handleLogIn could be cleaned up

Why is dbHash a let? You only assign it a value once and only use it in one place, so just declare it there:

      getHash(data[0], (err, result) => {
        if (err) console.log(err);
        else if (result.length === 0) {
          res.writeHead(200, {'Content-Type': 'text/html'});
          res.end('Username does not exist');
        }
        else {
          const dbHash = result[0].password;
          //Compare the retrieved hash with the one provided by user.
          bcrypt.compare(data[1], dbHash, (err, result) => {

You're not handling actual errors from your queries:

getHash(data[0], (err, result) => {
        if (err) {
          res.writeHead(500, {'Content-Type': 'text/html'});
          return res.end('Sorry there was an error on our end');
        }
        else if (result.length === 0) {
          res.writeHead(200, {'Content-Type': 'text/html'});
          return res.end('Username does not exist');
        }
        // ...The rest of your code
        // ...

You could make just one call to the database that gets the user's username and password at the same time, rather than having two nested calls.

req.on('end', ()=>{
      const [username, password] = data.split(',');
      getUsernameAndPassword(username, (err, result) => {
        if (err) {
          res.writeHead(500, {'Content-Type': 'text/html'});
          return res.end('Sorry there was an error on our end');
        }
        else if (result.length === 0) {
          res.writeHead(200, {'Content-Type': 'text/html'});
          return res.end('User does not exist');
        }
        const [usernameDb, hashDb] = result[0];
        //Compare the retrieved hash with the one provided by user.
        bcrypt.compare(password, hashDb, (err, result) => {
        //... The rest
        //...

Again, handle the errors in the bcrypt callback:

bcrypt.compare(password, hashDb, (err, result) => {
          if (err) // HANDLE ME
          else {
            if (result === false) {
              res.writeHead(200, {'Content-Type': 'text/html'});
              res.end('Wrong Password');
            } else {

Also there's some very weird nesting in that callback. Why is there if(err) {stuff;} else { if(result === false) { ... } rather than just an else if

FACN3 / spend500

handleLogIn could be cleaned up #39