JavaScript promise not working with resolve()

I was having some problem with promise in JavaScript. So what I am trying to do is I wanted to retrieve from firebase, then store all the result returned into array. After that I will perform some sorting on the array. Here is my code:

    let promise = new Promise((resolve, reject) => {
    var query = firebase.database().ref('');
        query.once( 'value', data => {
            data.forEach(subtypeSnapshot => {
                var itemData = ;

                var query = firebase.database().ref('').child(itemKey);
                query.once( 'value', data => {
                    var itemDetail = ;
                    datasetarr.push();
                });
            }); 
            resolve(datasetarr);
        });             
    });

With this set of code, from the first console.log inside the promise, I managed to get these in my console:

With these, it means there is nothing wrong with my firebase retrieval. Afterwhich, I wanted to store each of them into array and this is the part:

datasetarr.push({type: subtype, quantity: quantity});

After done everything and I resolved the promise, when promise is done, I then print out the items in array. However, nothing is printed out at the for loop inside .then(). Any ideas?