40 lines
1.6 KiB
JavaScript
40 lines
1.6 KiB
JavaScript
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import toDate from "../toDate/index.js";
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import differenceInCalendarYears from "../differenceInCalendarYears/index.js";
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import compareAsc from "../compareAsc/index.js";
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import requiredArgs from "../_lib/requiredArgs/index.js";
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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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* @param {Date|Number} dateLeft - the later date
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* @param {Date|Number} dateRight - the earlier date
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* @returns {Number} the number of full years
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* @throws {TypeError} 2 arguments required
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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 default function differenceInYears(dirtyDateLeft, dirtyDateRight) {
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requiredArgs(2, arguments);
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var dateLeft = toDate(dirtyDateLeft);
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var dateRight = toDate(dirtyDateRight);
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var sign = compareAsc(dateLeft, dateRight);
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var difference = Math.abs(differenceInCalendarYears(dateLeft, dateRight)); // 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); // 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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var isLastYearNotFull = compareAsc(dateLeft, dateRight) === -sign;
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var result = sign * (difference - Number(isLastYearNotFull)); // Prevent negative zero
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return result === 0 ? 0 : result;
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}
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