Firebase Functions - NodeJS: Asynchronious Array handling

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!

Frank van Puffelen · Accepted Answer · 2017-10-26 10:54:37Z

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
});

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