lib/jsdom/living/helpers/dates-and-times.js
function weekNumberOfLastDay(year) {
  const jan1 = new Date(year, 0);
  return jan1.getDay() === 4 || (isLeapYear(year) && jan1.getDay() === 3) ? 53 : 52;
}
function weekNumberOfLastDay(year) {
  const jan1 = new Date(year, 0);
  return jan1.getDay() === 4 || (isLeapYear(year) && jan1.getDay() === 3) ? 53 : 52;
}
https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#week-number-of-the-last-day https://stackoverflow.com/a/18538272/1937836