---
name: angular-best-practices
description: Angular performance optimization and best practices guide. Use when writing, reviewing, or refactoring Angular code for optimal performance, bundle size, and rendering efficiency.
risk: safe
source: self
---
# 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)
