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

Modern JavaScript date utility library

import toInteger from '../toInteger/index.js';
import toDate from '../../toDate/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 setUTCISODay(dirtyDate, dirtyDay) {
  if (arguments.length < 2) {
    throw new TypeError('2 arguments required, but only ' + arguments.length + ' present');
  }

  var day = toInteger(dirtyDay);

  if (day % 7 === 0) {
    day = day - 7;
  }

  var weekStartsOn = 1;
  var date = toDate(dirtyDate);
  var currentDay = date.getUTCDay();
  var remainder = day % 7;
  var dayIndex = (remainder + 7) % 7;
  var diff = (dayIndex < weekStartsOn ? 7 : 0) + day - currentDay;
  date.setUTCDate(date.getUTCDate() + diff);
  return date;
}