Creating a promise callback

Question

So I'm having a very hard time figuring out how to create a function that will work for different firebase queries / different promises.

Consider this promise, that will turn an empty output array to the returned data of the query / promise.

//userKeys is an array with my 'queries' 
Promise.all(userKeys.map(function(key) {
        return database.child(key).once("value") //Returns a promise
    })).then(function(respond) {
        userData = respond;
    });

This works perfectly. So I thought I would create a function that will do just that for different queries, and thats where I just reached a dead end.

So I will just wrap my previous code in a function

function query(keys, array) {
    return Promise.all(keys.map(function(key) {
        return ref.child(key).once("value")
    })).then(function(respond) {
        array = respond; //This won't work
        return respond;
    });
}

So I tried returning the promise which I can then do this on

someQuery = query(queryKeys);
someQuery.then(function(data){ //Manual written array to store data here
queryResults = data})

I still need to store that data in an array programmatically, which I just cannot figure out.

So look back at my second snippet of code above (My function) The second parameter was meant to be an array that I want to pass, an array of my choice that will be filled with the data, but that won't work either. If I simplify my function even more I start seeing strange things.

output = [];
function modify(arrayName) {
    var data = ["1", "2", "3", "4"];
    arrayName = data;  //Doesnt work
    arrayName = data.slice() //Doesnt work
    arrayName.push(data) //Works, but I don't want to push I want to copy
}
modify(output); //Output will stay empty

Why does my output array stay empty? Is this normal? Also when I console log arrayName it returns an object, when its clearly an array. Can someone please elaborate ?

I have a feeling I will eventually have to write a promise manually for every query I will be making in the future, I just wonder if theres a more efficient way of doing that.

Answer 1

In JavaScript any non-primitive value is passed by a "copy of reference" (more details in this answer).

You can update the contents of a given Array using Array#splice(), e.g.:

function mutate(arr) { arr.splice(0, arr.length, 1, 2, 3); }
var a = [];
mutate(a);
console.log(a); // [1,2,3]

The splice function is a bit inconvenient for your case, as it expects each new item to be passed as a separate argument. This can be worked around by using Function#apply() (or the spread operator in the modern JS):

function mutate(dest, src) { dest.splice.apply(dest, [0, dest.length].concat(src)); }
var arr = [1,2,3];
mutate(arr, [6]);
console.log(arr); // [ 6 ]

An easier way would be to pass an object, containing the array to your mutator function:

var myData = { array: [] };
function mutate(obj, newArray) {
  obj.array = newArray;
}
mutate(myData, [1,2,3]);

That said, are you sure you don't need the callback that the promises provide? If you use a helper function like you describe, you won't get any feedback about when/if the data is received. In other words, what was wrong with the "So I tried returning the promise which I can then do this on" step?