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I hit a really weird issue with a simple JavaScript array last night. I am working on a React Native app powered by Firebase and getting data from the real-time database with the SDK.

Here is the code:

    var app = this

    this.props.fb.auth().onAuthStateChanged(function(user) {
        if (user) {
            app.props.fb.database().ref('/users/' + user.uid + "/timeline").once('value').then(function(snapshot) {
                var posts = snapshot.val()
                return posts
            }).then((posts) => {                
                var statuses = [];
                for (i in posts) {

                    app.props.fb.database().ref('/posts/' + posts[i]).once('value').then(function(snapshot) {
                       statuses.push(snapshot.val())
                    });
                }

                console.log(statuses)
            })
        }

    });

The above code is supposed to get the data from the timeline of each user, iterate through each of the posts on the timeline and then get the data of the post from posts. It then simply pushes the data to the statuses array, which is being console logged at the end.

This is what shows up on the console. enter image description here

Until the array is expanded, the console doesn't show the items in the array. Additionally, when I try to get the length of the array, it returns 0 and I also can't iterate through the items on the array.

What am I doing wrong?

Frank van Puffelen
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hackerman
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  • 99/100 JS errors seem to be async issues - this looks the same. u cannot log in the console an array that is being populated by an async call. – Sten Muchow Oct 25 '17 at 10:31
  • You shouldn't iterate an array using a [`for..in`](https://stackoverflow.com/questions/500504/why-is-using-for-in-with-array-iteration-a-bad-idea) construct. Use [`Array.prototype.forEach()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach). – Phylogenesis Oct 25 '17 at 10:34

1 Answers1

2

If posts is an iterable I would look into using some version of q.all and then when all promises are resolved you can reliably log the result of push all those pieces into the statuses array.

some pseudo code:

if (user) {
        app.props.fb.database().ref(`/users/${user.uid}/timeline`)
        .once('value')
        .then(snapshot => snapshot.val())
        .then(posts => {                
            const statuses = [];

            const promises = Object.keys(posts).map(key => {
                return app.props.fb.database().ref(`/posts/${posts[key]}`).once('value')
                    .then(snapshot => statuses.push(snapshot.val()));
            }

            return Promises.all(promises)
        })
        .then(() => console.log(statuses));
    }
Sten Muchow
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