JS--計算某一年某一月某一日所處的週數

<h2>計算週數</h2>
<input id="date" type="text" value="2019-03-19" />
<button onclick="Count($('#date').val())">計算</button>
<script src="~/Scripts/jquery-1.10.2.min.js"></script>
<script>

    var firstStartWeek = 0; //得到今年1月1為週幾，星期日為起點  星期天=0

    var year = "";          //年

    var month = "";         //月

    var day = "";           //日

    var week = 0;           //週數

    var allDay = 0;         //總天數

    var monthDay = new Array();

    function Count(val) {

        year = val.substring(0, 4);

        month = val.substring(5, 7);

        day = val.substring(8, 10);

        week = 0;

        allDay = 0;
        
        //減掉第一週剩餘的天數
        firstStartWeek = 7 - new Date(year + "/01/01").getDay();

        //計算潤平年

        if ((parseInt(year) % 4 == 0 || parseInt(year) % 400 == 0) && (parseInt(year) % 100 != 0)) {

            //閏年

            monthDay = new Array(31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

        } else {

            //平年

            monthDay = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

        }

        if (firstStartWeek == 0) {
            for (var i = 0; i < parseInt(month) - 1 ; i++) {

                allDay += monthDay[i];

            }

            allDay += parseInt(day);//加上當前月的天數

            week = Math.ceil(allDay / 7);

        } else {
            week++;//補回一週

            for (var i = 0; i < parseInt(month) - 1 ; i++) {
                allDay += monthDay[i];
            }

            allDay += parseInt(day);//加上當前月的天數
            allDay -= firstStartWeek;//扣除第一週多餘的天數

            week += Math.ceil(allDay / 7);

        }

        alert(week);

    }

</script>