Node.JS Rethink-DB check if username and email is already exist

I working on a login/register system with Node based on the RethinkDB Chat example when I found that it doesn't check if the user exists with email or username something that's a problem. When I was looking to solve this I was not able to find out why because of running a database check would require a callback with a function something that makes it really hard to achieve.

  if (typeof req.user !== 'undefined') {
    res.redirect('/account');
    return;
  }
  if (!validateEmail(req.param('email'))) {
    req.flash('error', 'Not a valid email address!')
    res.redirect('/register');
    return;
  }

  // Add a check for EMAIL/USERNAME here.

  if (req.param('password') !== req.param('password2')) {
    req.flash('error', 'Passwords does not match!')
    res.redirect('/register');
    return;
  }

What I need help with it to if a user exists with a username or mail that's equal with the one in the form it will send a:

  if (alreadyExists) {
    req.flash('error', 'That username/email is already in use.')
    res.redirect('/register');
    return;
  }

So the main problem is I have to get to know if it exists in the same functions as the other ones and not in a callback. Any help is appreciated!

To check if a user with the given email address exists, you will have to do a check in your RethinkDB-database, which is asynchronous. This can not be achieved without a callback, but it's not that bad!

var r = require('rethinkdbdash')();

function getUserByEmailAddress(emailAddress) {
  return r.db('test').table('user')
    .getAll(emailAddress, {index: 'emailAddress'}).run();
}

app.post('/register', function(req, res) {
  // User already has a session. Not allowed to log in.
  if(req.user) {
    return res.redirect('/account');
  } else if(!validateEmail(req.body.emailAddress)) {
    return res.status(500).send('Not a valid email address'); 
  } else if(req.body.password !== req.body.password2) {
    return res.status(500).send('Passwords do not match'); 
  }

  getUserByEmailAddress(req.body.emailAddress).then(function(user) {
    if(user) {
      res.status(500).send('User with given email address already exists');
    } else {
      // New email address! Encrypt password, add to db, etc.
    }
  })
}

Note that you will have to create a secondary index for this.

You should probably also consider posting the form with a POST-request instead of a GET-request.