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import toInteger from "../_lib/toInteger/index.js";
import toDate from "../toDate/index.js";
import requiredArgs from "../_lib/requiredArgs/index.js";
/**
* @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:
* const result = addMonths(new Date(2014, 8, 1), 5)
* //=> Sun Feb 01 2015 00:00:00
*/
export default function addMonths(dirtyDate, dirtyAmount) {
requiredArgs(2, arguments);
var date = toDate(dirtyDate);
var 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;
}
var 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, 0, 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.
var endOfDesiredMonth = new Date(date.getTime());
endOfDesiredMonth.setMonth(date.getMonth() + amount + 1, 0);
var 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;
}
}
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