I am trying to work in authentification in an organized way having a controller and service as follows. Although it has been a struggle to find a way to pass the error messages from services to controllers. The user is saved, but if I want to check it the email already exists nothing happens.

Controller:

const { isEmail, isEmpty } = require("../middleware/authMiddlewareValidators");
const { newUser } = require("../services/authServices");

// Register
exports.register = async (req, res) => {
  const { firstName, lastName, email, password } = req.body;

  if (isEmpty(email)) {
    res.status(400).json({ message: "Email must not be empty" });
    return;
  }
  if (!isEmail(email)) {
    res.status(400).json({ message: "Please provide a valid email adress" });
    return;
  }

  if (isEmpty(password)) {
    res.status(400).json({ message: "Password must not be empty" });
    return;
  }
  if (password.length < 6) {
    res
      .status(400)
      .json({ message: "Password must have at least 6 characters" });
    return;
  }

  try {
    await newUser(firstName, lastName, email, password)
    res.status(200).json({ message: "signup success! please login." });
  } catch (error) {
    console.log(error);
    res.status(500).json({message: error})
  }
};

Service:

const User = require("../models/User-model");
const bcrypt = require("bcryptjs");
const passport = require("passport");
const { transporter } = require("../configs/nodemailer");

exports.newUser = async (firstName, lastName, email, password) => {
  try {
    let user = await User.findOne({ email });
    
    if (user) {
      throw new Error("This email is already taken");
    } else {
      const salt = bcrypt.genSaltSync(10);
      const hashPass = bcrypt.hashSync(password, salt);

      const newUser = new User({
        firstName: firstName,
        lastName: lastName,
        email: email,
        password: hashPass,
      });

      newUser.save(() => {
        transporter.sendMail({
          to: newUser.email,
          from: process.env.SCHOOLSCOOL_EMAIL,
          subject: "Succefull register!",
          html: `<p>Welcome to School's Cool ${newUser.firstName} ${newUser.lastName}, <br>
                  <br> Please login to use the web application. <br><br> Thank you.</p>`,
        });
      });
    }
  } catch (error) {
    throw new Error(error.message);
  }
};

Not sure what am I doing wrong but I can't pass the message if the user already exists...