Passport authentication via ajax

What I'm trying to accomplish is to do user authentication via ajax using passport. The recommended way of using passport is to trigger the authorisation process via a GET request which gets triggered using a regular <a> tag. After successful (or failed) authentication there is a redirect to a new page. Now instead of using a html link to trigger the route I want to do it via ajax. The problem here is that when I try this with a third party authentication strategy such as passport-facebook I get a CORS error:

XMLHttpRequest cannot load [the facebook auth URL]' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource. 
Origin 'http://localhost:8080' is therefore not allowed access.

Here is the relevant server side code:

// route for facebook authentication and login
app.get('/auth/facebook', passport.authenticate('facebook', { scope : ['email'] }));

// handle the callback after facebook has authenticated the user
app.get('/auth/facebook/callback', (req, res, next) => {
  passport.authenticate('facebook', (err, user, info) => {
    req.login(user, function(err) {
      if(err) return next(err);
      return res.status(200).json({
        success: "All good man!"
      });
    })
  })(req, res, next);
});

While in the browser I simply do:

axios.get('/auth/facebook').then((response) => {
  console.log(response);
});

I found several similar questions but none of them seemed to have a satisfying answer. Is there a way to actually do this? Or a smart workaround? Maybe a way to still send back a JSON response even if the route was triggered using an html link?

via Flavio