How to hide error message coming from an API using Ajax, Fetch or Axios

Question

Good evening guys, I’m beginner with Javascript, and I’m developing the functionality of a CRUD, I want to use the correct HTTP verbs for each function (Obs: I know that using the HTTP verbs is only convention) anyway, my application for testing has the following environment:

• Javascript Vanilla
• Nodejs
• Express
• Mongodb

The Backend part is done, and it’s working normally, I’ll take a route to illustrate:

router.post('/add', async (req, res) => {
  if (await User.findOne({ email: req.body.email }))
    return res.status(400).send('Error user already exists.');

  await User.create(req.body, (err, user) => {
    if (err)
      return res.status(400).send({ error: 'Failed to insert documents.' });

    return res.json(user);
  });
});

As the code above informs, is a route to add the user, first check if the user exists in the database, as parameter use the field email because in my Model it is a single item, if it exists it will return an error if not, it will add the user.

As already informed, I tried in several ways to get the following error message:

fetch('/test/add', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json; charset=utf-8' },
  body: JSON.stringify({
    name: 'test',
    age: '20',
    email: '[email protected]',
    gender: 'male',
    developer: 'yes',
  }),
})
  .then(res => {
    if (res.ok) {
      return res.json();
    } else {
      return Promise.reject({ status: res.status, statusText: res.statusText });
    }
  })
    .then(res => console.log(res))
    .catch(err => console.log('Error, with message:', err.statusText));

I forced an error by adding the same data that has already been entered and as already said the field email is unique, so internally Mongodb would blow an error, I treat him in Backend, but in Frontend I have no control.

I’d really like to use Async/Await and be able to control error messages, and also wanted the answer in which my Backend returns to be seen on Frontend too, someone could help me?