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# angular-best-practices

> Angular performance optimization covering change detection, signals, zoneless, SSR hydration, bundle optimization, and rendering patterns. Use when writing, reviewing, or refactoring Angular code for optimal performance, bundle size, and rendering efficiency.

# Angular Best Practices

Comprehensive performance optimization guide for Angular applications. Contains prioritized rules for eliminating performance bottlenecks, optimizing bundles, and improving rendering.

## When to Apply

Reference these guidelines when:

- Writing new Angular components or pages
- Implementing data fetching patterns
- Reviewing code for performance issues
- Refactoring existing Angular code
- Optimizing bundle size or load times
- Configuring SSR/hydration

---

## Rule Categories by Priority

| Priority | Category              | Impact     | Focus                           |
| -------- | --------------------- | ---------- | ------------------------------- |
| 1        | Change Detection      | CRITICAL   | Signals, OnPush, Zoneless       |
| 2        | Async Waterfalls      | CRITICAL   | RxJS patterns, SSR preloading   |
| 3        | Bundle Optimization   | CRITICAL   | Lazy loading, tree shaking      |
| 4        | Rendering Performance | HIGH       | @defer, trackBy, virtualization |
| 5        | Server-Side Rendering | HIGH       | Hydration, prerendering         |
| 6        | Template Optimization | MEDIUM     | Control flow, pipes             |
| 7        | State Management      | MEDIUM     | Signal patterns, selectors      |
| 8        | Memory Management     | LOW-MEDIUM | Cleanup, subscriptions          |

---

## 1. Change Detection (CRITICAL)

### Use OnPush Change Detection

```typescript
// 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;
}
```

### Prefer Signals Over Mutable Properties

```typescript
// 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;
}
```

### Enable Zoneless for New Projects

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

**Benefits:**

- No zone.js patches on async APIs
- Smaller bundle (~15KB savings)
- Clean stack traces for debugging
- Better micro-frontend compatibility

---

## 2. Async Operations & Waterfalls (CRITICAL)

### Eliminate Sequential Data Fetching

```typescript
// 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();
```

### Avoid Client-Side Waterfalls in SSR

```typescript
// 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();
  }
}
```

---

## 3. Bundle Optimization (CRITICAL)

### Lazy Load Routes

```typescript
// 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
];
```

### Use @defer for Heavy Components

```html
<!-- 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()" />
```

### Avoid Barrel File Re-exports

```typescript
// 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";
```

### Dynamic Import Third-Party Libraries

```typescript
// 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';
```

---

## 4. Rendering Performance (HIGH)

### Always Use trackBy with @for

```html
<!-- 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" />
}
```

### Use Virtual Scrolling for Large Lists

```typescript
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>
  `
})
```

### Prefer Pure Pipes Over Methods

```typescript
// 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) { ... }
```

### Use computed() for Derived Data

```typescript
// 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)
  );
}
```

---

## 5. Server-Side Rendering (HIGH)

### Configure Incremental Hydration

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

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

### Defer Non-Critical Content

```html
<!-- 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 />
}
```

### Use TransferState for SSR Data

```typescript
@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);
        }
      }),
    );
  }
}
```

---

## 6. Template Optimization (MEDIUM)

### Use New Control Flow Syntax

```html
<!-- 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>
```

### Avoid Complex Template Expressions

```typescript
// 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) { ... }
```

---

## 7. State Management (MEDIUM)

### Use Selectors to Prevent Re-renders

```typescript
// 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);
}
```

### Colocate State with Features

```typescript
// 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
}
```

---

## 8. Memory Management (LOW-MEDIUM)

### Use takeUntilDestroyed for Subscriptions

```typescript
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
  }
}
```

### Prefer Signals Over Subscriptions

```typescript
// 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();
  }
}
```

---

## Quick Reference Checklist

### New Component

- [ ] `changeDetection: ChangeDetectionStrategy.OnPush`
- [ ] `standalone: true`
- [ ] Signals for state (`signal()`, `input()`, `output()`)
- [ ] `inject()` for dependencies
- [ ] `@for` with `track` expression

### Performance Review

- [ ] No methods in templates (use pipes or computed)
- [ ] Large lists virtualized
- [ ] Heavy components deferred
- [ ] Routes lazy-loaded
- [ ] Third-party libs dynamically imported

### SSR Check

- [ ] Hydration configured
- [ ] Critical content renders first
- [ ] Non-critical content uses `@defer (hydrate on ...)`
- [ ] TransferState for server-fetched data

---

## Resources

- [Angular Performance Guide](https://angular.dev/best-practices/performance)
- [Zoneless Angular](https://angular.dev/guide/experimental/zoneless)
- [Angular SSR Guide](https://angular.dev/guide/ssr)
- [Change Detection Deep Dive](https://angular.dev/guide/change-detection)
