48 lines
1.7 KiB
JavaScript
48 lines
1.7 KiB
JavaScript
import { compareAsc } from "./compareAsc.mjs";
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import { differenceInCalendarYears } from "./differenceInCalendarYears.mjs";
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import { toDate } from "./toDate.mjs";
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/**
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* @name differenceInYears
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* @category Year Helpers
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* @summary Get the number of full years between the given dates.
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*
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* @description
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* Get the number of full years between the given dates.
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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 years
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*
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* @example
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* // How many full years are between 31 December 2013 and 11 February 2015?
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* const result = differenceInYears(new Date(2015, 1, 11), new Date(2013, 11, 31))
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* //=> 1
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*/
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export function differenceInYears(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(differenceInCalendarYears(_dateLeft, _dateRight));
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// Set both dates to a valid leap year for accurate comparison when dealing
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// with leap days
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_dateLeft.setFullYear(1584);
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_dateRight.setFullYear(1584);
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// Math.abs(diff in full years - diff in calendar years) === 1 if last calendar year is not full
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// If so, result must be decreased by 1 in absolute value
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const isLastYearNotFull = compareAsc(_dateLeft, _dateRight) === -sign;
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const result = sign * (difference - +isLastYearNotFull);
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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 differenceInYears;
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