nodejs modal for login instead of login route - node.js

Question

Im building a nodejs (express) app using a user model for commenting thins etc.
For certain activities a user must have an account and has to be logged in. I'm using a middleware (function isLoggedIn) to check if a user is logged in. At first, I did this with a form that's shown if you go to the /login route. Instead of using a route, I want to use a bootstrap modal that shows up.
This is my login form that shows up when the modal is being invoked:
<form action="/login" method="POST">
<div class="modal fade" id="loginModal" tabindex="-1" role="dialog" aria-labelledby="loginModalTitle" aria-hidden="true">
<div class="modal-dialog modal-dialog-centered" role="document">
<div class="modal-content">
<div class="modal-header">
<h5 class="modal-title" id="loginModalTitle">Login</h5>
<button type="button" class="close" data-dismiss="modal" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
</div>
<div class="modal-body">
<div class="form-group" id="formGroupLogin">
<input type="email" class="form-control" name="username" placeholder="E-Mail" required>
<input type="password" class="form-control" name="password" placeholder="Passwort" required>
</div>
</div>
<div class="modal-footer">
<input type="submit" class="btn btn-primary btn-block" value="Login">
</div>
</div>
</div>
</div>
</form>
It works perfektly fine when I put this on my navbar so that it's being invoked when you click the "Login" Button in the navbar.
But how can I invoke this modal when I want to guide to the login form within a middleware that checks if the user is authenticated?
In the past, I did this by routing to a route that shows the login form (using res.redirect("/login"). Now I don't want to route, I just need to expand the modal.
That's how my middleware looked so far:
function isLoggedIn(req, res, next) {
if (req.isAuthenticated()) {
return next();
}
res.redirect("/login");
}
How do I have to change it? Any Ideas?