Firebase Functions - NodeJS: Asynchronious Array handling - node.js

Question

I´ve checked numerous posts but could not solve the issue.
I want to return the array before going on. I tried using a function with a callback but that did not work either.
My code looks as following:
exports.GetHelmets = functions.database.ref('onTrack/{userID}').onCreate(event => {
var helmets = [];
let userID = event.params.userID;
let friendRef = admin.database().ref("friends").child(userID);
friendRef.once("value").then(snapshot => {
snapshot.forEach(function(snapshot2){
let rider = snapshot2.val();
let riderID = rider.id;
let rhRef = admin.database().ref("User").child(riderID);
rhRef.once("value", function(snapshot3){
let rider2 = snapshot3.val();
let helmetID = rider2.helmet;
if (helmetID != "STANDARD"){
if(helmets.indexOf(helmetID) < 0){
helmets.push(helmetID);
};
};
});
});
return helmets;
}).then(helmets => {
//WORK WITH ARRAY
});
I hope you can help me, thanks!

Answer 1

You want the last then() to get all the inner data, each of which requires its own call to once(). In such a case, you'll want to use Promise.all() to wait for all the onces.
exports.GetHelmets = functions.database.ref('onTrack/{userID}').onCreate(event => {
let userID = event.params.userID;
let friendRef = admin.database().ref("friends").child(userID);
friendRef.once("value").then(snapshot => {
var promises = []l
snapshot.forEach(function(snapshot2){
let rider = snapshot2.val();
let riderID = rider.id;
let rhRef = admin.database().ref("User").child(riderID);
promises.push(rhRef.once("value");
});
});
return Promise.all(promises);
}).then(snapshots => {
var helmets = [];
snapshots.forEach((snapshot) => {
let rider2 = snapshot.val();
let helmetID = rider2.helmet;
if (helmetID != "STANDARD"){
if(helmets.indexOf(helmetID) < 0){
helmets.push(helmetID);
};
};
});
// WORK WITH helmets
});