For loop not working after Firebase Database Read in JavaScript

Question

I have many articles in my firebase realtime database and I have named their node in a series of numbers.

I want to retrieve them using a for-loop. Before that I check how many articles are available and then run the FOR loop accordingly. But when I receive number of articles available the program stops and the for loop won't run.

Here is my code:

let numOfArticlesAvailable;

const numOfArticlesRef = firebase.database().ref('/articles/available-count');
numOfArticlesRef.on('value', function (snapshot) {
    console.log('Articles Available: ' + snapshot.val());
    numOfArticlesAvailable = snapshot.val();
    console.log(numOfArticlesAvailable);
})

for (let index = numOfArticlesAvailable; index >= 1; index--) {
    const articlesRef = firebase.database().ref(`/articles/${index}`);
    console.log(articlesRef);
    articlesRef.on('value', function (snapshot) {
        const title = snapshot.val().title;
        const content = snapshot.val().content;
        const timestamp = snapshot.val().timestamp;

        console.log(`${title} has contents "${content}" and timestamp ${timestamp}`)
    })
}

The code log the correct value of articles available but the for-loop just won't start.

You need to run the loop under numOfArticlesRef.on('value', function (snapshot) {}) callback — Ashish Ranjan
– Ashish Ranjan, Commented May 3, 2020 at 11:33
@AshishRanjan my code is going to have more further steps. Am I supposed to enclose all of it in that callback then? — Dharmaraj
– Dharmaraj, Commented May 3, 2020 at 11:34
Yeah, maybe enclose the logic under different functions and call those functions from under the callback — Ashish Ranjan
– Ashish Ranjan, Commented May 3, 2020 at 11:35

Lazar Nikolic · Accepted Answer · 2020-05-03 11:36:33Z

Your for loop is not starting because numOfArticlesRef.on('value', function (snapshot) { is async function. This means that in the moment you for loop starts, value of numOfArticlesRef is either undefined or 0. or something like that. Try to do this:

let numOfArticlesAvailable;

const numOfArticlesRef = firebase.database().ref('/articles/available-count');
numOfArticlesRef.on('value', function (snapshot) {
    console.log('Articles Available: ' + snapshot.val());
    numOfArticlesAvailable = snapshot.val();
    console.log(numOfArticlesAvailable);
for (let index = numOfArticlesAvailable; index >= 1; index--) {
    const articlesRef = firebase.database().ref(`/articles/${index}`);
    console.log(articlesRef);
    articlesRef.on('value', function (snapshot) {
        const title = snapshot.val().title;
        const content = snapshot.val().content;
        const timestamp = snapshot.val().timestamp;

        console.log(`${title} has contents "${content}" and timestamp ${timestamp}`)
    })
}

})

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