js 获取某年的某天是第几周

 /**
  * 判断年份是否为润年
  *
  * @param {Number} year
  */
 function isLeapYear(year) {
     return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
 }
 /**
  * 获取某一年份的某一月份的天数
  *
  * @param {Number} year
  * @param {Number} month
  */
 function getMonthDays(year, month) {
     return [31, null, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month] || (isLeapYear(year) ? 29 : 28);
 } /**
  * 获取某年的某天是第几周
  * @param {Number} y
  * @param {Number} m
  * @param {Number} d
  * @returns {Number}
  */
 function getWeekNumber(y, m, d) {
     var now = new Date(y, m - 1, d),
         year = now.getFullYear(),
         month = now.getMonth(),
         days = now.getDate();
     //那一天是那一年中的第多少天
     for (var i = 0; i < month; i++) {
         days += getMonthDays(year, i);
     }

     //那一年第一天是星期几
     var yearFirstDay = new Date(year, 0, 1).getDay() || 7;

     var week = null;
     if (yearFirstDay == 1) {
         week = Math.ceil(days / yearFirstDay);
     } else {
         days -= (7 - yearFirstDay + 1);
         week = Math.ceil(days / 7) + 1;
     }

     return week;
 }