Source: differenceInYears/index.js

import toDate from '../toDate/index'
import differenceInCalendarYears from '../differenceInCalendarYears/index'
import compareAsc from '../compareAsc/index'
import requiredArgs from '../_lib/requiredArgs/index'

/**
 * @name differenceInYears
 * @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.
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * @param {Date|Number} dateLeft - the later date
 * @param {Date|Number} dateRight - the earlier date
 * @returns {Number} the number of full years
 * @throws {TypeError} 2 arguments required
 *
 * @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
 */
export default function differenceInYears(dirtyDateLeft, dirtyDateRight) {
  requiredArgs(2, arguments)

  var dateLeft = toDate(dirtyDateLeft)
  var dateRight = toDate(dirtyDateRight)

  var sign = compareAsc(dateLeft, dateRight)
  var difference = Math.abs(differenceInCalendarYears(dateLeft, dateRight))

  // Set both dates to a valid leap year for accurate comparison when dealing
  // with leap days
  dateLeft.setFullYear('1584')
  dateRight.setFullYear('1584')

  // 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
  var result = sign * (difference - isLastYearNotFull)
  // Prevent negative zero
  return result === 0 ? 0 : result
}