Comment by Izkata

I'd like to propose weird alternative to this:

  function throwError(error) {
      throw new Error(error);
  }
  
  async function createUser(user) {
      validateUserInput(user) || throwError(err.userValidationFailed);
      isPasswordValid(user.password) || throwError(err.invalidPassword);
      !(await userService.getUserByEmail(user.email)) || throwError(err.userExists);

What if...

      [
          [() => validateUserInput(user), err.userValidationFailed],
          [() => isPasswordValid(user.password), err.invalidPassword],
          [() => !(await userService.getUserByEmail(user.email)), err.userExists],
      ].forEach(function([is_good, error]) {
          if (!is_good()) {
              throw new Error(error);
          }
      });

Also on the regex:

  const rules = [/[a-z]{1,}/, /[A-Z]{1,}/, /[0-9]{1,}/, /\W{1,}/];

No one caught that in all four of these, "{1,}" could be replaced with the much more common "+". A bit odd considering the desire for brevity. I do personally prefer "[0-9]" over "\d", especially considering the other rules, but can go either way on "\W".

I might have also added a fifth regex for length though, instead of doing it differently, if my head was in that mode: /.{8,}/