Get the number of the week with #GoogleAppsScript

Javascript does not have a method to get the number of the week.

It seems that the fourth of January always is in the first week of a year. If that is correct GAS can help to get the number of the week we are living in.

Very interesting to know.

function getWeekNumber(){
  
  var thisYear = (new Date()).getFullYear();
  var fourthDayOfTheYear = new Date(thisYear,0,4);
  
  var dayFirstMonday = fourthDayOfTheYear.getDate() + 8 - fourthDayOfTheYear.getDay();
  var dateFirstMonday = new Date(thisYear,0,dayFirstMonday);
  var msFirstMonday = dateFirstMonday.getTime(); 
  
  var msDateToday = (new Date()).getTime(); 
  var msOneWeek = 7*24*60*60*1000; 

  for(var i=0;i<52;i++){
    
    var msNextMonday = msFirstMonday + (i * msOneWeek);
    var msComingNextMonday = msNextMonday + msOneWeek;
    
    if(msNextMonday <= msDateToday && msDateToday < msComingNextMonday){
      
      var weekNumber = i+2;
      
    }
  }
  return weekNumber;
}