js 获取某年的某天是第几周

/**
2 * 判断年份是否为润年
3 *
4 * @param {Number} year
5  */
6 function isLeapYear(year) {
7     return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
8 }
9 /**
10 * 获取某一年份的某一月份的天数
11 *
12 * @param {Number} year
13 * @param {Number} month
14  */
15 function getMonthDays(year, month) {
16     return [31, null, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month] || (isLeapYear(year) ? 29 : 28);
17 }26 /**
27 * 获取某年的某天是第几周
28 * @param {Number} y
29 * @param {Number} m
30 * @param {Number} d
31 * @returns {Number}
32  */
33 function getWeekNumber(y, m, d) {
34     var now = new Date(y, m - 1, d),
35         year = now.getFullYear(),
36         month = now.getMonth(),
37         days = now.getDate();
38     //那一天是那一年中的第多少天
39     for (var i = 0; i < month; i++) {
40         days += getMonthDays(year, i);
41     }
42
43     //那一年第一天是星期几
44     var yearFirstDay = new Date(year, 0, 1).getDay() || 7;
45
46     var week = null;
47     if (yearFirstDay == 1) {
48         week = Math.ceil(days / yearFirstDay);
49     } else {
50         days -= (7 - yearFirstDay + 1);
51         week = Math.ceil(days / 7) + 1;
52     }
53
54     return week;
55 }