[Fixed] Sending message from express backend to vuejs frontend

Issue

Hi so it’s kind of complicated for me, hope anyone can help.
Here’s the situation : i have an app divided server side with node/express and front side with Vuejs,
what I’m doing in the back is creating a user here’s the code :

            const createUser=(req, res, next) => {
             console.log("register");
             let con=req.con
          let { email,password } = req.body;
      console.log(req.body)
  con.query(
    `SELECT * FROM users
      WHERE email = $1`,
    [email],
  (err, results) => {
      if (err) {
        console.log(err);
        res.status(404).json({error: err});
      }
      console.log(results);

      if (results.rows.length > 0) {
          //throw new error_types.InfoError("user already exists");
          res.status(200).json({error: "user already exists"});
      } else {

      const hashedPassword = bcrypt.hashSync(password, parseInt(process.env.BCRYPT_ROUNDS));
        con.query('INSERT INTO users (email,password) VALUES ($1, $2)',
          [email,password],
          (err, results) => {
            if (err) {
             next(err);
             
            }
            res.json({info: "User inseted" });
          }
        );
      }
    }
  );
 }

so im checking if it already exists else register it in DB,all good here.
Now in my Vuejs part i have this :

          REGISTER({ commit, dispatch, rootState }, { payload }) {
 
  const {email,password} = payload
    console.log(payload)

  commit('SET_STATE', {
    loading: true,
  })

  const register = mapAuthProviders[rootState.settings.authProvider].register
  register(email,password)
  .then(success => {
    if (success) {
      notification.success({
        message: "Succesful Registered",
        description: "You have successfully registered!",
      })
      router.push('/auth/login')
      commit('SET_STATE', {
        loading: false,
      })
    }
    if (!success) {
      commit('SET_STATE', {
        loading: false,
      })
    }
  })
},

Now the problem happens here as the registration is done all okay but when i use the same email again for another registration it said the same message successfully registred but do not get saved to DB now what i want is the message user aleady exists that appears.
Anyone can help me please?
Edited :added axios part

 export async function register(email,password) {
  return axios
 .post('/register', {emailpassword,})
 .then(response => {
  if (response) {
    const { token } = response.data
    if (token) {
      store.set('accessToken', token)
    }
    return response.data
  }
  return false
})
.catch(err => console.log(err))

}

Solution

Without seeing the actual source code that does the HTTP request from the client, it’s hard to say exactly what the error handling looks like. The most obvious culprit is this:

res.status(200).json({error: "user already exists"});

You’re responding with HTTP 200 OK when an error occurs. Typically, a client implementation will treat this as success. You should signal to clients that an error has occurred – for example, respond with a "409 Conflict". Also make sure the client’s fetch() call (or whatever the client uses for talking to the server) does proper error handling (checks statusCode).

The code has another issue, however – a race condition. This is a case of a TOCTTOU (Time-of-Check to Time-of-Use), where a non-zero amount of time passes between the existence check (SELECT) and the INSERT. If two users are registering for the same e-mail at the same time, they could both get a success.

You should remove the check altogether and use uniqueness constraints offered by the database instead (UNIQUE INDEX). This way, the DB guarantees there can be no duplicates and you don’t have to worry about race conditions.