Facebook api – Handling linking accounts in Firebase

I am following Firebase’s instruction on social login. Below is an example of what I am using and it is all working fine from a login authentication perspective.

I have, however, both Google and Facebook login working independently.

What I would like now to be able to do is link the accounts. You can see below in fact where this might go (see the comment):

If you are using multiple auth providers on your app you should handle linking the user’s accounts here.

I have tried many variations of what I think should go here, but to no avail. Can anyone guide me in relation to what they think should go here? Thanks!

function initFBApp() {
    // Result from Redirect auth flow.
    // [START getidptoken]
    firebase.auth().getRedirectResult().then(function (result) {
        if (result.credential) {
            // This gives you a Facebook Access Token. You can use it to access the Facebook API.
            var token = result.credential.accessToken;
            // [START_EXCLUDE]
            document.getElementById('FBquickstart-oauthtoken').textContent = token;
        }
        else {
            document.getElementById('FBquickstart-oauthtoken').textContent = 'null';
            // [END_EXCLUDE]
        }
        // The signed-in user info.
        var user = result.user;
    }).catch(function (error) {
        // Handle Errors here.
        var errorCode = error.code;
        var errorMessage = error.message;
        // The email of the user's account used.
        var email = error.email;
        // The firebase.auth.AuthCredential type that was used.
        var credential = error.credential;
        // [START_EXCLUDE]
        if (errorCode === 'auth/account-exists-with-different-credential') {
            alert('You have already signed up with a different auth provider for that emails.');
            // If you are using multiple auth providers on your app you should handle linking
            // the user's accounts here.
        }
        else {
            console.error(error);
        }
        // [END_EXCLUDE]
    });
    // [END getidptoken]
    // Listening for auth state changes.
    // [START authstatelistener]
    firebase.auth().onAuthStateChanged(function (user) {
        if (user) {
            // User is signed in.
            var displayName = user.displayName;
            var email = user.email;
            var emailVerified = user.emailVerified;
            var photoURL = user.photoURL;
            var isAnonymous = user.isAnonymous;
            var uid = user.uid;
            var providerData = user.providerData;
            // [START_EXCLUDE]
            document.getElementById('FBquickstart-sign-in-status').textContent = 'Signed in';
            document.getElementById('FBquickstart-sign-in').textContent = 'Log out';
            document.getElementById('FBquickstart-account-details').textContent = JSON.stringify(user, null, '  ');
            // [END_EXCLUDE]
        }
        else {
            // User is signed out.
            // [START_EXCLUDE]
            document.getElementById('FBquickstart-sign-in-status').textContent = 'Signed out';
            document.getElementById('FBquickstart-sign-in').textContent = 'Log in with Facebook';
            document.getElementById('FBquickstart-account-details').textContent = 'null';
            document.getElementById('FBquickstart-oauthtoken').textContent = 'null';
            // [END_EXCLUDE]
        }
        // [START_EXCLUDE]
        document.getElementById('FBquickstart-sign-in').disabled = false;
        // [END_EXCLUDE]
    });
    // [END authstatelistener]
    document.getElementById('FBquickstart-sign-in').addEventListener('click', toggleFBSignIn, false);
}