// CORRECT - OnPush with Signals
@Component({
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `<div>{{ count() }}</div>`,
})
export class CounterComponent {
  count = signal(0);
}

// WRONG - Default change detection
@Component({
  template: `<div>{{ count }}</div>`, // Checked every cycle
})
export class CounterComponent {
  count = 0;
}

// CORRECT - Signals trigger precise updates
@Component({
  template: `
    <h1>{{ title() }}</h1>
    <p>Count: {{ count() }}</p>
  `,
})
export class DashboardComponent {
  title = signal("Dashboard");
  count = signal(0);
}

// WRONG - Mutable properties require zone.js checks
@Component({
  template: `
    <h1>{{ title }}</h1>
    <p>Count: {{ count }}</p>
  `,
})
export class DashboardComponent {
  title = "Dashboard";
  count = 0;
}

// main.ts - Zoneless Angular (v20+)
bootstrapApplication(AppComponent, {
  providers: [provideZonelessChangeDetection()],
});

// WRONG - Nested subscriptions create waterfalls
this.route.params.subscribe((params) => {
  // 1. Wait for params
  this.userService.getUser(params.id).subscribe((user) => {
    // 2. Wait for user
    this.postsService.getPosts(user.id).subscribe((posts) => {
      // 3. Wait for posts
    });
  });
});

// CORRECT - Parallel execution with forkJoin
forkJoin({
  user: this.userService.getUser(id),
  posts: this.postsService.getPosts(id),
}).subscribe((data) => {
  // Fetched in parallel
});

// CORRECT - Flatten dependent calls with switchMap
this.route.params
  .pipe(
    map((p) => p.id),
    switchMap((id) => this.userService.getUser(id)),
  )
  .subscribe();

// CORRECT - Use resolvers or blocking hydration for critical data
export const route: Route = {
  path: "profile/:id",
  resolve: { data: profileResolver }, // Fetched on server before navigation
  component: ProfileComponent,
};

// WRONG - Component fetches data on init
class ProfileComponent implements OnInit {
  ngOnInit() {
    // Starts ONLY after JS loads and component renders
    this.http.get("/api/profile").subscribe();
  }
}

// CORRECT - Lazy load feature routes
export const routes: Routes = [
  {
    path: "admin",
    loadChildren: () =>
      import("./admin/admin.routes").then((m) => m.ADMIN_ROUTES),
  },
  {
    path: "dashboard",
    loadComponent: () =>
      import("./dashboard/dashboard.component").then(
        (m) => m.DashboardComponent,
      ),
  },
];

// WRONG - Eager loading everything
import { AdminModule } from "./admin/admin.module";
export const routes: Routes = [
  { path: "admin", component: AdminComponent }, // In main bundle
];

<!-- CORRECT - Heavy component loads on demand -->
@defer (on viewport) {
<app-analytics-chart [data]="data()" />
} @placeholder {
<div class="chart-skeleton"></div>
}

<!-- WRONG - Heavy component in initial bundle -->
<app-analytics-chart [data]="data()" />

// WRONG - Imports entire barrel, breaks tree-shaking
import { Button, Modal, Table } from "@shared/components";

// CORRECT - Direct imports
import { Button } from "@shared/components/button/button.component";
import { Modal } from "@shared/components/modal/modal.component";

// CORRECT - Load heavy library on demand
async loadChart() {
  const { Chart } = await import('chart.js');
  this.chart = new Chart(this.canvas, config);
}

// WRONG - Bundle Chart.js in main chunk
import { Chart } from 'chart.js';

<!-- CORRECT - Efficient DOM updates -->
@for (item of items(); track item.id) {
<app-item-card [item]="item" />
}

<!-- WRONG - Entire list re-renders on any change -->
@for (item of items(); track $index) {
<app-item-card [item]="item" />
}

import { CdkVirtualScrollViewport, CdkFixedSizeVirtualScroll } from '@angular/cdk/scrolling';

@Component({
  imports: [CdkVirtualScrollViewport, CdkFixedSizeVirtualScroll],
  template: `
    <cdk-virtual-scroll-viewport itemSize="50" class="viewport">
      <div *cdkVirtualFor="let item of items" class="item">
        {{ item.name }}
      </div>
    </cdk-virtual-scroll-viewport>
  `
})

// CORRECT - Pure pipe, memoized
@Pipe({ name: 'filterActive', standalone: true, pure: true })
export class FilterActivePipe implements PipeTransform {
  transform(items: Item[]): Item[] {
    return items.filter(i => i.active);
  }
}

// Template
@for (item of items() | filterActive; track item.id) { ... }

// WRONG - Method called every change detection
@for (item of getActiveItems(); track item.id) { ... }

// CORRECT - Computed, cached until dependencies change
export class ProductStore {
  products = signal<Product[]>([]);
  filter = signal('');

  filteredProducts = computed(() => {
    const f = this.filter().toLowerCase();
    return this.products().filter(p =>
      p.name.toLowerCase().includes(f)
    );
  });
}

// WRONG - Recalculates every access
get filteredProducts() {
  return this.products.filter(p =>
    p.name.toLowerCase().includes(this.filter)
  );
}

// app.config.ts
import {
  provideClientHydration,
  withIncrementalHydration,
} from "@angular/platform-browser";

export const appConfig: ApplicationConfig = {
  providers: [
    provideClientHydration(withIncrementalHydration(), withEventReplay()),
  ],
};

<!-- Critical above-the-fold content -->
<app-header />
<app-hero />

<!-- Below-fold deferred with hydration triggers -->
@defer (hydrate on viewport) {
<app-product-grid />
} @defer (hydrate on interaction) {
<app-chat-widget />
}

@Injectable({ providedIn: "root" })
export class DataService {
  private http = inject(HttpClient);
  private transferState = inject(TransferState);
  private platformId = inject(PLATFORM_ID);

  getData(key: string): Observable<Data> {
    const stateKey = makeStateKey<Data>(key);

    if (isPlatformBrowser(this.platformId)) {
      const cached = this.transferState.get(stateKey, null);
      if (cached) {
        this.transferState.remove(stateKey);
        return of(cached);
      }
    }

    return this.http.get<Data>(`/api/${key}`).pipe(
      tap((data) => {
        if (isPlatformServer(this.platformId)) {
          this.transferState.set(stateKey, data);
        }
      }),
    );
  }
}

<!-- CORRECT - New control flow (faster, smaller bundle) -->
@if (user()) {
<span>{{ user()!.name }}</span>
} @else {
<span>Guest</span>
} @for (item of items(); track item.id) {
<app-item [item]="item" />
} @empty {
<p>No items</p>
}

<!-- WRONG - Legacy structural directives -->
<span *ngIf="user; else guest">{{ user.name }}</span>
<ng-template #guest><span>Guest</span></ng-template>

// CORRECT - Precompute in component
class Component {
  items = signal<Item[]>([]);
  sortedItems = computed(() =>
    [...this.items()].sort((a, b) => a.name.localeCompare(b.name))
  );
}

// Template
@for (item of sortedItems(); track item.id) { ... }

// WRONG - Sorting in template every render
@for (item of items() | sort:'name'; track item.id) { ... }

// CORRECT - Selective subscription
@Component({
  template: `<span>{{ userName() }}</span>`,
})
class HeaderComponent {
  private store = inject(Store);
  // Only re-renders when userName changes
  userName = this.store.selectSignal(selectUserName);
}

// WRONG - Subscribing to entire state
@Component({
  template: `<span>{{ state().user.name }}</span>`,
})
class HeaderComponent {
  private store = inject(Store);
  // Re-renders on ANY state change
  state = toSignal(this.store);
}

// CORRECT - Feature-scoped store
@Injectable() // NOT providedIn: 'root'
export class ProductStore { ... }

@Component({
  providers: [ProductStore], // Scoped to component tree
})
export class ProductPageComponent {
  store = inject(ProductStore);
}

// WRONG - Everything in global store
@Injectable({ providedIn: 'root' })
export class GlobalStore {
  // Contains ALL app state - hard to tree-shake
}

import { takeUntilDestroyed } from '@angular/core/rxjs-interop';

@Component({...})
export class DataComponent {
  private destroyRef = inject(DestroyRef);

  constructor() {
    this.data$.pipe(
      takeUntilDestroyed(this.destroyRef)
    ).subscribe(data => this.process(data));
  }
}

// WRONG - Manual subscription management
export class DataComponent implements OnDestroy {
  private subscription!: Subscription;

  ngOnInit() {
    this.subscription = this.data$.subscribe(...);
  }

  ngOnDestroy() {
    this.subscription.unsubscribe(); // Easy to forget
  }
}

// CORRECT - No subscription needed
@Component({
  template: `<div>{{ data().name }}</div>`,
})
export class Component {
  data = toSignal(this.service.data$, { initialValue: null });
}

// WRONG - Manual subscription
@Component({
  template: `<div>{{ data?.name }}</div>`,
})
export class Component implements OnInit, OnDestroy {
  data: Data | null = null;
  private sub!: Subscription;

  ngOnInit() {
    this.sub = this.service.data$.subscribe((d) => (this.data = d));
  }

  ngOnDestroy() {
    this.sub.unsubscribe();
  }
}