33 lines
1.2 KiB
JavaScript
33 lines
1.2 KiB
JavaScript
var startOfISOYear = require('../start_of_iso_year/index.js')
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var addWeeks = require('../add_weeks/index.js')
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var MILLISECONDS_IN_WEEK = 604800000
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/**
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* @category ISO Week-Numbering Year Helpers
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* @summary Get the number of weeks in an ISO week-numbering year of the given date.
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*
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* @description
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* Get the number of weeks in an ISO week-numbering year of the given date.
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*
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* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
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*
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* @param {Date|String|Number} date - the given date
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* @returns {Number} the number of ISO weeks in a year
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*
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* @example
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* // How many weeks are in ISO week-numbering year 2015?
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* var result = getISOWeeksInYear(new Date(2015, 1, 11))
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* //=> 53
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*/
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function getISOWeeksInYear (dirtyDate) {
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var thisYear = startOfISOYear(dirtyDate)
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var nextYear = startOfISOYear(addWeeks(thisYear, 60))
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var diff = nextYear.valueOf() - thisYear.valueOf()
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// Round the number of weeks to the nearest integer
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// because the number of milliseconds in a week is not constant
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// (e.g. it's different in the week of the daylight saving time clock shift)
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return Math.round(diff / MILLISECONDS_IN_WEEK)
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}
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module.exports = getISOWeeksInYear
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