auth.js

const authRouter = require('express').Router();
const User = require('../models/user');

authRouter.post('/checkCredentials', (req, res) => {
  const { email, password } = req.body;
  User.findByEmail(email).then((user) => {
    if (!user) res.status(401).send('Invalid credentials');
    else {
      User.verifyPassword(password, user.hashedPassword).then(
        (passwordIsCorrect) => {
          if (passwordIsCorrect) res.status(200).send('Your credentials are valid !');
          else res.status(401).send('Invalid credentials');
        }
      );
    }
  });
});

module.exports = authRouter;

user.js

const create = ({ firstname, lastname, city, language, email, password }) => {
  return hashPassword(password).then((hashedPassword) => {
    return db
      .query('INSERT INTO users SET ?', {
        firstname,
        lastname,
        city,
        language,
        email,
        hashedPassword,
      })
      .then(([result]) => {
        const id = result.insertId;
        return { firstname, lastname, city, language, email, id };
      });
  });
};

const hashingOptions = {
  type: argon2.argon2id,
  memoryCost: 2 ** 16,
  timeCost: 5,
  parallelism: 1,
};

const hashPassword = (plainPassword) => {
  return argon2.hash(plainPassword, hashingOptions);
};

const verifyPassword = (plainPassword, hashedPassword) => {
  return argon2.verify(hashedPassword, plainPassword, hashingOptions);
};