date-fns/src/differenceInCalendarMonths/index.ts


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

/**
 * @name differenceInCalendarMonths
 * @category Month Helpers
 * @summary Get the number of calendar months between the given dates.
 *
 * @description
 * Get the number of calendar months 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 calendar months
 * @throws {TypeError} 2 arguments required
 *
 * @example
 * // How many calendar months are between 31 January 2014 and 1 September 2014?
 * var result = differenceInCalendarMonths(
 *   new Date(2014, 8, 1),
 *   new Date(2014, 0, 31)
 * )
 * //=> 8
 */
export default function differenceInCalendarMonths(
  dirtyDateLeft: Date | number,
  dirtyDateRight: Date | number
): number {
  requiredArgs(2, arguments)

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

  const yearDiff = dateLeft.getFullYear() - dateRight.getFullYear()
  const monthDiff = dateLeft.getMonth() - dateRight.getMonth()

  return yearDiff * 12 + monthDiff
}