Firebase cloud function accesing Database plus trigger Push notification

index.js

var functions = require('firebase-functions');
var admin = require('firebase-admin');
var serviceAccount = require("./serviceAccountKey.json");
 // admin.initializeApp(functions.config().firebase);
 admin.initializeApp({
    credential: admin.credential.cert(serviceAccount),
    databaseURL: "https://tutsalert.firebaseio.com"
  });

exports.sendNotification = functions.database.ref('/articles/{articleId}')
    .onWrite((change, context) => {

        // Grab the current value of what was written to the Realtime Database.
        // var eventSnapshot = event.data.ref;
        var str1 = "Author is ";
        // var str = str1.concat(eventSnapshot.child("author").val());
        console.log(str1+change.after.val());
 
        var topic = "android";
        var payload = {
            data: {
                title: change.after.child("title").val(),
                author:   change.after.child("author").val()
            }
        };
 
        // Send a message to devices subscribed to the provided topic.
        return admin.messaging().sendToTopic(topic, payload)
            .then(function (response) {
                // See the MessagingTopicResponse reference documentation for the
                // contents of response.
                console.log("Successfully sent message:", response);
            })
            .catch(function (error) {
                console.log("Error sending message:", error);
            });
        });