date-fns/esm/_lib/getUTCISOWeekYear/index.js

Modern JavaScript date utility library

import toDate from '../../toDate/index.js';
import startOfUTCISOWeek from '../startOfUTCISOWeek/index.js'; // This function will be a part of public API when UTC function will be implemented.
// See issue: https://github.com/date-fns/date-fns/issues/376

export default function getUTCISOWeekYear(dirtyDate) {
  if (arguments.length < 1) {
    throw new TypeError('1 argument required, but only ' + arguments.length + ' present');
  }

  var date = toDate(dirtyDate);
  var year = date.getUTCFullYear();
  var fourthOfJanuaryOfNextYear = new Date(0);
  fourthOfJanuaryOfNextYear.setUTCFullYear(year + 1, 0, 4);
  fourthOfJanuaryOfNextYear.setUTCHours(0, 0, 0, 0);
  var startOfNextYear = startOfUTCISOWeek(fourthOfJanuaryOfNextYear);
  var fourthOfJanuaryOfThisYear = new Date(0);
  fourthOfJanuaryOfThisYear.setUTCFullYear(year, 0, 4);
  fourthOfJanuaryOfThisYear.setUTCHours(0, 0, 0, 0);
  var startOfThisYear = startOfUTCISOWeek(fourthOfJanuaryOfThisYear);

  if (date.getTime() >= startOfNextYear.getTime()) {
    return year + 1;
  } else if (date.getTime() >= startOfThisYear.getTime()) {
    return year;
  } else {
    return year - 1;
  }
}