FirebaseUI User verification/logic as part of auth flow

auth.js

/*

//js file handling FirebaseUI
//TO-DO: FirebaseUI init code.......

documentation: https://github.com/firebase/firebaseui-web#available-callbacks

Flow of Data:
-- Pass params from signInSuccess callback (auth.js) 
-- to cloud function getUserClaims (cloudFunctions.js) 
-- via rewrites (firebase.json)


*/

const signInSuccessUrl = '/';

var uiConfig = {
        signInSuccessUrl: signInSuccessUrl,
        callbacks: {
            signInSuccess: function (currentUser, credential, redirectUrl) {
                const user = currentUser.toJSON();
                firebase.auth().currentUser.getIdToken(true)
                    .then(function (idToken) {
                        var url = '/login?uid=' + user.uid + '&exponge=true';  //TO-DO: change url to rewrite => in firebase.json & pass params
                        location.href = url;  //calls cloudFunction getUserClaims via rewrite
                        return;
                    });
            },
            signInFailure: function (error) {
                console.log('ERROR: failed to sign in user', error);
                location.href = signInSuccessUrl;
            },
        }
    };

cloundFunctions.js

//TO-DO: firebase.admin() init code.....

const welcomeUrl = '/path/to/page';
const index = '/';

/*
auth().setCustomUserClaims()
documentation: https://firebase.google.com/docs/auth/admin/custom-claims
*/

exports.newUser = functions.auth.user().onCreate((event) => {
    const user = event.data;
    const uid = user.uid;

    admin.auth().setCustomUserClaims(uid, { isNewUser: true })
        .then(() => {
            console.log('successful setCustomUserClaims', uid);
        })
        .catch(err => {
            console.log('Couldn\'t set custom claims', err, uid)
        });

    //Do stuff
});

exports.getUserClaims = functions.https.onRequest((request, response) => {
    var query = request.query;
    var token = query.token;
    var uid = query.uid;
    var exponge = query.exponge;
    admin.auth().getUser(uid).then((userRecord) => {
        var claims = userRecord.customClaims;

        if (claims && claims.isNewUser) {
            response.redirect(welcomeUrl);
        }
        else {
            response.redirect(index);
        }
      //Add lots of logic and params to the query string to enable versatile redirecting

        if (exponge) {
            admin.auth().setCustomUserClaims(uid, { isNewUser: null })
                .catch(err => {
                    console.log('Couldn\'t set custom claims', err, uid)
                });
        }
    }).catch(err => {
        console.log('Couldn\'t get custom claims', err, uid)
    });
});

firebase.json

{
  "database": {
    "rules": "database.rules.json"
  },
  "hosting": {
    "public": "public",
    "rewrites": [   
      {
        /* 
        This rewrite calls the cloud funtion getUserClaims when hosting location.href = '/login'
        documentation: https://firebase.google.com/docs/hosting/functions
        */
        "source": "/login",
        "function": "getUserClaims"
      }
    ],

    "ignore": [
      "firebase.json",
      "**/ .*", "**/node_modules/**"
    ]
  },
  "storage": {
    "rules": "storage.rules"
  }
}