[Angular & Unit Testing] TestBed.get vs Injector

Both what "TestBed.get" & "injector" trying to do is get service for the test component. 

But there is some diffenece which determined when use which.

Register the service: 

You need to mock the service, not using the real service!

You can use mock class, or just an object.

userServiceStub = {
  isLoggedIn: true,
  user: { name: 'Test User'}
};

TestBed.configureTestingModule({
   declarations: [ WelcomeComponent ],
// providers:    [ UserService ]  // NO! Don't provide the real service!
                                  // Provide a test-double instead
   providers:    [ {provide: UserService, useValue: userServiceStub } ]
});

"Root Injector"

You may be able to get the service from the root injector via TestBed.get. This is easier to remember and less verbose. But it only works when Angular injects the component with the service instance in the test's root injector. 

// UserService from the root injector
userService = TestBed.get(UserService);

"Injected service for Component"

// UserService actually injected into the component
userService = fixture.debugElement.injector.get(UserService);

Code Example:

beforeEach(() => {
  // stub UserService for test purposes
  userServiceStub = {
    isLoggedIn: true,
    user: { name: 'Test User'}
  };

  TestBed.configureTestingModule({
     declarations: [ WelcomeComponent ],
     providers:    [ {provide: UserService, useValue: userServiceStub } ]
  });

  fixture = TestBed.createComponent(WelcomeComponent);
  comp    = fixture.componentInstance;

  // UserService from the root injector
  userService = TestBed.get(UserService);

  //  get the "welcome" element by CSS selector (e.g., by class name)
  de = fixture.debugElement.query(By.css('.welcome'));
  el = de.nativeElement;
});

it('should welcome the user', () => {
  fixture.detectChanges();
  const content = el.textContent;
  expect(content).toContain('Welcome', '"Welcome ..."');
  expect(content).toContain('Test User', 'expected name');
});

it('should welcome "Bubba"', () => {
  userService.user.name = 'Bubba'; // welcome message hasn't been shown yet
  fixture.detectChanges();
  expect(el.textContent).toContain('Bubba');
});

it('should request login if not logged in', () => {
  userService.isLoggedIn = false; // welcome message hasn't been shown yet
  fixture.detectChanges();
  const content = el.textContent;
  expect(content).not.toContain('Welcome', 'not welcomed');
  expect(content).toMatch(/log in/i, '"log in"');
});