Week numbers in JavaScript

weeknumber.js

/// How to get the week number from a date
// The code below adds two new functions to the Date object. Add this to your source code.
// To get the ISO week number (1-53) of a Date object, mydate, use mydate.getWeek().
// To get the corresponding four-digit year, use mydate.getWeekYear().

// This script is released to the public domain and may be used, modified and
// distributed without restrictions. Attribution not necessary but appreciated.
// @SOURCE: https://weeknumber.net/how-to/javascript

// Returns the ISO week of the date.
Date.prototype.getWeek = function() {
  var date = new Date(this.getTime());
  date.setHours(0, 0, 0, 0);
  // Thursday in current week decides the year.
  date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7);
  // January 4 is always in week 1.
  var week1 = new Date(date.getFullYear(), 0, 4);
  // Adjust to Thursday in week 1 and count number of weeks from date to week1.
  return 1 + Math.round(((date.getTime() - week1.getTime()) / 86400000
                        - 3 + (week1.getDay() + 6) % 7) / 7);
}

// Returns the four-digit year corresponding to the ISO week of the date.
Date.prototype.getWeekYear = function() {
  var date = new Date(this.getTime());
  date.setDate(date.getDate() + 3 - (date.getDay() + 6) % 7);
  return date.getFullYear();
}