Неделя года

const weekOfYear = date => {
  const startOfYear = new Date(date.getFullYear(), 0, 1);
  startOfYear.setDate(startOfYear.getDate() + (startOfYear.getDay() % 7));
  return Math.round((date - startOfYear) / (7 * 24 * 3600 * 1000));
};

weekOfYear(new Date('2021-06-18')); // 23