nodejs REST API firebase login

I am the beginner of nodejs and firebase. I am using post method to sign in with email and password, after the signInWithEmailAndPassword process completed will go to listener onAuthStateChanged. I would like to response the status code in onAuthStateChanged, can i response outside the post?

var express = require('express');
var app = express();
var router = express.Router();
var firebase = require('../db');
var bodyParser = require('body-parser');

// for forms
router.use(bodyParser.urlencoded({extended: true}));
router.use(bodyParser.json()); 

router.post('/', function (req, res) {
    var email = req.body.email;
    var password = req.body.password;

    firebase.auth().signInWithEmailAndPassword(email, 
password).catch(function(error) {
        if(error) {
            res.status(404).json();
        } 
    });

});

firebase.auth().onAuthStateChanged(function(user) {
                if (user) {
                    user.getToken().then(function(token) {
                        // I want to response 200 status code and token here
                        res.status(200).json({token:token})
        });
    }
});


module.exports = router;

via Ong Kah Kit