Function getISOWeek

Source

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