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 王天杨
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"use strict";
exports.addMonths = addMonths;
var _index = require("./toDate.js");
var _index2 = require("./constructFrom.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.
*
* @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments. Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc).
*
* @param date - The date to be changed
* @param amount - The amount of months to be added.
*
* @returns The new date with the months added
*
* @example
* // Add 5 months to 1 September 2014:
* const result = addMonths(new Date(2014, 8, 1), 5)
* //=> Sun Feb 01 2015 00:00:00
*
* // Add one month to 30 January 2023:
* const result = addMonths(new Date(2023, 0, 30), 1)
* //=> Tue Feb 28 2023 00:00:00
*/
function addMonths(date, amount) {
const _date = (0, _index.toDate)(date);
if (isNaN(amount)) return (0, _index2.constructFrom)(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, 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.
const endOfDesiredMonth = (0, _index2.constructFrom)(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;
}
}
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