52 lines
1.7 KiB
JavaScript
52 lines
1.7 KiB
JavaScript
import { constructFrom } from "./constructFrom.mjs";
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import { startOfISOWeek } from "./startOfISOWeek.mjs";
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import { toDate } from "./toDate.mjs";
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/**
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* @name getISOWeekYear
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* @category ISO Week-Numbering Year Helpers
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* @summary Get the ISO week-numbering year of the given date.
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*
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* @description
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* Get the ISO week-numbering year of the given date,
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* which always starts 3 days before the year's first Thursday.
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*
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* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
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*
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* @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments. Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc).
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*
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* @param date - The given date
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*
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* @returns The ISO week-numbering year
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*
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* @example
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* // Which ISO-week numbering year is 2 January 2005?
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* const result = getISOWeekYear(new Date(2005, 0, 2))
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* //=> 2004
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*/
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export function getISOWeekYear(date) {
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const _date = toDate(date);
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const year = _date.getFullYear();
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const fourthOfJanuaryOfNextYear = constructFrom(date, 0);
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fourthOfJanuaryOfNextYear.setFullYear(year + 1, 0, 4);
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fourthOfJanuaryOfNextYear.setHours(0, 0, 0, 0);
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const startOfNextYear = startOfISOWeek(fourthOfJanuaryOfNextYear);
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const fourthOfJanuaryOfThisYear = constructFrom(date, 0);
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fourthOfJanuaryOfThisYear.setFullYear(year, 0, 4);
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fourthOfJanuaryOfThisYear.setHours(0, 0, 0, 0);
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const startOfThisYear = startOfISOWeek(fourthOfJanuaryOfThisYear);
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if (_date.getTime() >= startOfNextYear.getTime()) {
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return year + 1;
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} else if (_date.getTime() >= startOfThisYear.getTime()) {
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return year;
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} else {
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return year - 1;
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}
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}
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// Fallback for modularized imports:
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export default getISOWeekYear;
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