Determining whether data is added or deleted to Fi

I'm trying to push notification to android app whenever new posts are added. But the notifications arrive whenever the data is "changed" i.e even when posts are deleted which i don't require. How can i put a condition so that FCM sends notification only if the posts are added. Here is my index.js file

const functions = require('firebase-functions');
let admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendPush = functions.database.ref('/promos').onWrite(event => {
var topic = "deals_notification";
let projectStateChanged = false;
let projectCreated = true;
let projectData = event.data.val();
if (!event.data.previous.exists()) {
    // Do things here if project didn't exists before
}
if (projectCreated && event.data.changed()) {
    projectStateChanged = true;
}
let msg = "";
if (projectCreated) {
    msg = "A project state was changed";
}
if (!event.data.exists()) {
    return;
  }
let payload = {
        notification: {
            title: 'Firebase Notification',
            body: msg,
            sound: 'default',
            badge: '1'
        }
};

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);
});
});

You're doing two things wrong:

Your function is now triggered whenever any data is written under /promos. You want it to be triggered whenever a specific promo is written: /promo/{promoid}.
You're complete ignoring whether the data already existed: if (!event.data.previous.exists()) {, so will need to wire that up.

So something closer to this:

const functions = require('firebase-functions');
let admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendPush = functions.database.ref('/promos/{promoId}').onWrite(event => {
    if (!event.data.previous.exists()) {
        let topic = "deals_notification";
        let payload = {
            notification: {
                title: 'Firebase Notification',
                body: "A project state was changed",
                sound: 'default',
                badge: '1'
            }
        };

        return admin.messaging().sendToTopic(topic, payload);
    }
    return true; // signal that we're done, since we're not sending a message
});