cloud functions firebase return value is always null?

Question

I used cloud function in firebase

function idea is to set leader for the session if there is no leader else return leader's user-id I use javascript

it always returns null even I return snapshot.val or the user id the function receive

console.log(snapshot.val) give me [object Object]

code

exports.SetLeader = functions.https.onCall((data, context) => {
userID = data.ID
sessionID = data.text
sessionref = "/Sessions/".concat(sessionID);
sessionref = sessionref.concat("/Leader");

var db = admin.database();
var ref = db.ref(sessionref);
console.log("ID" + userID)


        ref.once("value", function (snapshot) {
            console.log("snapchot"+ snapshot.val()); //x
            if (snapshot.val() == null) {
                admin.database()
                    .ref(sessionref).update({ userID })
                console.log("inside if " + userID)
                return { leader: userID };
            }else{
               console.log("inside else " + snapshot.val())
                return { leader: snapshot.val()};
            }

        }, function (errorObject) {
            console.log("The read failed: " + errorObject.code);
        });

        });

Log if there is no leader, set the new leader and return id

Log if there is a leader and I need to return his id

Answer 1

As explained in the doc for Callable Cloud Functions, "to return data after an asynchronous operation, return a promise". By using the "callback version" of the once() method, you don't return a Promise.

You should use the "promise version" as follows:

return ref.once("value")
        .then(snapshot => {...});

In addition, since you have to deal with different cases, depending on the value of the snapshot, it is more readable to use async/await in your Cloud Function. The following should therefore do the trick (untested):

exports.SetLeader = functions.https.onCall(async (data, context) => {

    try {
        const userID = data.ID
        const sessionID = data.text
        const sessionref = `/Sessions/${sessionID}/Leader`  // See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals

        const db = admin.database();
        const ref = db.ref(sessionref);
        console.log("ID" + userID)

        const snapshot = await ref.once("value");
        console.log("snapchot" + snapshot.val());

        if (snapshot.val() == null) {
            await admin.database().ref(sessionref).update({ userID })
            console.log("inside if " + userID)
            return { leader: userID };
        } else {
            console.log("inside else " + snapshot.val())
            return { leader: snapshot.val() };
        }
    } catch (error) {
        // See https://firebase.google.com/docs/functions/callable#handle_errors
        // for more fine grained error management
        console.log(error);
        return null;;
    }

});