JS :: Get Week Number (ISO 8601)

get_week_number.js

//Returns ISO 8601 week number and year
Date.prototype.getFullWeek = function(){
    var jan1, w, d = new Date(this);
    d.setDate(d.getDate()+4-(d.getDay()||7));		// Set to nearest Thursday: current date + 4 - current day number, make Sunday's day number 7
    jan1 = new Date(d.getFullYear(),0,1);		// Get first day of year
    w = Math.ceil((((d-jan1)/86400000)+1)/7);		// Calculate full weeks to nearest Thursday
    return {y: d.getFullYear(), w: w };
};
//Returns ISO 8601 week number
Date.prototype.getWeek = function(){ 
	return this.getFullWeek().w; 
};
var getWeeksInYear = function(y){ 
	return new Date(y,11,28).getFullWeek().w; 
};

I think you might want to add a line to getFullWeek just before the return to correct ISO dates currently coming out as W0 rather than the last week of the previous year:

    var y = d.getFullYear();
    if (w) return { y: y, w: w };
    return getWeeksInYear(y - 1); // 2000-01-01 = 1999W52, for instance

If I run this code today (4th January 2016), it returns week number 2 and not 1...