Source: getISOWeeksInYear/index.js

import startOfISOWeekYear from '../startOfISOWeekYear/index'
import addWeeks from '../addWeeks/index'
import requiredArgs from '../_lib/requiredArgs/index'

var MILLISECONDS_IN_WEEK = 604800000

/**
 * @name getISOWeeksInYear
 * @category ISO Week-Numbering Year Helpers
 * @summary Get the number of weeks in an ISO week-numbering year of the given date.
 *
 * @description
 * Get the number of weeks in an ISO week-numbering year of the given date.
 *
 * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * @param {Date|Number} date - the given date
 * @returns {Number} the number of ISO weeks in a year
 * @throws {TypeError} 1 argument required
 *
 * @example
 * // How many weeks are in ISO week-numbering year 2015?
 * var result = getISOWeeksInYear(new Date(2015, 1, 11))
 * //=> 53
 */
export default function getISOWeeksInYear(dirtyDate) {
  requiredArgs(1, arguments)

  var thisYear = startOfISOWeekYear(dirtyDate)
  var nextYear = startOfISOWeekYear(addWeeks(thisYear, 60))
  var diff = nextYear.valueOf() - thisYear.valueOf()
  // Round the number of weeks to the nearest integer
  // because the number of milliseconds in a week is not constant
  // (e.g. it's different in the week of the daylight saving time clock shift)
  return Math.round(diff / MILLISECONDS_IN_WEEK)
}