index.js
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var parse = require('../parse/index.js')
var differenceInCalendarYears = require('../difference_in_calendar_years/index.js')
var compareAsc = require('../compare_asc/index.js')
/**
* @category Year Helpers
* @summary Get the number of full years between the given dates.
*
* @description
* Get the number of full years between the given dates.
*
* @param {Date|String|Number} dateLeft - the later date
* @param {Date|String|Number} dateRight - the earlier date
* @returns {Number} the number of full years
*
* @example
* // How many full years are between 31 December 2013 and 11 February 2015?
* var result = differenceInYears(
* new Date(2015, 1, 11),
* new Date(2013, 11, 31)
* )
* //=> 1
*/
function differenceInYears (dirtyDateLeft, dirtyDateRight) {
var dateLeft = parse(dirtyDateLeft)
var dateRight = parse(dirtyDateRight)
var sign = compareAsc(dateLeft, dateRight)
var difference = Math.abs(differenceInCalendarYears(dateLeft, dateRight))
dateLeft.setFullYear(dateLeft.getFullYear() - sign * difference)
// Math.abs(diff in full years - diff in calendar years) === 1 if last calendar year is not full
// If so, result must be decreased by 1 in absolute value
var isLastYearNotFull = compareAsc(dateLeft, dateRight) === -sign
return sign * (difference - isLastYearNotFull)
}
module.exports = differenceInYears