Argument of type '(snap: DataSnapshot) => void

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

The forEach method in the DataSnapshot has this signature:

forEach(action: (a: firebase.database.DataSnapshot) => boolean): boolean;

as the action can return true to short-circuit the enumeration and return early. If a falsy value is returned, enumeration continues normally. (This is mentioned in the documentation.)

To appease the TypeScript compiler, the simplest solution would be to return false (to continue enumerating the child snapshots):

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