date-fns/src/startOfISOWeekYear/index.ts


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import getISOWeekYear from '../getISOWeekYear/index'
import startOfISOWeek from '../startOfISOWeek/index'
import requiredArgs from '../_lib/requiredArgs/index'

/**
 * @name startOfISOWeekYear
 * @category ISO Week-Numbering Year Helpers
 * @summary Return the start of an ISO week-numbering year for the given date.
 *
 * @description
 * Return the start of an ISO week-numbering year,
 * which always starts 3 days before the year's first Thursday.
 * The result will be in the local timezone.
 *
 * 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 original date
 * @returns {Date} the start of an ISO week-numbering year
 * @throws {TypeError} 1 argument required
 *
 * @example
 * // The start of an ISO week-numbering year for 2 July 2005:
 * const result = startOfISOWeekYear(new Date(2005, 6, 2))
 * //=> Mon Jan 03 2005 00:00:00
 */
export default function startOfISOWeekYear(dirtyDate: Date | number): Date {
  requiredArgs(1, arguments)

  const year = getISOWeekYear(dirtyDate)
  const fourthOfJanuary = new Date(0)
  fourthOfJanuary.setFullYear(year, 0, 4)
  fourthOfJanuary.setHours(0, 0, 0, 0)
  const date = startOfISOWeek(fourthOfJanuary)
  return date
}