Source: addMonths/index.ts

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

/**
 * @name addMonths
 * @category Month Helpers
 * @summary Add the specified number of months to the given date.
 *
 * @description
 * Add the specified number of months to the given date.
 *
 * ### 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} date - the date to be changed
 * @param {Number} amount - the amount of months to be added. Positive decimals will be rounded using `Math.floor`, decimals less than zero will be rounded using `Math.ceil`.
 * @returns {Date} the new date with the months added
 * @throws {TypeError} 2 arguments required
 *
 * @example
 * // Add 5 months to 1 September 2014:
 * var result = addMonths(new Date(2014, 8, 1), 5)
 * //=> Sun Feb 01 2015 00:00:00
 */
export default function addMonths(
  dirtyDate: Date | number,
  dirtyAmount: number
): Date {
  requiredArgs(2, arguments)

  const date = toDate(dirtyDate)
  const amount = toInteger(dirtyAmount)
  if (isNaN(amount)) {
    return new Date(NaN)
  }
  if (!amount) {
    // If 0 months, no-op to avoid changing times in the hour before end of DST
    return date
  }
  const dayOfMonth = date.getDate()

  // The JS Date object supports date math by accepting out-of-bounds values for
  // month, day, etc. For example, new Date(2020, 1, 0) returns 31 Dec 2019 and
  // new Date(2020, 13, 1) returns 1 Feb 2021.  This is *almost* the behavior we
  // want except that dates will wrap around the end of a month, meaning that
  // new Date(2020, 13, 31) will return 3 Mar 2021 not 28 Feb 2021 as desired. So
  // we'll default to the end of the desired month by adding 1 to the desired
  // month and using a date of 0 to back up one day to the end of the desired
  // month.
  const endOfDesiredMonth = new Date(date.getTime())
  endOfDesiredMonth.setMonth(date.getMonth() + amount + 1, 0)
  const daysInMonth = endOfDesiredMonth.getDate()
  if (dayOfMonth >= daysInMonth) {
    // If we're already at the end of the month, then this is the correct date
    // and we're done.
    return endOfDesiredMonth
  } else {
    // Otherwise, we now know that setting the original day-of-month value won't
    // cause an overflow, so set the desired day-of-month. Note that we can't
    // just set the date of `endOfDesiredMonth` because that object may have had
    // its time changed in the unusual case where where a DST transition was on
    // the last day of the month and its local time was in the hour skipped or
    // repeated next to a DST transition.  So we use `date` instead which is
    // guaranteed to still have the original time.
    date.setFullYear(
      endOfDesiredMonth.getFullYear(),
      endOfDesiredMonth.getMonth(),
      dayOfMonth
    )
    return date
  }
}