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38 lines
1.3 KiB
38 lines
1.3 KiB
var parse = require('../parse/index.js') |
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var differenceInCalendarYears = require('../difference_in_calendar_years/index.js') |
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var compareAsc = require('../compare_asc/index.js') |
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/** |
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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|String|Number} dateLeft - the later date |
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* @param {Date|String|Number} dateRight - the earlier date |
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* @returns {Number} 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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* var result = differenceInYears( |
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* new Date(2015, 1, 11), |
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* new Date(2013, 11, 31) |
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* ) |
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* //=> 1 |
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*/ |
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function differenceInYears (dirtyDateLeft, dirtyDateRight) { |
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var dateLeft = parse(dirtyDateLeft) |
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var dateRight = parse(dirtyDateRight) |
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var sign = compareAsc(dateLeft, dateRight) |
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var difference = Math.abs(differenceInCalendarYears(dateLeft, dateRight)) |
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dateLeft.setFullYear(dateLeft.getFullYear() - sign * difference) |
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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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var isLastYearNotFull = compareAsc(dateLeft, dateRight) === -sign |
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return sign * (difference - isLastYearNotFull) |
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} |
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module.exports = differenceInYears
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