ionic framework - Argument of type '(snap: DataSnapshot) => void' is not assignable to parameter of type '(a: DataSnapshot) => boolean'

Question

I've already read several questions and answers about this problem but wasn't able to solve it.

I'm using Ionic2 and I have a method which retrieves data from Firebase Database v3. I don't understand why I get following error in console when I do ionic serve:

Error TS2345: Argument of type '(snap: DataSnapshot) => void' is not assignable to parameter of type '(a: DataSnapshot) => boolean'.
  Type 'void' is not assignable to type 'boolean'.

This is the method:

constructor(private http: Http) {

    firebase.database().ref('users').orderByChild("id").on("value", function(snapshot){
                    let items = [];
                    snapshot.forEach(snap => {
                        items.push({
                            uid: snap.val().uid,
                            username: snap.val().username,
                        });
                    });
                });
            }
}

Answer 1

Waitting for answers