Sample showing how to use Firebase Cloud Function to send push notification for a topic.

firebase-cloud-function-notification-to-topic.js

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

exports.sendNotification = functions.database.ref('/messages/{pushId}/text').onWrite((event) => {
    const data = event.data;
    console.log('Message received');
    if(!data.changed()){
        console.log('Nothing changed');
        return;
    }else{
        console.log(data.val());
    }

    const payLoad = {
        notification:{
            title: 'Message received',
            body: 'You received a new message',
            sound: "default"
        }
    };

    const options = {
        priority: "high",
        timeToLive: 60*60*2
    };

    return admin.messaging().sendToTopic("Message_Notifications", payLoad, options);
});