How to retrieve the child/child value in Firebase when the key is unknown

[mk01111000]

The following code enables me to return all the Expressions of the current user. In this example the following Expressions are shown in the console:

But I am not sure how to list all the titles under Expressions for the current user. The desired outcome should be: KIKKER, CPM2MAN

function f_returnUserDetails(a){
    var key;
    var childData;
    return new Promise(function(resolve, reject) { //return promise
        firebase.database().ref('/users/').orderByChild("uid").equalTo(a).on('value', function (snapshot) {
            snapshot.forEach(function(childSnapshot) {
                key = childSnapshot.key;
                childData = childSnapshot.val();
                resolve([childData, childSnapshot.key]);
            });
        });
    });
};

firebase.auth().onAuthStateChanged(function(user){
    var user = firebase.auth().currentUser;
    var uid = user.uid;
    f_returnUserDetails(uid).then((dbItems) => {
        console.log(dbItems[0].Expressions);
    });
});

[mk01111000]

The missing image from my previous post