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import toDate from "../toDate/index.js";
import startOfISOWeek from "../startOfISOWeek/index.js";
import requiredArgs from "../_lib/requiredArgs/index.js";
/**
* @name getISOWeekYear
* @category ISO Week-Numbering Year Helpers
* @summary Get the ISO week-numbering year of the given date.
*
* @description
* Get the ISO week-numbering year of the given date,
* which always starts 3 days before the year's first Thursday.
*
* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
*
* @param {Date|Number} date - the given date
* @returns {Number} the ISO week-numbering year
* @throws {TypeError} 1 argument required
*
* @example
* // Which ISO-week numbering year is 2 January 2005?
* const result = getISOWeekYear(new Date(2005, 0, 2))
* //=> 2004
*/
export default function getISOWeekYear(dirtyDate) {
requiredArgs(1, arguments);
var date = toDate(dirtyDate);
var year = date.getFullYear();
var fourthOfJanuaryOfNextYear = new Date(0);
fourthOfJanuaryOfNextYear.setFullYear(year + 1, 0, 4);
fourthOfJanuaryOfNextYear.setHours(0, 0, 0, 0);
var startOfNextYear = startOfISOWeek(fourthOfJanuaryOfNextYear);
var fourthOfJanuaryOfThisYear = new Date(0);
fourthOfJanuaryOfThisYear.setFullYear(year, 0, 4);
fourthOfJanuaryOfThisYear.setHours(0, 0, 0, 0);
var startOfThisYear = startOfISOWeek(fourthOfJanuaryOfThisYear);
if (date.getTime() >= startOfNextYear.getTime()) {
return year + 1;
} else if (date.getTime() >= startOfThisYear.getTime()) {
return year;
} else {
return year - 1;
}
}
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