javascript date toISOString with timezone

localeIsoDateString.js

function localeIsoDateString(date = new Date())
{
  let timezoneOffsetHours = (new Date()).getTimezoneOffset() * 60 * 1000;
  let timezoneAsString = ["+", "+", "-"][Math.sign(timezoneOffsetHours) + 1] + (new Date(Math.abs(timezoneOffsetHours)).toISOString().substr(11, 5));
  return new Date(date.getTime() - timezoneOffsetHours).toISOString().substr(0, 23) + timezoneAsString;
};

// Expected output (for Central European Summer Time): 2021-07-14T10:34:01.830+02:00