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import toDate from '../toDate/index'
import differenceInCalendarMonths from '../differenceInCalendarMonths/index'
import compareAsc from '../compareAsc/index'
import requiredArgs from '../_lib/requiredArgs/index'
import isLastDayOfMonth from '../isLastDayOfMonth/index'
/**
* @name differenceInMonths
* @category Month Helpers
* @summary Get the number of full months between the given dates.
*
* @description
* Get the number of full 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 full months
* @throws {TypeError} 2 arguments required
*
* @example
* // How many full months are between 31 January 2014 and 1 September 2014?
* var result = differenceInMonths(new Date(2014, 8, 1), new Date(2014, 0, 31))
* //=> 7
*/
export default function differenceInMonths(
dirtyDateLeft: Date | number,
dirtyDateRight: Date | number
): number {
requiredArgs(2, arguments)
const dateLeft = toDate(dirtyDateLeft)
const dateRight = toDate(dirtyDateRight)
const sign = compareAsc(dateLeft, dateRight)
const difference = Math.abs(differenceInCalendarMonths(dateLeft, dateRight))
let result
// Check for the difference of less than month
if (difference < 1) {
result = 0
} else {
if (dateLeft.getMonth() === 1 && dateLeft.getDate() > 27) {
// This will check if the date is end of Feb and assign a higher end of month date
// to compare it with Jan
dateLeft.setDate(30)
}
dateLeft.setMonth(dateLeft.getMonth() - sign * difference)
// Math.abs(diff in full months - diff in calendar months) === 1 if last calendar month is not full
// If so, result must be decreased by 1 in absolute value
let isLastMonthNotFull = compareAsc(dateLeft, dateRight) === -sign
// Check for cases of one full calendar month
if (
isLastDayOfMonth(toDate(dirtyDateLeft)) &&
difference === 1 &&
compareAsc(dirtyDateLeft, dateRight) === 1
) {
isLastMonthNotFull = false
}
result = sign * (difference - Number(isLastMonthNotFull))
}
// Prevent negative zero
return result === 0 ? 0 : result
}
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