This code is for device to device firebase notifications, Thanks for TVAC Studio channel for great tutorial on youtube https://www.youtube.com/watch?v=Abh3r9hh5gw&list=PLGCjwl1RrtcRHjHyZAxm_Mq4qvtnundo0&index=4

index.js

'use strict' 
// The Cloud Functions for Firebase SDK to create Cloud Functions and setup triggers.
const functions = require('firebase-functions');

// The Firebase Admin SDK to access the Firebase Realtime Database.
const admin = require('firebase-admin');
admin.initializeApp();

exports.sendNotification = functions.firestore.document('Users/{user_id}/Notifications/{notification_id}').onWrite ((change, context) => {
    
    const user_id = context.params.user_id;
    const notification_id = context.params.notification_id;
   
    console.log(" user_id " + user_id);
    console.log("notification_id" + notification_id);

 return admin.firestore().collection("Users").doc(user_id).collection("Notifications").doc(notification_id).get().then(queryResult=> {

  const from_user_id = queryResult.data().from;
  const from_message = queryResult.data().message;
   

  const from_data = admin.firestore().collection("Users").doc(from_user_id).get();
  const to_data = admin.firestore().collection("Users").doc(user_id).get();
  return Promise.all([from_data, to_data]).then(results =>{
   const from_name = results[0].data().name;
   const to_name = results[1].data().name;
   const token_id = results[1].data().token_id;
   
   console.log("from :"+ from_name+" To "+ to_name);

   const payLoad = {
       notification: {
           title : "Notification From: " + from_name,
           body : from_message,
           icon : "default"
       }

    }

   return admin.messaging().sendToDevice(token_id,payLoad).then(results=>{
console.log("Notification sent")

 return true;
   });
  
  });


 });
});