Check Firebase auth token expiry with AngularJS

app.routes.js

function Run($cookies, $firebaseAuth, $rootScope, $state, FBURL) {
    var authRef = new Firebase(FBURL),
        clock = parseInt(getServerTime(authRef) / 1000),
        expires = 0;

    $rootScope.auth = $firebaseAuth(authRef);

    $rootScope.logout = function() {
        $rootScope.auth.$logout();
        $rootScope.checkSession();
    };

    $rootScope.checkSession = function() {
        if ($cookies.authToken) {
            authRef.authWithCustomToken($cookies.authToken, function(error, authData) {
                if (error) {
                    $state.transitionTo('auth');
                } else if (authData) {
                    expires = authData.expires;
                    $state.transitionTo('home');

                    $rootScope.checkExpiry(clock, expires);
                } else {
                    $state.transitionTo('auth');
                }
            });
        } else {
            authRef.unauth();
            $state.transitionTo('auth');
        }
    };

    $rootScope.checkSession();

    $rootScope.checkExpiry = function(clock, expires) {
        if (clock >= expires) {
            $state.transitionTo('auth');
        }
        
        setTimeout(function() {
            $rootScope.checkExpiry(clock + 1, expires)
        }, 1000);
    }
}