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var parse = require('../parse/index.js')
var startOfISOWeek = require('../start_of_iso_week/index.js')
/**
* @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|String|Number} date - the given date
* @returns {Number} the ISO week-numbering year
*
* @example
* // Which ISO-week numbering year is 2 January 2005?
* var result = getISOYear(new Date(2005, 0, 2))
* //=> 2004
*/
function getISOYear (dirtyDate) {
var date = parse(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
}
}
module.exports = getISOYear