get week of the year in JavaScript

getWeek.js

function( d ) { 

  // Create a copy of this date object  
  var target  = new Date(d.valueOf());  
  
  // ISO week date weeks start on monday  
  // so correct the day number  
  var dayNr   = (d.getDay() + 6) % 7;  

  // Set the target to the thursday of this week so the  
  // target date is in the right year  
  target.setDate(target.getDate() - dayNr + 3);  

  // ISO 8601 states that week 1 is the week  
  // with january 4th in it  
  var jan4    = new Date(target.getFullYear(), 0, 4);  

  // Number of days between target date and january 4th  
  var dayDiff = (target - jan4) / 86400000;    

  // Calculate week number: Week 1 (january 4th) plus the    
  // number of weeks between target date and january 4th    
  var weekNr = 1 + Math.ceil(dayDiff / 7);    

  return weekNr;    

}