Retrieve an Array Data from Firebase

Question

I have a code like this to retrieve data from Firebase using Javascript.

for (let i = 0; i < 60; i++) {
            var getMenit = ('0' + i).slice(-2);
            label_water_level_per_menit.push(getMenit);

            firebaseRef.child(`${hours}-${getMenit}/Level Ketinggian Air`).once("value", function (snapshot) {
                if(snapshot.exists()){
                    var data = snapshot.val();
                    data_water_level_per_menit.push(data);
                }
                else{
                    data_water_level_per_menit.push(0);
                }
            });
        }

console.log(data_water_level_per_menit);

The console log is show like this.

And when i retrieve back the data using this code:

for (i = 0; i < 60; i++){
      console.log(data_water_level_per_menit[i]);
}

I have undefined value.