index.js
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var parse = require('../parse/index.js')
var differenceInCalendarISOYears = require('../difference_in_calendar_iso_years/index.js')
var compareAsc = require('../compare_asc/index.js')
var subISOYears = require('../sub_iso_years/index.js')
/**
* @category ISO Week-Numbering Year Helpers
* @summary Get the number of full ISO week-numbering years between the given dates.
*
* @description
* Get the number of full ISO week-numbering years between the given dates.
*
* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
*
* @param {Date|String|Number} dateLeft - the later date
* @param {Date|String|Number} dateRight - the earlier date
* @returns {Number} the number of full ISO week-numbering years
*
* @example
* // How many full ISO week-numbering years are between 1 January 2010 and 1 January 2012?
* var result = differenceInISOYears(
* new Date(2012, 0, 1),
* new Date(2010, 0, 1)
* )
* //=> 1
*/
function differenceInISOYears (dirtyDateLeft, dirtyDateRight) {
var dateLeft = parse(dirtyDateLeft)
var dateRight = parse(dirtyDateRight)
var sign = compareAsc(dateLeft, dateRight)
var difference = Math.abs(differenceInCalendarISOYears(dateLeft, dateRight))
dateLeft = subISOYears(dateLeft, sign * difference)
// Math.abs(diff in full ISO years - diff in calendar ISO years) === 1
// if last calendar ISO year is not full
// If so, result must be decreased by 1 in absolute value
var isLastISOYearNotFull = compareAsc(dateLeft, dateRight) === -sign
return sign * (difference - isLastISOYearNotFull)
}
module.exports = differenceInISOYears