let err, res, userInviter;
if (emails && emails.length) {
  //to returns {res, err} object
  ({res: userInviter, err} = await to(User.findById(userId)));
  //failOnError returns just result
  res = await to(
    _.map(emails, email => CalendarService.inviteByEmail(email, userInviter, event.title))
  ).failOnError();
}