I am receiving this error while getting notifications from firebase - firebase

Question

'use strict'
const functions = require('firebase-functions');
const admin=require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification=functions.database.ref('/notifications/{user_id}/{notification_id }').onWrite((change,context) =>{
const user_id=context.params.user_id;
const notification_id=context.params.notification_id;
console.log('The user ID is :',user_id);
if(!change.after.val()){
return console.log('A notification has been deleted from database:',notification_id);
}
const fromUser=admin.database().ref(`/notifications/${user_id}/${notification_id}`).once('value');
return fromUser.then(fromUserResult=>{
const from_user_id=fromUserResult.val().from;
console.log('You have new notification from: : ', from_user_id);
const userQuery=admin.database().ref(`users/${from_user_id}/name`).once('value');
return userQuery.then(userResult=>{
const userName=userResult.val();
const deviceToken=admin.database().ref(`/users/${user_id}/device_token`).once('value');
return deviceToken.then(result =>{
const token_id=result.val();
const payload={
notification:{
title:"Friend Request",
body:`${userName} has sent you request`,
icon:"default"
}
};
return admin.messaging().sendToDevice(token_id, payload);
});
});
});
});
TypeError: Cannot read property 'from' of null
at fromUser.then.fromUserResult (/user_code/index.js:22:47)
at process._tickDomainCallback (internal/process/next_tick.js:135:7)

Answer 1

The only line of code where you're accessing a property called from is here:
const from_user_id=fromUserResult.val().from;
Therefore, fromUserResult.val() must be returning null.
fromUserResult is a DataSnapshot type object. According to the API documentation for the val() method, it can return null if there is no data at the location of the query. So, you will have to check for that case in your code.

Answer 2

I have achieved sending a notification with sender's name using this code:
'use strict'
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
exports.sendNotification = functions.database.ref('/Notifications/{receiver_user_id}/{notification_id}')
.onWrite((data, context) =>
{
const receiver_user_id = context.params.receiver_user_id;
const notification_id = context.params.notification_id;
if(!data.after.val())
{
console.log('A notification has been deleted :' , notification_id);
return null;
}
const sender_user_id = admin.database().ref(`/Notifications/${receiver_user_id}/${notification_id}`).once('value');
return sender_user_id.then(fromUserResult =>
{
const from_sender_user_id = fromUserResult.val().from;
const userQuery = admin.database().ref(`/Users/${from_sender_user_id}/name`).once('value');
return userQuery.then(userResult =>
{
const senderUserName = userResult.val();
console.log('You have notification from :' , senderUserName);
const DeviceToken = admin.database().ref(`/Users/${receiver_user_id}/device_token`).once('value');
console.log('Checkpoint2');
return DeviceToken.then(result =>
{
const token_id = result.val();
const payload =
{
notification:
{
//from_sender_user_id : from_sender_user_id,
title: "New Chat Request",
body: `${senderUserName} wants to connect with you`,
icon: "default"
}
};
return admin.messaging().sendToDevice(token_id, payload).then(response =>
{
console.log('This was the notification Feature');
return null;
}).catch(error => {
console.error(error);
res.error(500);
});
});
});
});
});