date-fns/getWeekYear.js

import { getDefaultOptions } from "./_lib/defaultOptions.js";
import { constructFrom } from "./constructFrom.js";
import { startOfWeek } from "./startOfWeek.js";
import { toDate } from "./toDate.js";

/**
 * The {@link getWeekYear} function options.
 */

/**
 * @name getWeekYear
 * @category Week-Numbering Year Helpers
 * @summary Get the local week-numbering year of the given date.
 *
 * @description
 * Get the local week-numbering year of the given date.
 * The exact calculation depends on the values of
 * `options.weekStartsOn` (which is the index of the first day of the week)
 * and `options.firstWeekContainsDate` (which is the day of January, which is always in
 * the first week of the week-numbering year)
 *
 * Week numbering: https://en.wikipedia.org/wiki/Week#The_ISO_week_date_system
 *
 * @param date - The given date
 * @param options - An object with options.
 *
 * @returns The local week-numbering year
 *
 * @example
 * // Which week numbering year is 26 December 2004 with the default settings?
 * const result = getWeekYear(new Date(2004, 11, 26))
 * //=> 2005
 *
 * @example
 * // Which week numbering year is 26 December 2004 if week starts on Saturday?
 * const result = getWeekYear(new Date(2004, 11, 26), { weekStartsOn: 6 })
 * //=> 2004
 *
 * @example
 * // Which week numbering year is 26 December 2004 if the first week contains 4 January?
 * const result = getWeekYear(new Date(2004, 11, 26), { firstWeekContainsDate: 4 })
 * //=> 2004
 */
export function getWeekYear(date, options) {
  const _date = toDate(date, options?.in);
  const year = _date.getFullYear();

  const defaultOptions = getDefaultOptions();
  const firstWeekContainsDate =
    options?.firstWeekContainsDate ??
    options?.locale?.options?.firstWeekContainsDate ??
    defaultOptions.firstWeekContainsDate ??
    defaultOptions.locale?.options?.firstWeekContainsDate ??
    1;

  const firstWeekOfNextYear = constructFrom(options?.in || date, 0);
  firstWeekOfNextYear.setFullYear(year + 1, 0, firstWeekContainsDate);
  firstWeekOfNextYear.setHours(0, 0, 0, 0);
  const startOfNextYear = startOfWeek(firstWeekOfNextYear, options);

  const firstWeekOfThisYear = constructFrom(options?.in || date, 0);
  firstWeekOfThisYear.setFullYear(year, 0, firstWeekContainsDate);
  firstWeekOfThisYear.setHours(0, 0, 0, 0);
  const startOfThisYear = startOfWeek(firstWeekOfThisYear, options);

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

// Fallback for modularized imports:
export default getWeekYear;