70 lines
2.3 KiB
JavaScript
70 lines
2.3 KiB
JavaScript
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import { compareAsc } from "./compareAsc.mjs";
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import { differenceInCalendarMonths } from "./differenceInCalendarMonths.mjs";
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import { isLastDayOfMonth } from "./isLastDayOfMonth.mjs";
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import { toDate } from "./toDate.mjs";
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/**
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* @name differenceInMonths
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* @category Month Helpers
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* @summary Get the number of full months between the given dates.
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*
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* @description
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* Get the number of full months between the given dates using trunc as a default rounding method.
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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 dateLeft - The later date
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* @param dateRight - The earlier date
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*
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* @returns The number of full months
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*
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* @example
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* // How many full months are between 31 January 2014 and 1 September 2014?
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* const result = differenceInMonths(new Date(2014, 8, 1), new Date(2014, 0, 31))
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* //=> 7
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*/
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export function differenceInMonths(dateLeft, dateRight) {
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const _dateLeft = toDate(dateLeft);
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const _dateRight = toDate(dateRight);
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const sign = compareAsc(_dateLeft, _dateRight);
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const difference = Math.abs(
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differenceInCalendarMonths(_dateLeft, _dateRight),
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);
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let result;
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// Check for the difference of less than month
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if (difference < 1) {
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result = 0;
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} else {
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if (_dateLeft.getMonth() === 1 && _dateLeft.getDate() > 27) {
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// This will check if the date is end of Feb and assign a higher end of month date
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// to compare it with Jan
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_dateLeft.setDate(30);
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}
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_dateLeft.setMonth(_dateLeft.getMonth() - sign * difference);
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// Math.abs(diff in full months - diff in calendar months) === 1 if last calendar month is not full
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// If so, result must be decreased by 1 in absolute value
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let isLastMonthNotFull = compareAsc(_dateLeft, _dateRight) === -sign;
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// Check for cases of one full calendar month
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if (
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isLastDayOfMonth(toDate(dateLeft)) &&
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difference === 1 &&
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compareAsc(dateLeft, _dateRight) === 1
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) {
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isLastMonthNotFull = false;
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}
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result = sign * (difference - Number(isLastMonthNotFull));
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}
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// Prevent negative zero
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return result === 0 ? 0 : result;
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}
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// Fallback for modularized imports:
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export default differenceInMonths;
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