angular-best-practices

What this skill does

# Angular Best Practices

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

## When to Use
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" }