toISOStringWithTimezone

Converts a date to extended ISO format (ISO 8601), including timezone offset.

• Use Date.prototype.getTimezoneOffset() to get the timezone offset and reverse it, storing its sign in diff.
• Define a helper function, pad, that normalizes any passed number to an integer using Math.floor() and Math.abs() and pads it to 2 digits, using String.prototype.padStart().
• Use pad() and the built-in methods in the Date prototype to build the ISO 8601 string with timezone offset.

const toISOStringWithTimezone = date => {
  const tzOffset = -date.getTimezoneOffset();
  const diff = tzOffset >= 0 ? '+' : '-';
  const pad = n => `${Math.floor(Math.abs(n))}`.padStart(2, '0');
  return date.getFullYear() +
    '-' + pad(date.getMonth() + 1) +
    '-' + pad(date.getDate()) +
    'T' + pad(date.getHours()) +
    ':' + pad(date.getMinutes()) +
    ':' + pad(date.getSeconds()) +
    diff + pad(tzOffset / 60) +
    ':' + pad(tzOffset % 60);
};

toISOStringWithTimezone(new Date()); // '2020-10-06T20:43:33-04:00'