Javascript variable is an array of objects, but does not have access to elements

Question

I use a Firebase and Javascript database, and I have code that will receive every question in each category. I have an object called category, which will contain the name, questions and the number of questions, then it will move to the list of categories (questionsPerCategory). Inside the callback function, I just do console.log(questionsPerCategory). It prints an object (array) containing categories and questions. Now my problem is that when I do console.log(questionsPerCategory[0])it says undefined, I also tried console.log(questionsPerCategory.pop()), since it is an array, but also undefined. Why is this? Below is the code and image of the console log. Additional Note: CODE A and C are asynchronous, CODE B and D are synchronous.

this.getQuestionsForEachCategory = function(callback, questions, questionsPerCategory) {
    var ref = firebase.database().ref('category');
    var questionRef = firebase.database().ref('question');
    console.log('get questions for each category');

    // CODE A
    ref.once("value", function(snapshot) {

        // CODE B
        snapshot.forEach(function(childSnapshot) {
            var key = childSnapshot.key;

            var childData = childSnapshot.val();

            var category = {
                category_name: childData.category_name
            };

            // CODE C               
            questionRef.orderByChild("category_name").equalTo(childData.category_name).once("value", function(questionSnapshot){
                var count = 0;
                var q = [];

                // CODE D
                questionSnapshot.forEach(function(childQuestionSnapshot) {
                    var questionObj = childQuestionSnapshot.val();
                    count++;

                    questions.push(questionObj.question);
                    q.push(questionObj.question);
                });

                category.questions = q;
                category.questionCount = count;
                questionsPerCategory.push(category);

            });

        });
        callback(questionsPerCategory); 

    });
};

Answer 1

callback(questionsPerCategory); , .

questionsPerCategory . Promise API, .

Promise, , , . bluebird, , map.

Answer 2

:

this.getQuestionsForEachCategory = function(callback, questions) {
var ref = firebase.database().ref('category');
var questionRef = firebase.database().ref('question');
console.log('get questions for each category');
var questionsPerCategory = [];
// CODE A
ref.once("value", function(snapshot) {

    // CODE B
    snapshot.forEach(function(childSnapshot) {
        var key = childSnapshot.key;

        var childData = childSnapshot.val();

        var category = {
            category_name: childData.category_name
        };

        // CODE C               
        questionRef.orderByChild("category_name").equalTo(childData.category_name).once("value", function(questionSnapshot){
            var count = 0;
            var q = [];

            // CODE D
            questionSnapshot.forEach(function(childQuestionSnapshot) {
                var questionObj = childQuestionSnapshot.val();
                count++;

                questions.push(questionObj.question);
                q.push(questionObj.question);
            });

            category.questions = q;
            category.questionCount = count;
            questionsPerCategory.push(category);

        });
    callback(questionsPerCategory); 
    });
});
};