How to return Array from Firebase Snapshot

Question

I'm trying to retrieve some Data from Firebase (which works perfectly), but my colleague needs that I return the value of a snapshot as an Array, so that he can use it in another file. And as far as I know, it's impossible knowing that the value event is Asynchronous.

here's an example:

function getData() {
let current = [];
qRef.on("value", function (snapshot) { //qRef is the reference to my DB
    current = snapshot.val();
}, function (errorObject) {
    console.log("The read failed: " + errorObject.code);
});
return current; //expected value is the returned data from the snapshot
}

Is there any alternative solution? Thanks.

Inside the value event it returns the data as I want it.. But in the return statement I only have an empty array, which is normal because it's executed before the asynchronous event. — Haitham Hirt
– Haitham Hirt, Commented Aug 11, 2018 at 18:24

Sidney · Accepted Answer · 2018-08-11 18:27:06Z

4

You're right, you can't directly return the value because it's asynchronous.

Instead, you should make use of Promises:

function getData() {
  return qRef
    .once('value')
    .then(snapshot => snapshot.val())
    .then(value => [ value ])
}

Notice that I'm using .once() instead of .on(). .on() is for listening for changes, .once() is for just retrieving the value on-demand. Also, .on() doesn't use promises because a promise means "some async work will happen, then you'll get a result". Listening for a value might mean the callback is triggered many times.

answered Aug 11, 2018 at 18:27

Sidney

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3 Comments

Haitham Hirt Over a year ago

Thanks a lot it works! I have just one issue, if I want to return an edited array of the snapshot, where can I add it? (Because I actually need 10 shuffled values of the snapshot)

Sidney Over a year ago

The function in the second .then is taking the value, and sticking it in an array. You can do any manipulation to the snapshot value you'd like in that function.

user2286756 Over a year ago

Hi, im new with firebase+ nodejs. Why when i console.log(getData()); Its return Promise { <pending> }. Any advise?

Atul · Accepted Answer · 2018-08-11 18:52:49Z

0

if its not a snapshot , suppose a get function,then you can use this:-

function getData() {
    return new Promise(function(resolve, reject){
        let current = [];
        qRef.on("value", function (snapshot) { //qRef is the reference to my DB
            current = snapshot.val();
            resolve(current);
        }, function (errorObject) {
            console.log("The read failed: " + errorObject.code);
            reject({
                error: true
            })
        });

    })
}

call it like

getData.then(function(current){
    desired_variable = current
})

since its a snapshot you can pass a callbak function and call it everytime. callback function will set the data to the desired variable

 function getData(callback) {
    let current = [];
    qRef.on("value", function (snapshot) { //qRef is the reference to my DB
        current = snapshot.val();
        // write logic for current array
        callback(current);
    }, function (errorObject) {
        console.log("The read failed: " + errorObject.code);
        reject({
            error: true
        })
    });
}

it will be called like

getData(function(current){
       desired_variable = current;
})

hope it solves your issue

answered Aug 11, 2018 at 18:52

Atul

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1 Comment

Haitham Hirt Over a year ago

Perfect! Thanks.

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