Retry logic

todo.service.spec.ts

it('getSingleTodo should throw an error if 3 request attempts fail', (done) => {
    const id = 1;
    const errMessage = `Failed to fetch item with id ${id}`;
    service.getSingleTodo(id).subscribe(
      data => { fail('The request is supposed to throw an error') },
      (error: string) => { expect(error).toEqual(errMessage); done(); },
    );

    const retryCount = 3;
    for (let i = 0; i <= retryCount; i++) {
      httpMock
        .expectOne({url: 'https://jsonplaceholder.typicode.com/todos/1', method: 'GET'})
        .flush({}, { status: 404, statusText: errMessage });
    }
  });