How to send firebase notifications using nodejs? - node.js

Question

I want to implement push notifications automatically and I've used javascript (node.js) but I got this error
Function returned undefined, expected Promise or value
I am not node js developer I am a flutter developer and I don't know what is promises.
this is my code:
const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);
var notificationMessageData;
exports.fcmTester = functions.firestore.document('posts/{postID}').onCreate((snapshot , context) => {
notificationMessageData = snapshot.data();
admin.firestore().collection('pushTokens').get().then(async (snapshot) => {
var tokens = [];
if (snapshot.empty) {
console.log('No Devices');
} else {
for (var token of snapshot.docs) {
tokens.push(token.data().tokenID);
}
var payload = {
"notification": {
"title": "from" + notificationMessageData.writer,
"body": "from" + notificationMessageData.name,
"sound": "default"
},
"data": {
"sendername": notificationMessageData.writer,
"message": notificationMessageData.name
}
}
return await admin.messaging().sendToDevice(tokens , payload).then((response) => {
console.log('nice');
}).catch((err) => {
console.log(err);
})
}
})
})
Everything is going okay and I upload it without any problem but when adding a document to the posts collection it outputs in the logs the above error.
I have created a user registration form and I've registered the users and put their token id in a collection called pushTokens and then sending a notification for each user inside that collection but this didn't work.

Answer 1

There are two problems in your code:
You don't return the promises returned by the asynchronous Firebase methods (get() and sendToDevice());
You mix-up the use of async/await with the then() method.
I would suggest you watch the 3 official videos about "JavaScript Promises" from the Firebase video series, and then that you first try with using the then() method to correctly chain your Promises and return the chain.
The following code should work.
exports.fcmTester = functions.firestore.document('posts/{postID}').onCreate((snapshot, context) => {
const notificationMessageData = snapshot.data();
return admin.firestore().collection('pushTokens').get()
.then(snapshot => {
var tokens = [];
if (snapshot.empty) {
console.log('No Devices');
throw new Error('No Devices');
} else {
for (var token of snapshot.docs) {
tokens.push(token.data().tokenID);
}
var payload = {
"notification": {
"title": "from" + notificationMessageData.writer,
"body": "from" + notificationMessageData.name,
"sound": "default"
},
"data": {
"sendername": notificationMessageData.writer,
"message": notificationMessageData.name
}
}
return admin.messaging().sendToDevice(tokens, payload)
}
})
.catch((err) => {
console.log(err);
return null;
})
});
Then, after having experienced the "management" of asynchronous methods with then() (and catch()), you may give a try with async/await: again, this official video from Doug Stevenson will be of great help.

Answer 2

It says:
Function returned undefined, expected Promise or value
Have you tried returning null, or true or false, etc. from your functions?
A promise is something you pass around your program while you wait for it to become a real value.
It seems like the error message is focused on wanting you to return something from your function. Without knowing more, this is the best advice I can give.