Adding Proper Types to MockInstance When Using jest.fn

[pcbowers]

Currently, when I use MockInstance with jest.fn:

MockInstance(UserService, 'getUser', jest.fn())

The resulting type of this function is:

jest.Mock<any, any, any>

Ideally, it would actually be mocked to be:

jest.MockedFn<() => Observable<User>>

The best way I've found to get it to be typed properly is to add this somewhere in a typescript file, probably some global .d.ts file:

declare module 'ng-mocks' {
  function MockInstance<T extends object, K extends keyof T, S extends T[K]>(
    instance: AnyType<T>,
    name: K,
    stub: jest.Mock
  ): jest.MaybeMocked<S>;
}

That said, I was curious if anyone had a better solution out there to ensure it was typed properly, or if maybe I was missing something. Is there a better way to do this? If not, is this an overload that could be added to ng-mocks, or, since it's specific to jest, is that not something that is done?