How to use the @angular/fire/database.snapshotChanges function in @angular/fire

it('should handle empty sets', done => {
    const aref = createRef(rando());
    aref.set({});
    snapshotChanges(aref).pipe(take(1)).subscribe(data => {
      expect(data.length).toEqual(0);
    }).add(done);
  });

namefilter$.pipe(switchMap(name => {
      const filteredRef = name ? aref.child('name').equalTo(name) : aref
      return snapshotChanges(filteredRef);
    }),take(2)).subscribe(data => {
      count = count + 1;

function prepareSnapshotChanges(opts: { events?: ChildEvent[], skipnumber: number } = { skipnumber: 0 }) {
    const { events, skipnumber } = opts;
    const aref = createRef(rando());
    const snapChanges = snapshotChanges(aref, events);
    return {
      snapChanges: snapChanges.pipe(skip(skipnumber)),
      ref: aref
    };
  }