js计算当月有几周并显示对应的日期

需求说明：一个月按7天算，周日到周六，会存在有的月份是4周，有的月份是5周。本需求是跨月的情况下，算周三在哪个月份，就算入哪个月份。

////获取本月的总天数
function getMonthDayTotal(year,month){
    let nextMonth = month + 1;
    let d = new Date(year, nextMonth, 0);
    return d.getDate();
}

////获取本月最后一天是周几
function getMonthLastDayWeek(year,month) {
    let lastDay = getMonthDayTotal(year,month);
    let d = new Date(year, month, lastDay);
    return d.getDay();
}


$('#dateTimePicker').datetimepicker({
    timeZone:'Asia/Seoul',
    autoclose:true,
    format: 'yyyy-mm',
    startView: 'year',
    minView:'year',
}).on('changeMonth',function (ev) {

    let getUTCFullYear = ev.date.getUTCFullYear();
    let getUTCMonth = ev.date.getUTCMonth();          //获取月份，0 到 11 之间的整数
    let getUTCDate = ev.date.getUTCDate();              //1 到 31 之间的整数
    let getUTCDay = ev.date.getUTCDay();                //获取当天是周几,0-6

    let curMonthDatTotal = getMonthDayTotal(getUTCFullYear,getUTCMonth);    //本月总共有多少天
    let preMonthDayTotal = getMonthDayTotal(getUTCFullYear,getUTCMonth-1);  //上个月总共有多少天，也就是上个月最后一天的日期

    ////判断本月1号是星期几，从而获取本月第一周的开始日期
    let firstWeekStartDate = '';             //本月第一周的开始日期
    let firstWeekEndDate = '';               //本月第一周的结束日期
    let monthDay = curMonthDatTotal;         //计算本月周数的有效天数
    if (getUTCDay == 0){//如果是星期日
        firstWeekStartDate = getUTCDate;
        firstWeekEndDate = 7;
    }else if(getUTCDay>3){//如果大于星期三即是（四、五、六）,归上个月
        firstWeekStartDate = getUTCDate+7-getUTCDay;
        monthDay = curMonthDatTotal-firstWeekStartDate+1;
        firstWeekEndDate = firstWeekStartDate+6;
    }else{//小于等于星期三即是（一、二、三）,取上个月
        firstWeekStartDate = preMonthDayTotal-getUTCDay+1;
        firstWeekEndDate = 7-getUTCDay;
    }

    let weekTotal = parseInt(monthDay/7);
    ////判断最后一天是星期几，从而获取本月的总周数
    let lastDayWeek = getMonthLastDayWeek(getUTCFullYear,getUTCMonth);
    if (lastDayWeek >= 3 && lastDayWeek < 6){weekTotal += 1;}

    let weekOption = '';
    for (let i=1;i<=weekTotal;i++){
        let j;
        let between;
        switch (i) {
            case 1:
                  j="一";
                  between = firstWeekStartDate + '~' + firstWeekEndDate;
                  break;
            case 2:
                j="二";
                between = (firstWeekEndDate+1) + '~' + (firstWeekEndDate+7);
                firstWeekEndDate +=7;
                break;
            case 3:
                j="三";
                between = (firstWeekEndDate+1) + '~' + (firstWeekEndDate+7);
                firstWeekEndDate +=7;
                break;
            case 4:
                j="四";
                if (getUTCMonth == 1){//2月份特殊处理
                    between = (firstWeekEndDate+1) + '~' + (7-lastDayWeek-1);
                }else{
                    between = (firstWeekEndDate+1) + '~' + (firstWeekEndDate+7);
                    firstWeekEndDate +=7;
                }
                break;
            case 5:
                j="五";
                between = (firstWeekEndDate+1) + '~' + (7-lastDayWeek-1);
                break;

        }
        weekOption += '第'+j+'周（'+between+'）';
    }

    $('select[name="zhoushu"]').html(weekOption);

});

效果如下：