Recognize simultaneous logins with Firebase Presence

// https://stackoverflow.com/questions/21321234/how-to-prevent-simultaneous-logins-of-the-same-user-with-firebase

const getUserRef = userId => firebase.database().ref(`/users/${userId}`);

const ensureUserSession = userId => {
    const userRef = getUserRef(userId);

    return userRef.once(“value”).then(snapshot => {
        if (!snapshot.exists()) {
            return userRef.set({
                last_seen: firebase.database.ServerValue.TIMESTAMP
            });
        }

        const user = snapshot.val();

        if (user.is_online) {
            throw new SessionAlreadyExists(...);
        }

        return Promise.resolve();
    });
};

firebase.auth().onAuthStateChanged(user => {
    if (user) {
        return ensureUserSession(user.uid)
            .then(() => {
                const userRef = getUserRef(user.uid);

                return userRef
                    .onDisconnect()
                    .set({
                        is_online: false,
                        last_seen: firebase.database.ServerValue.TIMESTAMP
                    }).then(() =>
                        userRef.set({
                            is_online: true,
                            last_seen: firebase.database.ServerValue.TIMESTAMP
                        });
                    );
            }).catch(err => {
                console.error(err);

                if (err instanceof SessionAlreadyExists) {
                    return firebase.auth().signOut();
                }
            });
    }
});