JS | Return week number

Date.prototype.getWeek = function() {
  var date = new Date(this.getTime());
   date.setHours(0, 0, 0, 0);
  date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7);
  var week1 = new Date(date.getFullYear(), 0, 4);
  return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000
                        - 3 + (week1.getDay() + 6) % 7) / 7);
}
return new Date().getWeek();
// "12" on March 22 2018