45 lines
1.5 KiB
JavaScript
45 lines
1.5 KiB
JavaScript
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import toDate from "../toDate/index.js";
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import startOfISOWeek from "../startOfISOWeek/index.js";
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import requiredArgs from "../_lib/requiredArgs/index.js";
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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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* @param {Date|Number} date - the given date
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* @returns {Number} the ISO week-numbering year
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* @throws {TypeError} 1 argument required
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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 default function getISOWeekYear(dirtyDate) {
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requiredArgs(1, arguments);
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var date = toDate(dirtyDate);
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var year = date.getFullYear();
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var fourthOfJanuaryOfNextYear = new 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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var startOfNextYear = startOfISOWeek(fourthOfJanuaryOfNextYear);
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var fourthOfJanuaryOfThisYear = new 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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var 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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