Create key-value pair on Real Time Database

index.js

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 => {
  // 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');
});