71 lines
2.8 KiB
JavaScript
71 lines
2.8 KiB
JavaScript
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import { toDate } from "./toDate.mjs";
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import { constructFrom } from "./constructFrom.mjs";
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/**
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* @name addMonths
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* @category Month Helpers
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* @summary Add the specified number of months to the given date.
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*
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* @description
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* Add the specified number of months to the given date.
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*
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* @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).
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*
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* @param date - The date to be changed
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* @param amount - The amount of months to be added.
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*
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* @returns The new date with the months added
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*
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* @example
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* // Add 5 months to 1 September 2014:
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* const result = addMonths(new Date(2014, 8, 1), 5)
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* //=> Sun Feb 01 2015 00:00:00
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*
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* // Add one month to 30 January 2023:
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* const result = addMonths(new Date(2023, 0, 30), 1)
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* //=> Tue Feb 28 2023 00:00:00
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*/
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export function addMonths(date, amount) {
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const _date = toDate(date);
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if (isNaN(amount)) return constructFrom(date, NaN);
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if (!amount) {
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// If 0 months, no-op to avoid changing times in the hour before end of DST
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return _date;
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}
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const dayOfMonth = _date.getDate();
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// The JS Date object supports date math by accepting out-of-bounds values for
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// month, day, etc. For example, new Date(2020, 0, 0) returns 31 Dec 2019 and
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// new Date(2020, 13, 1) returns 1 Feb 2021. This is *almost* the behavior we
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// want except that dates will wrap around the end of a month, meaning that
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// new Date(2020, 13, 31) will return 3 Mar 2021 not 28 Feb 2021 as desired. So
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// we'll default to the end of the desired month by adding 1 to the desired
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// month and using a date of 0 to back up one day to the end of the desired
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// month.
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const endOfDesiredMonth = constructFrom(date, _date.getTime());
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endOfDesiredMonth.setMonth(_date.getMonth() + amount + 1, 0);
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const daysInMonth = endOfDesiredMonth.getDate();
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if (dayOfMonth >= daysInMonth) {
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// If we're already at the end of the month, then this is the correct date
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// and we're done.
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return endOfDesiredMonth;
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} else {
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// Otherwise, we now know that setting the original day-of-month value won't
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// cause an overflow, so set the desired day-of-month. Note that we can't
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// just set the date of `endOfDesiredMonth` because that object may have had
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// its time changed in the unusual case where where a DST transition was on
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// the last day of the month and its local time was in the hour skipped or
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// repeated next to a DST transition. So we use `date` instead which is
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// guaranteed to still have the original time.
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_date.setFullYear(
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endOfDesiredMonth.getFullYear(),
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endOfDesiredMonth.getMonth(),
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dayOfMonth,
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);
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return _date;
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}
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}
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// Fallback for modularized imports:
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export default addMonths;
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