Presence feature using google firestore, realtime database and the cloud function

Presence status - google cloud function.js

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();

const firestore = functions.firestore;

exports.onUserPresenceStatusChanged = functions.database
	.ref('/online/{userId}')
	.onUpdate((change, context) => {

		var db = admin.firestore();
		const beforeData = change.before.val(); // data before the write
		const afterData = change.after.val(); // data after the write

		console.log('userId:: ' + context.params.userId);
		console.log('BeforeData:: ' + beforeData);
		console.log('AfterData:: ' + afterData);

		presenceStatus = (afterData === 'online') ? true : false;

		console.log('PresenceStatus:: ' + presenceStatus);

		// const usersRef = firestore.document('/users/' + context.params.userId);
		// const usersRef = db.collection("users");

		db.doc('users/' + context.params.userId).set({
			online: presenceStatus,
			last_online_at: Date.now(),
		}, {merge: true})
		.then(function() {
			console.log("Document successfully written.");
		})
		.catch(function(error) {
			console.error("Error writing document: ", error);
		});
});