Myujth

I use firebase real time database and firebase cloud I tried the if users pushid below have expoToken,I wrote the message the pushnotification/id and send the message with expoToken,(I write react native with expo I try the send a push notification)that is my use case anyway just my problem is I run this function and give me an TypeError: Cannot read property 'ref' of undefined error, how can I fix this

const functions = require('firebase-functions');
var fetch = require('node-fetch');

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

exports.sendPushNotification = functions.database
 .ref('pushnotification/id')
 .onCreate(event => 
 const root = event.data.ref.root;

 var messages = ;
 return root
 .child('/users')
 .once('value')
 .then(function(snapshot) 
 snapshot.forEach(function(childSnapshot) 
 var expoToken = childSnapshot.val().expoToken;
 if (expoToken) 
 messages.push(
 to: expoToken,
 body: 'New node Added'
 );
 
 );
 return Promise.all(messages);
 )
 .then(() => 
 fetch('https://exp.host/--/api/v2/push/send', 
 method: 'POST',
 headers: 
 Accept: 'application/json',
 'Content-Type': 'application/json'
 ,
 body: JSON.stringfy(messages)
 );
 );
 );