let promises = [];
//Return first promise from DB save to zone_obj list
firebase.database().ref('node-client/images').once('value').then(function(snapshot) {
    promises.push(snapshot.val());
});
Promise.all(promises).then(values => {
    console.log(values); // zone_obj
});

let promises = [];
//Return first promise from DB save to zone_obj list
firebase.database().ref('node-client/images').once('value').then(function(snapshot) {
    // Push the promises to the array
    promises.push(snapshot.val());
})
.then(function() {
    // Log the values when all promises are resolved
    Promise.all(promises).then(values => {
        console.log(values); // zone_obj
    });
});