NodeJS change session variable after redirect

Question

I have a login system, user posts data to /login route and it saves data if logged correctly.

router.post('/login', function(req, res, next) {
  if(req.method == 'POST') {
    const username = req.body.username;
    const password = req.body.password;
    sql.query('SELECT * FROM `users` WHERE `username` = "'+username+'"', function(err, results) {
      if(err) { 
        req.session.message = {status: 'danger', message: 'Failed to login, check password.'}
        res.redirect('/')
      } else if (results.length != 0) {
        const passw = results[0].password;
        bcrypt.compare(password, passw, function(err, result) {
          if(result) {
            req.session.loggedin = true;
            req.session.data = results[0];
            req.session.message = {status: 'success', message: `Welcome back, ${username}.`}
            req.session.save(function(err) {
                res.redirect(req.headers.referer);
            })
          } else {
            req.session.message = {status: 'danger', message: 'Failed to login, check password.'}
            res.redirect('/')
          }
        });
      } else {
        req.session.message = {status: 'danger', message: 'Failed to login, check username.'}
        res.redirect('/')
      }
    })
  }
});

so it saves the data and redirects to index but everytime a user goes on the index page it shows the Welcome message, it obviously does this because the variable is still defined so my goal is to change that variable too null after they login and the welcome message shows up once.

ive tried:

delete req.session.message

but this just deletes the variable before it gets passed to the next request.

How im displaying the message:

<script type="text/javascript">
   var message = JSON.parse('<%- JSON.stringify(message) %>');
   $(document).ready(function(){
      if(message != null) {
         $('.toast').addClass('bg-' + message.status);
         $('.toast-body').html(message.message)
         $(".toast").toast('show');
      }
   });        
</script>

How im rendering the data:

if(req.session.loggedin) {
    res.render('index', {
        loggedin: true,
        data: req.session.data,
        message: req.session.message
    });
} else {
    res.render('index', {
        loggedin: false,
        data: null,
        message: req.session.message
  });
}