How to spy on nested methods, jasmine

Question

I need to test a function and the only test case I need help with is to check if the dataCollection.add() method has been called . I presume a spy is needed here, but any recommendations will be much appreciated.

Here is the function I need to test containing the dataCollection.add() method:

...
constructor(private _firebase: AngularFirestore, private _read: ReadService) {}

createRootCollectionDocument$(collection: string, data: any): Observable<firebase.firestore.DocumentReference> {
   const testt = this._firebase.collection(collection);
   const dataCollection: AngularFirestoreCollection<any> = this._firebase.collection(collection);
    return Observable.from(
       dataCollection.add({ ...data }).catch((err: firebase.firestore.FirestoreError) => {
          throw err.message;
        })
     );
 }

Here is my test case as yet:

...
beforeEach(() => {
   service = TestBed.get(CrudService);
}

it('uses the firebase collection.add method', () => {
      return service.createRootCollectionDocument$(collectionStub, dataStub).toPromise().then((result) => {
         ...
      })
 });           
...

Thanks in advance!

Answer 1

You can stub out this._firebase.collection to return a mock object and then see that its add function was called.

it('uses the firebase collection.add method', () => {

      let mockAdder = jasmine.createSpyObj("adder", ["add"]);
      jasmine.spyOn(service["_firebase"], "collection").andReturn(mockAdder);

      return service.createRootCollectionDocument$(collectionStub, dataStub).toPromise().then((result) => {

          expect(mockAdder.add).toHaveBeenCalled();

      });
});