angular-best-practices

Angular Best Practices

Critical rules (non-obvious)

• Always unsubscribe from Observables in ngOnDestroy — use takeUntilDestroyed() (Angular 16+) or Subject + takeUntil
• ChangeDetectionStrategy.OnPush: component only updates when input reference changes or async pipe emits — use for all leaf components
• Never mutate input objects/arrays: OnPush won't detect mutation; create new reference instead
• **trackBy is mandatory on *ngFor** with dynamic lists — without it, every change re-renders all DOM nodes
• async pipe auto-unsubscribes — prefer it over manual subscription in templates

Component with OnPush + signals (Angular 17+)

@Component({
  selector: "app-product-list",
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `
    @for (product of products(); track product.id) {
      <app-product-card [product]="product" />
    }
    @if (loading()) { <app-spinner /> }
  `,
})
export class ProductListComponent {
  products = input.required<Product[]>();
  loading = input(false);

  // Computed signal
  total = computed(() => this.products().length);
}

Service with signals store pattern

@Injectable({ providedIn: "root" })
export class CartService {
  private _items = signal<CartItem[]>([]);

  items = this._items.asReadonly();
  total = computed(() => this._items().reduce((sum, i) => sum + i.price * i.qty, 0));

  addItem(item: CartItem) {
    this._items.update(items =>
      items.some(i => i.id === item.id)
        ? items.map(i => i.id === item.id ? { ...i, qty: i.qty + 1 } : i)
        : [...items, { ...item, qty: 1 }]
    );
  }
}

HTTP with interceptors

// auth interceptor
export const authInterceptor: HttpInterceptorFn = (req, next) => {
  const token = inject(AuthService).token();
  if (!token) return next(req);
  return next(req.clone({ setHeaders: { Authorization: `Bearer ${token}` } })).pipe(
    catchError(err => {
      if (err.status === 401) inject(Router).navigate(["/login"]);
      return throwError(() => err);
    })
  );
};

// Register in app.config.ts
provideHttpClient(withInterceptors([authInterceptor]))

RxJS: key operators (non-obvious behavior)

// switchMap: cancels previous — good for search, bad for saves
search$.pipe(
  debounceTime(300),
  distinctUntilChanged(),
  switchMap(term => this.api.search(term))  // cancels in-flight request on new input
)

// exhaustMap: ignores new while processing — good for login button
loginClick$.pipe(
  exhaustMap(() => this.auth.login(credentials))  // prevents double-submit
)

// mergeMap: parallel — good for independent operations
ids$.pipe(mergeMap(id => this.api.fetch(id), 3))  // 3 concurrent max

// combineLatest vs withLatestFrom:
// combineLatest: emits when ANY source emits
// withLatestFrom: emits only when primary source emits, takes latest from secondary
primary$.pipe(withLatestFrom(secondary$))  // common for "take latest filter value on button click"

Auto-unsubscribe pattern

// Angular 16+ (preferred)
@Component({...})
export class MyComponent {
  private destroyRef = inject(DestroyRef);

  ngOnInit() {
    this.data$.pipe(takeUntilDestroyed(this.destroyRef)).subscribe(...);
  }
}

// Before Angular 16
export class MyComponent implements OnDestroy {
  private destroy$ = new Subject<void>();
  ngOnInit() { this.data$.pipe(takeUntil(this.destroy$)).subscribe(...); }
  ngOnDestroy() { this.destroy$.next(); this.destroy$.complete(); }
}

Lazy loading + standalone components

// app.routes.ts
export const routes: Routes = [
  {
    path: "admin",
    loadChildren: () => import("./admin/admin.routes").then(m => m.ADMIN_ROUTES),
    canMatch: [adminGuard],
  },
];

// Standalone component (Angular 15+)
@Component({
  standalone: true,
  imports: [CommonModule, RouterModule, ReactiveFormsModule],
  template: `...`,
})
export class ProfileComponent {}

Common pitfalls

| Pitfall | Fix | |---|---| | Memory leak from unsubscribed Observable | Use takeUntilDestroyed() or async pipe | | ExpressionChangedAfterChecked error | Defer with afterNextRender() or move to signals | | Heavy computation in template | Move to computed() signal or pipe(map(...)) | | *ngIf with async pipe fetches twice | Use as syntax: *ngIf="data$ \| async as data" | | Zone.js performance in loops | Use ChangeDetectionStrategy.OnPush + signals |