const firestoreDb = firebase.firestore();
const oldRealTimeDb = firebase.database();

const usersRef = firestoreDb.collection('users'); // Get a reference to the Users collection;
const onlineRef = oldRealTimeDb.ref('.info/connected'); // Get a reference to the list of connections

onlineRef.on('value', snapshot => {

  oldRealTimeDb
    .ref(`/status/${userId}`)
    .onDisconnect() // Set up the disconnect hook
    .set('offline') // The value to be set for this key when the client disconnects
    .then(() => {
        // Set the Firestore User's online status to true
        usersRef
          .doc(userId)
          .set({
            online: true,
          }, { merge: true});

        // Let's also create a key in our real-time database
        // The value is set to 'online'
        oldRealTimeDb.ref(`/status/${userId}`).set('online');
    });

});