//*get current calendarweek in the year

 var calendarweek = dt.calendarweek();

or

//*get current calendarweek in the year

var calendarweek = new Date().calendarweek();

    Date.prototype.calendarweek = function () {

        //------< Date.prototype.calendarweek() >--------

        //*extends a date variable by a calendarweek() property

        var currentdate = this;     //current extend date-variable

        var january_01 = new Date(currentdate.getFullYear(), 0, 1);

        //*calendar days already passed

        var calendardays = Math.floor((currentdate - january_01) / (24 * 60 * 60 * 1000));

        //*week by 7days

        var calendarweek = Math.ceil((currentdate.getDay() + 1 + calendardays) / 7);

        //return current calendarweek

        return calendarweek;

        //------</ Date.prototype.calendarweek() >--------

    };