Angular 7 Component Test is using original service instead of mock

I see two issues in the code above:

• You have an extra set of square brackets in the providers array of your TestBed. I don't think this is a major issue, it is just semantically incorrect.
• When you provide HelloWorldService in the providers array, you have specified useClass, but have provided an object (which is what jasmine.createSpyObj() produces), so you should instead specify useValue.

I hope this helps.

Update:

Ok, you have come quite a long ways! You corrected the two problems I outlined above, have done the overrideProvider() before the compileComponents() as I suggested in a comment to another answer as well as wrapped the beforeEach() inside an async().

The reason I asked to see all the other code was so I could quickly put it up in the following StackBlitz for testing. As you can see in that StackBlitz, the test is now passing.

I only added one line in the beforeEach() to declare a returnValue from your spy, so that the subscribe in your component has an Observable to subscribe to:

mockHelloWorldService.getHelloWorld.and.returnValue(of('Test Message'));

I added this before the call to fixture.detectChanges() since this will invoke ngOnInit(), and the spy needs a return value set BEFORE executing ngOnInit() so that it will execute correctly and not give you the error you were seeing.

I also added a line in the spec to show how you can test that the result of the Observable is correctly set to your component variable:

expect(component.helloWorldMessage).toEqual('Test Message');

Note that you are significantly complicating your testing by specifying the HelloWorldService in the providers array of your component declaration. If you instead provide this service as a singleton in root that will simplify things a lot, including how you test. See details in the Official Docs.

Maybe your HelloWorldService is declared in the component providers. So you need to use https://angular.io/api/core/testing/TestBed#overrideComponent to replace the provider.