Function getISOWeekYear

pkgs/core/src/getISOWeekYear/index.ts:31–55

(
  date: DateArg<Date> & {},
  options?: GetISOWeekYearOptions | undefined,
)

 * //=> 2004
 */
export function getISOWeekYear(
  date: DateArg<Date> & {},
  options?: GetISOWeekYearOptions | undefined,
): number {
  const _date = toDate(date, options?.in);
  const year = _date.getFullYear();

  const fourthOfJanuaryOfNextYear = constructFrom(_date, 0);
  fourthOfJanuaryOfNextYear.setFullYear(year + 1, 0, 4);
  fourthOfJanuaryOfNextYear.setHours(0, 0, 0, 0);
  const startOfNextYear = startOfISOWeek(fourthOfJanuaryOfNextYear);

  const fourthOfJanuaryOfThisYear = constructFrom(_date, 0);
  fourthOfJanuaryOfThisYear.setFullYear(year, 0, 4);
  fourthOfJanuaryOfThisYear.setHours(0, 0, 0, 0);
  const startOfThisYear = startOfISOWeek(fourthOfJanuaryOfThisYear);

  if (_date.getTime() >= startOfNextYear.getTime()) {
    return year + 1;
  } else if (_date.getTime() >= startOfThisYear.getTime()) {
    return year;
  } else {
    return year - 1;
  }
}