Why should you care?

DI enables swapping real APIs for mocks in tests and keeps constructors declarable.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

@Component({ standalone: true, template: `<pre>{{ summary() }}</pre>` })
export class SummaryComponent {
  private accounts = inject(AccountService);
  summary = toSignal(
    this.accounts.list().pipe(map(list => `${list.length} accounts`)),
    { initialValue: 'Loading...' }
  );
}
// Test override
TestBed.configureTestingModule({
  providers: [{ provide: AccountService, useValue: mockAccountService }]
});

What happened?

• inject() resolves dependencies from injector tree.
• TestBed replaces AccountService with a mock — no real HTTP in unit tests.

Practice next

1. Use inject() in one standalone component.
2. Write a test that provides a fake service.
3. Try optional inject(Logger, { optional: true }).
4. Provide API_BASE_URL injection token for environment URLs.
5. Use factory provider for Logger with environment log level.

Remember

DI decouples components from concrete implementations. inject() is preferred in standalone components. Tests override providers for isolation.

Multi-region API

MeanVerse EU vs US builds inject different API_BASE_URL tokens.

Outcome: Same components, different backend URL per deployment.