code: function yearWeek (y, yd) {
      const dow = isoDow(y, 1, 1);
      const start = dow > 4 ? 9 - dow : 2 - dow;
      if ((Math.abs(yd - start) % 7) !== 0) {
          // y yd is not the start of any week
          return [];
      }
      if (yd < start) {
          // The week starts before the first week of the year => go back one year
          yd += Math.round((Date.UTC(y, 0, 1) - Date.UTC(y - 1, 0, 1)) / MS_PER_DAY);
          return yearWeek(y - 1, yd);
      } else if (Date.UTC(y, 0, 1) + (yd - 1 + 3) * MS_PER_DAY >= Date.UTC(y + 1, 0, 1)) {
          // The Wednesday (start of week + 3) lies in the next year => advance one year
          yd -= Math.round((Date.UTC(y + 1, 0, 1) - Date.UTC(y, 0, 1)) / MS_PER_DAY);
          return yearWeek(y + 1, yd);
      }
      return [y, 1 + Math.round((yd - start) / 7)];
  }
docs: Return year and week number given year and day number