Mocking constructor of a class with jasmine, jest or ts-mockito

Question

I have a group of classes that receive some dependencies outside angular in this way.

import {TypeATest} from '...../TypeA.model'
import { TypeBTest } from '..../TypeB.model'
import { SomeDependency } from './services/SomeDependency'
import { SomeAnother } from './services/SomeAnother'
  // ....

@Injectable({
  providedIn: 'root'
})
export class TestingService {
  this.activeTest: AnyTestType;

  constructor(private readonly injectorService: InjectorService) {}

  loadTest(TypeOfTest) {
    const someDependency = this.injectorService.get(SomeDependency)
    const someAnother = this.injectorService.get(SomeAnother)
      switch(TypeOfTest) {
        case TypeA:
            injector
            this.activeTest = new TypeATest(someDependency, someAnother);
            break;
        case TypeB:
            this.activeTest = new TypeBTest(someAnother);
            break;
      }
  }

  startTest(){
    this.activeTest.start()
  }

// .. more this.activeTest uses...
}

I am unit testing the service that loads that external classes but I do not want to create TypeATest , TypeBTest or similar but just mock the result (they all have the same API) but I am not able to find how to mock them. Is there a way to do this?

Answer 1

The two constructors are named exports of their respective modules.

You can mock the entire module using jest.mock with a module factory:

jest.mock('...../TypeA.model', () => {
  const start = jest.fn();
  const result = { start };
  return jest.fn(() => result);  
});

test('something', () => {
  // ...
});

...or mock only the named export of the module using jest.spyOn :

import * as TypeB from '..../TypeB.model';

test('something', () => {
  const spy = jest.spyOn(TypeB, 'TypeBTest');
  const start = jest.fn();
  spy.mockReturnValue({ start });
  // ...
})