日期获取自定义周开始日的周数

如果我有自定义周开始日而不是星期一，应该如何更改 Date 类的 getWeekNumber() 原型。

当前查找 ISO 周数的代码：

Date.prototype.getWeekNumber = function() {
    // Create a copy of this date object
    var target = new Date(this.valueOf());

    // ISO week date weeks start on monday
    // so correct the day number
    var dayNr = (this.getDay() + 6) % 7;

    // ISO 8601 states that week 1 is the week
    // with the first thursday of that year.
    // Set the target date to the thursday in the target week
    target.setDate(target.getDate() - dayNr + 3);

    // Store the millisecond value of the target date
    var firstThursday = target.valueOf();

    // Set the target to the first thursday of the year
    // First set the target to january first
    target.setMonth(0, 1);
    // Not a thursday? Correct the date to the next thursday
    if (target.getDay() !== 4) {
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
    }

    // The weeknumber is the number of weeks between the
    // first thursday of the year and the thursday in the target week
    return 1 + Math.ceil((firstThursday - target) / 604800000); // 604800000 = 7 * 24 * 3600 * 1000
};

使用现有函数并添加可选参数weekstart指定一周从哪一天开始：0 是星期日，1 是星期一，依此类推。默认值为星期一。

Date.prototype.getWeekNumber = function(weekstart) {
    var target = new Date(this.valueOf());

    // Set default for weekstart and clamp to useful range        
    if (weekstart === undefined) weekstart = 1;
    weekstart %= 7;

    // Replaced offset of (6) with (7 - weekstart)
    var dayNr = (this.getDay() + 7 - weekstart) % 7;
    target.setDate(target.getDate() - dayNr + 3);

    var firstThursday = target.valueOf();

    target.setMonth(0, 1);
    if (target.getDay() !== 4) {
        target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
    }

    return 1 + Math.ceil((firstThursday - target) / 604800000);
};