    d = 1
                } else {
                    m = m + 1
                    d = 1
                }
            } else {
                d = d + 1
            }
            break;
    default:
      break;
    }
    return dateFormat(new Date(`${y}-${m}-${d}`), fmt)
}
console.log(getDays(new Date('2021-1-1'), "prev", "-"));