CoreModule vs SharedModule Complete Angular Architecture Guide

Hey everyone, it’s CodingBear here! With over two decades of experience in Vue.js and Angular development, I’ve seen countless projects struggle with module organization. Today we’re diving deep into one of the most fundamental yet misunderstood concepts in Angular architecture: the difference between CoreModule and SharedModule. Whether you’re building enterprise applications or smaller projects, understanding this distinction is crucial for creating maintainable, scalable codebases. Let’s break down when to use each module type and how they can dramatically improve your application’s structure.

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Understanding CoreModule: The Foundation of Your Application

The CoreModule in Angular is designed to contain services and functionality that are essential to your application and should be loaded only once. Think of it as the foundation of your house - you only need to pour the foundation once, and everything else builds upon it.

What Belongs in CoreModule?

Singleton Services: These are services that should have only one instance throughout your application. Common examples include:

• Authentication services
• HTTP interceptors
• Logging services
• Configuration services
• User session management

@Injectable({
  providedIn: 'root'
})
export class AuthService {
  private isAuthenticated = false;
  
  login(credentials: any): Observable<boolean> {
    // Authentication logic
    return this.http.post('/api/auth', credentials);
  }
  
  logout(): void {
    this.isAuthenticated = false;
  }
}

Application-Wide Components: Components that appear only once in your app, typically in the main layout:

• Navigation bars
• Footers
• Sidebars
• Loading spinners
• Error handling components Why CoreModule is Loaded Once? The CoreModule should be imported only in your AppModule (the root module). This ensures that services provided in CoreModule are true singletons. If you import CoreModule in multiple feature modules, you’ll end up with multiple instances of services that should be singletons, leading to unpredictable behavior and bugs that are difficult to trace. CoreModule Implementation Example:

@NgModule({
  declarations: [
    NavbarComponent,
    FooterComponent,
    LoadingSpinnerComponent
  ],
  imports: [
    CommonModule,
    HttpClientModule
  ],
  providers: [
    AuthService,
    LoggerService,
    ConfigService,
    {
      provide: HTTP_INTERCEPTORS,
      useClass: AuthInterceptor,
      multi: true
    }
  ],
  exports: [
    NavbarComponent,
    FooterComponent
  ]
})
export class CoreModule {
  constructor(@Optional() @SkipSelf() parentModule: CoreModule) {
    if (parentModule) {
      throw new Error(
        'CoreModule is already loaded. Import it in the AppModule only');
    }
  }
}

This constructor pattern prevents CoreModule from being imported more than once, enforcing the singleton nature of its services.

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Exploring SharedModule: Your Reusable Component Library

The SharedModule serves a completely different purpose - it’s all about reusability and sharing common functionality across multiple parts of your application. If CoreModule is your foundation, SharedModule is your toolbox of reusable components and directives that different feature modules can use.

What Belongs in SharedModule?

Common Components: UI components that are used across multiple feature modules:

• Buttons
• Form controls
• Modals
• Data tables
• Cards
• Custom input fields Pipes and Directives: Reusable transformations and behavior modifiers:
• Custom pipes for formatting
• Structural directives
• Attribute directives Common Modules: Frequently used Angular modules that multiple feature modules need:
• CommonModule
• FormsModule
• ReactiveFormsModule
• Material modules (if using Angular Material)

// A reusable button component example
@Component({
  selector: 'app-primary-button',
  template: `
    <button 
      [class]="buttonClasses" 
      [disabled]="disabled"
      (click)="onClick.emit($event)">
      <ng-content></ng-content>
    </button>
  `,
  styleUrls: ['./primary-button.component.scss']
})
export class PrimaryButtonComponent {
  @Input() variant: 'primary' | 'secondary' = 'primary';
  @Input() size: 'small' | 'medium' | 'large' = 'medium';
  @Input() disabled: boolean = false;
  @Output() onClick = new EventEmitter<Event>();
  
  get buttonClasses(): string {
    return `btn btn-${this.variant} btn-${this.size}`;
  }
}

SharedModule Implementation:

@NgModule({
  declarations: [
    PrimaryButtonComponent,
    SecondaryButtonComponent,
    DataTableComponent,
    CustomModalComponent,
    HighlightDirective,
    CurrencyFormatPipe
  ],
  imports: [
    CommonModule,
    FormsModule,
    ReactiveFormsModule,
    MatButtonModule,
    MatDialogModule
  ],
  exports: [
    CommonModule,
    FormsModule,
    ReactiveFormsModule,
    PrimaryButtonComponent,
    SecondaryButtonComponent,
    DataTableComponent,
    CustomModalComponent,
    HighlightDirective,
    CurrencyFormatPipe,
    MatButtonModule,
    MatDialogModule
  ]
})
export class SharedModule { }

Key Benefits of SharedModule:

1. Consistency: Ensures UI and behavior consistency across your application
2. Maintainability: Changes to shared components only need to be made in one place
3. Reduced Bundle Size: Properly organized shared modules can help with lazy loading optimization
4. Team Collaboration: Different teams can work on different feature modules while sharing common components

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Architectural Patterns and Best Practices

When to Use Which Module?

CoreModule Use Cases:

• Authentication and authorization services
• HTTP interceptors for global request handling
• Application configuration that needs to be loaded once
• Analytics and logging services
• Web socket connections
• Global state management services SharedModule Use Cases:
• UI component library
• Form controls and validation
• Common pipes for data transformation
• Directives for common behaviors
• Layout components used across features
• Third-party module re-exports

Advanced Module Organization Strategies

Multiple Shared Modules: For large applications, consider creating multiple shared modules:

• UiComponentsModule for basic UI elements
• FormControlsModule for form-related components
• LayoutModule for layout components
• PipesModule for all custom pipes Lazy Loading Considerations: While CoreModule is always eager-loaded, SharedModule can be imported by both eager and lazy-loaded modules. Be mindful of bundle sizes when organizing shared functionality.

// Feature module using SharedModule
@NgModule({
  declarations: [UserProfileComponent],
  imports: [
    CommonModule,
    SharedModule, // Reusable components
    UserProfileRoutingModule
  ]
})
export class UserProfileModule { }

Common Anti-Patterns to Avoid

1. Putting Services in SharedModule: This can create multiple service instances
2. Importing CoreModule in Feature Modules: Breaks singleton pattern
3. Overloading SharedModule: Making it too large and including unnecessary dependencies
4. Circular Dependencies: Between SharedModule and feature modules

Testing Strategies

CoreModule Testing: Focus on singleton behavior and application-wide functionality

describe('AuthService', () => {
  let service: AuthService;
  
  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [CoreModule],
      providers: [AuthService]
    });
    service = TestBed.inject(AuthService);
  });
  
  it('should be singleton', () => {
    const anotherInstance = TestBed.inject(AuthService);
    expect(service).toBe(anotherInstance);
  });
});

SharedModule Testing: Focus on component reusability and isolation

describe('PrimaryButtonComponent', () => {
  let component: PrimaryButtonComponent;
  let fixture: ComponentFixture<PrimaryButtonComponent>;
  
  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [SharedModule]
    }).compileComponents();
  });
  
  beforeEach(() => {
    fixture = TestBed.createComponent(PrimaryButtonComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });
  
  it('should emit click event', () => {
    const emitSpy = spyOn(component.onClick, 'emit');
    fixture.nativeElement.querySelector('button').click();
    expect(emitSpy).toHaveBeenCalled();
  });
});

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Understanding the distinction between CoreModule and SharedModule is fundamental to building well-structured Angular applications. CoreModule handles your singleton services and application-wide components that form the foundation of your app, while SharedModule manages reusable components and directives that promote consistency across features. By following these patterns, you’ll create more maintainable, testable, and scalable applications. Remember, good architecture isn’t about following rules blindly, but understanding the principles behind them. Happy coding, and may your modules always be well-organized! Keep building amazing things, and don’t hesitate to reach out if you have questions about Angular architecture patterns. This is CodingBear, signing off!

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