Javascript: Calculate Week no

http://javascript.about.com/library/blweekyear.htm


function:
Date.prototype.getWeek = function()
{
var onejan = new Date(this.getFullYear(),0,1);
return Math.ceil((((this - onejan) / 86400000) + onejan.getDay()+1)/7);
}


call:
var today = new Date();
var weekno = today.getWeek();