If I have a custom week start day other than Monday, how should the getWeekNumber() prototype for the Date class be modified?
Here is the existing code for calculating the ISO Week Number:
Date.prototype.getWeekNumber = function() {
// Create a duplicate of this date object
var target = new Date(this.valueOf());
// Adjust the day number to suit custom week start day
var dayNr = (this.getDay() + 6) % 7;
// According to ISO standards, week 1 begins with the first Thursday of the year.
// Set the target date to the Thursday within the designated week
target.setDate(target.getDate() - dayNr + 3);
// Save millisecond value of the target date
var firstThursday = target.valueOf();
// Move target to the first Thursday of the year
target.setMonth(0, 1);
// If not already a Thursday, adjust to the next Thursday
if (target.getDay() !== 4) {
target.setMonth(0, 1 + ((4 - target.getDay()) + 7) % 7);
}
// The week number represents the weeks between the first Thursday of the year and the one in the target week
return 1 + Math.ceil((firstThursday - target) / 604800000); // 604800000 = 7 * 24 * 3600 * 1000
};