Well this is one of those posts that start out being as a reminder for myself, since I had to do a script that I don't want to waste my time having to write again later on. Basically I ran into a customer who wanted week numbers added to a calendar. This seemed basic enough, but apparently getting a weeknumber is not a native thing in javascript.
First I picked up a script to add a getWeek method to the Date prototype, and I was a happy man. Sadly not customer not quite as much. It turns out that we do not follow the ISO 8601 standard for defining weeknumbers here in Denmark. According to my projectmanager the rule of thumb is that the first thursday always falls in week 1. This made me add a little extention so I now have a getIsoWeek and a getWeek method on my Date objects and this is how it was done.
/**
* Get the ISO 8601 week date week number
*/
Date.prototype.getIsoWeek = 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;
// Set the target to the thursday of this week so the
// target date is in the right year
target.setDate(target.getDate() - dayNr + 3);
// ISO 8601 states that week 1 is the week
// with january 4th in it
var jan4 = new Date(target.getFullYear(), 0, 4);
// Number of days between target date and january 4th
var dayDiff = (target - jan4) / 86400000;
// Calculate week number: Week 1 (january 4th) plus the
// number of weeks between target date and january 4th
var weekNr = 1 + Math.ceil(dayDiff / 7);
return weekNr;
}
/**
* Get danish weeknumber.
* According to danish weeknumbers the first thursday always falls in week 1
*/
Date.prototype.getWeek = function() {
var weekNr = this.getIsoWeek();
var firstThursday = new Date(this.getFullYear(), 0, 1)
while (firstThursday.getDay() != 4) firstThursday.setDate(firstThursday.getDate() + 1);
if (firstThursday.getIsoWeek() == 2) weekNr--;
if (weekNr == 0) weekNr = 1;
return weekNr;
}
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