Week number of the year with JS (Starting on Sundays)

weekNumber.js

Date.prototype.getWeekStartingOnSunday = function() {
  var date = new Date(this.getTime());
  // Thursday in current week decides the year.
  // offset +1 because the week starts on Sunday
  date.setDate(date.getDate() + 4 - (date.getDay()+1 + 6) % 7);
  // January 4 is always in week 1.
  var week1 = new Date(date.getFullYear(), 0, 4);
  return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000
      - 4 + (week1.getDay()+1 + 6) % 7) / 7);
}