TDD Angular component with Jasmine using @angular/cli

calculator.component.spec.ts

  // update TestBed configuration to use MathService as one of the providers
  // in the DESCRIBE section
  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ CalculatorComponent ],
      providers: [MathService]          // <=== add this line
    })
    .compileComponents();
  }));

  // adding another test besides IT code block
  it('should call mathService to do subtraction', () => {
    const mathSvcInstance = TestBed.get(MathService);
    // Remember - what MathService SHOULD return, that's none of you business right now.
    // you are testing calculator component - Separation of Concerns
    spyOn(mathSvcInstance, 'subtract').and.returnValue(500);
    component.param1 = 5;
    component.param2 = 7;

    component.subtract();

    expect(mathSvcInstance.subtract).toHaveBeenCalledWith(component.param1, component.param2);
    expect(component.result).toBe(500);
  });