Sending data back to Android from Firebase Cloud Functions - node.js

Question

I want to send data back via functions.https.onCall, the code works and returns data to the console but i don't know why I can't send the data back to the device(in return number one, I understand can't why return number 2 is not working because the asynchronous task, so there is a way to to it after its finish?).
const admin = require('firebase-admin');
const functions = require('firebase-functions');
admin.initializeApp(functions.config().firebase);
var posts= new Array();
var finalposts = new Array();
function post (uid,text,user_name,vid,sendtime,vote,tag,pic,lan){
this.uid=uid;
this.text=text;
this.user_name=user_name;
this.vid=vid;
this.sendtime=sendtime;
this.vote=vote;
this.tag=tag;
this.pic=pic;
this.lan=lan;
}
exports.getWarm = functions.https.onCall((data, context) => {
finalposts = [];
posts = [];
db.collection('local uploads/heb/posts').where('vote', '<', 200).where('vote', '>', 100).get()
.then((snapshot) => {
snapshot.forEach((doc) => {
posts.push( new post(doc.data()["uid"],doc.data()["text"],doc.data()["user_name"],doc.data()["vid"],
doc.data()["sendtime"],doc.data()["vote"],doc.data()["tag"],doc.data()["pic"],doc.data()["lan"]));
});
posts.sort(function(a,b) {return (a.sendtime<b.sendtime)? 1:((a.sendtime>b.sendtime)? -1:0);});
for (var i =0; i<data.refresh_num*2; i++) {
finalposts.push(posts[i]);
}
console.log('hey', '=>', finalposts);
// working: the posts are shown in the console
// (1.) return { posts: finalposts };
// returns null
})
.catch((err) => {
console.log('Error getting documents', err);
});
// (2)return { posts: finalposts };
// returns {posts=[]}
return { posts: 'data' };
// returns {posts=data}
});

Answer 1

You need to return a Promise from your Firebase function in order to return data. Try adding return before your db.collection and see if that works.
From the docs: https://firebase.google.com/docs/functions/callable
To return data after an asynchronous operation, return a promise.
EDIT: I am not very familiar with Firebase Firestore, but any calls that generate a promise, add a return statement before it.
Also, from my experience, you will want to move your return { posts: 'data' }; to execute after your last promise by using then.

Answer 2

well it was simpler then I thought, I just needed to return it.
exports.getWarm = functions.https.onCall((data, context) => {
finalposts = [];
posts = [];
return db.collection('local uploads/heb/posts').where('vote', '<', 200).where('vote', '>', 100).get()
.then((snapshot) => {
snapshot.forEach((doc) => {
posts.push( new post(doc.data()["uid"],doc.data()["text"],doc.data()["user_name"],doc.data()["vid"],
doc.data()["sendtime"],doc.data()["vote"],doc.data()["tag"],doc.data()["pic"],doc.data()["lan"]));
});
posts.sort(function(a,b) {return (a.sendtime<b.sendtime)? 1:((a.sendtime>b.sendtime)? -1:0);});
for (var i =0; i<data.refresh_num*2; i++) {
finalposts.push(posts[i]);
}
console.log('hey', '=>', finalposts);
return { posts: finalposts };
})
.catch((err) => {
console.log('Error getting documents', err);
});
});