Javascript Date getFullWeek()

//sourced: http://stackoverflow.com/questions/12193626/week-year-picker-jquery-plugin

//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; };



//Usage:/*from  ww w  .j  av  a 2s .  c  om*/

//alert( getWeeksInYear( 2013 ) );
//alert( new Date().getWeek() );