Confused on how to properly use promises in http Firebase Cloud Function - node.js

Question

I am new to using cloud functions for Firebase with an http triggered function and I am confused on how to properly terminate the function. I'm not sure if I should be using res.sendStatus, returning a promise, or both.
The goal of my function is to loop through several documents in the collection 'communities'. Each community has a collection of documents where I query the document with the highest value of 'hotScore'. I then send an iOS push notification containing that document to a topic (all users in that given community).
Unfortunately, I am getting several errors when the code is run such as Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client and Unhandled rejection. I'm pretty such this is due to my negligence in handling function termination, although I have been confused by the online resources I have looked at so far. Would someone mind taking a look at my code/pointing me in the right direction? Thank you so much!
exports.sendNotificationTrendingPost = functions.https.onRequest(async (req, res) => {
//Get communities collection from Firestore
return admin.firestore().collection('communities').get().then((communities) => {
var communityPromises = [];
//Loop through each community
communities.forEach((community) => {
let communityID = community.get('communityID');
let communityName = community.get('name');
//Get the post with the highest hotScore
let communityPromise = admin.firestore().collection('communities').doc(communityID).collection('posts').orderBy('hotScore', 'desc').limit(1).get().then((posts) => {
let hottestPost = posts[0];
let postID = hottestPost.get('postID');
let postText = hottestPost.get('text');
let currentDate = Date.now() / 1000;
var message;
//Verify that the hottest post was posted in the past 24 hours
if (hottestPost.get('date') > (currentDate - 86400)) {
//Build the notification text (shortening if too long)
let shortenedPostText = postText.substring(0,60);
var textEnd = '';
if (postText.length > 60) {
textEnd = '...';
}
let notificationText = 'Trending post on ' + communityName + ': ' + shortenedPostText + textEnd;
//Build the push notification
message = {
apns: {
headers: {
'apns-push-type': 'alert'
},
payload: {
aps: {
alert: {
body: notificationText,
},
},
postID: postID,
},
},
topic: communityID
}
}
//Send the message and return the promise
if (message === null) {
return null;
} else {
return admin.messaging().send(message);
}
})
.catch(error => {
console.log(error);
res.status(500).send(error);
})
if (communityPromise !== null) {
communityPromises.push(communityPromise);
}
})
res.sendStatus(200);
return Promise.all(communityPromises);
})
.catch(error => {
console.log(error);
res.status(500).send(error);
})
})

Answer 1

As samthecodingman advised, it is much better to use async/await in your case, as it will simplify the code and will make it much easier to read.
The following changes should do the trick (untested). Note how we use an Array of Community names in order to pass the names from one loop to the other. This works because, with Promise.all(), the returned values are in order of the Promises passed, regardless of completion order.
exports.sendNotificationTrendingPost = functions.https.onRequest(async (req, res) => {
try {
const db = admin.firestore();
const communitiesQuerySnap = await db.collection('communities').get();
const communityPromises = [];
const communityNames = [];
communitiesQuerySnap.forEach((community) => {
let communityID = community.get('communityID');
let communityName = community.get('name');
communityNames.push(communityName);
communityPromises.push(db.collection('communities').doc(communityID).collection('posts').orderBy('hotScore', 'desc').limit(1).get())
});
const postsQuerySnapArray = await Promise.all(communityPromises);
const messagePromises = [];
postsQuerySnapArray.forEach((postsQuerySnap, index) => {
const hottestPost = postsQuerySnap.docs[0];
const postID = hottestPost.get('postID');
const postText = hottestPost.get('text');
const currentDate = Date.now() / 1000;
let message;
if (hottestPost.get('date') > (currentDate - 86400)) {
//Build the notification text (shortening if too long)
let shortenedPostText = postText.substring(0, 60);
var textEnd = '';
if (postText.length > 60) {
textEnd = '...';
}
const communityName = communityNames[index]; // The two Arrays postsQuerySnapArray and communityName have the same order, because Promise.all keeps the order.
let notificationText = 'Trending post on ' + communityName + ': ' + shortenedPostText + textEnd;
//Build the push notification
message = {
apns: {
headers: {
'apns-push-type': 'alert'
},
payload: {
aps: {
alert: {
body: notificationText,
},
},
postID: postID,
},
},
topic: communityID
}
messagePromises.push(admin.messaging().send(message));
}
})
await Promise.all(messagePromises);
res.status(200).send({ result: "completed" }); // Or res.end()
} catch (error) {
console.log(error);
res.status(500).send(error);
}
});