getWeekNumber.js

// `date` must be a Date object, otherwise you may
// need to change lines 6 and 10 to account
// for a different data type.
function(date) {
  // Create a copy of this date object  
  var target = new Date(date.valueOf());

  // ISO week date weeks start on Monday  
  // so correct the day number  
  var dayNumber = (date.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() - dayNumber + 3);

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

  // Number of days between target date and January 4th  
  // 1000 ms/sec * 60 sec/min * 60 min/hour * 24 hour/day = 86400000 ms/day
  var dayDiff = (target - janFourth) / 86400000;

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

  return weekNumber;
}