function ISO8601_week_no(dt)   
  {  
     var tdt = new Date(dt.valueOf());  
     var dayn = (dt.getDay() + 6) % 7;  
     tdt.setDate(tdt.getDate() - dayn + 3);  
     var firstThursday = tdt.valueOf();  
     tdt.setMonth(0, 1);  
     if (tdt.getDay() !== 4)   
       {  
      tdt.setMonth(0, 1 + ((4 - tdt.getDay()) + 7) % 7);  
        }  
     return 1 + Math.ceil((firstThursday - tdt) / 604800000);  
        }  
  
dt = new Date();  
console.log(ISO8601_week_no(dt));  
  
dt = new Date(2015, 10, 1);  
console.log(ISO8601_week_no(dt));  