Angular Best Practices

# 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: ProfileCom