Angular Agent Skill

Angular — Standalone, Signals, Zoneless (Angular 20/21+)

> Build Angular the way it ships in 2026: standalone components, signals as the reactivity model, zoneless change detection, built-in control flow, and inject() DI. Treat NgModules, *ngFor, and @Input() decorators as legacy you only touch to migrate.

Not this skill

AngularJS (1.x) is out of scope entirely — this skill is Angular 2+ only and the APIs do not map.

Route elsewhere for React → ../react/SKILL.md; Next.js App Router → ../nextjs/SKILL.md;

Vue/Nuxt, Svelte, SolidJS, Astro → ../vue-nuxt/SKILL.md, ../svelte/SKILL.md,

../solid-js/SKILL.md, ../astro/SKILL.md; a pure TypeScript language question (generics,

narrowing, tsconfig) with no Angular dimension → ../typescript/SKILL.md; a **standalone NestJS

API → ../nestjs/SKILL.md; a generic Node service → ../nodejs/SKILL.md; cross-framework

Playwright e2e strategy** → ../testing-web/SKILL.md / ../e2e-testing/SKILL.md. Angular Universal

SSR and Angular's own ng test (Vitest) setup stay here.

Decide first

| Situation | Do this | Why |

|-----------|---------|-----|

| Greenfield app / new feature | Zoneless + signals + standalone by default. ng new (Angular 21) already excludes Zone.js. | The defaults shipped stable in v20-v21; fight them and you write more code that the framework now does for you. |

| Brownfield NgModule + decorator app | Migrate incrementally with the schematics in references/migration.md (NgModule→standalone, control flow, decorator→signal inputs, Zone.js→zoneless, Karma→Vitest); do not rewrite. Keep Zone.js until you flip it on purpose. | A working app that uses *ngIf is not a bug. Churn introduces risk for no user value. |

| "View not updating" complaint | Jump to the change-detection section: signal not read in template, OnPush without a signal, or stale Zone.js assumption. | Zoneless means a mutation that no signal observes will never repaint — the fix is structural, not a detectChanges() call. |

The modern baseline

No NgModules. Bootstrap a standalone root component and configure providers in app.config.ts.

// main.ts
import { bootstrapApplication } from '@angular/platform-browser';
import { App } from './app/app';
import { appConfig } from './app/app.config';

bootstrapApplication(App, appConfig);

// app/app.config.ts
import { ApplicationConfig, provideZonelessChangeDetection } from '@angular/core';
import { provideRouter } from '@angular/router';
import { provideHttpClient, withFetch } from '@angular/common/http';
import { routes } from './app.routes';

export const appConfig: ApplicationConfig = {
  providers: [
    provideZonelessChangeDetection(), // no Zone.js; CD driven by signals + events
    provideRouter(routes),
    provideHttpClient(withFetch()),
  ],
};

• Rule: one bootstrapApplication call, providers in app.config.ts. Why: NgModule bootstrap (platformBrowserDynamic().bootstrapModule(AppModule)) is the legacy path — more files, slower to reason about.
• Rule: components are standalone by default (the standalone flag is implied in v20+; do not write standalone: true in new code, and never write standalone: false). Why: standalone is the framework default now; the flag is noise.

Bad → Good

// Bad — NgModule wiring for a single component
@NgModule({ declarations: [UserCard], imports: [CommonModule], exports: [UserCard] })
export class UserCardModule {}

// Good — standalone component imports only what it uses
@Component({
  selector: 'app-user-card',
  imports: [DatePipe],
  template: `<p>{{ joined() | date }}</p>`,
})
export class UserCard {
  joined = input.required<Date>();
}

Signals as the reactivity model

signal() holds state, computed() derives it, effect() runs side effects, linkedSignal() resets writable state when a source changes.

import { signal, computed, effect, linkedSignal } from '@angular/core';

const qty = signal(1);
const price = signal(9.99);
const total = computed(() => qty() * price());        // derived — recomputes lazily
const draftQty = linkedSignal(() => qty());            // writable, resets when qty changes

effect(() => console.log('total changed:', total())); // side effect ONLY (logging, DOM, sync)

• Rule: derive with computed(), never with effect(). Why: an effect() that writes a signal to "compute" a value creates a hidden dependency graph that loops or fires extra times — computed() is pull-based and memoized.
• Rule: effect() is for side effects (logging, localStorage, imperative DOM, 3rd-party libs), not for keeping two signals in sync. Why: synced state belongs in computed() or linkedSignal().

Component I/O is signal-based: input(), input.required(), output(), model() for two-way.

Bad → Good

// Bad — decorator I/O, mutable, no type-safety on required
@Input() userId!: string;
@Output() saved = new EventEmitter<User>();

// Good — signal inputs/outputs
userId = input.required<string>();          // read as userId()
saved = output<User>();                     // emit with saved.emit(user)
name = model('');                           // two-way: [(name)]="..."

Templates: built-in control flow

Use @if / @for / @switch / @defer. The legacy *ngIf / *ngFor / *ngSwitch structural directives are deprecated.

@if (user(); as u) {
  <h1>{{ u.name }}</h1>
} @else {
  <app-spinner />
}

@for (item of items(); track item.id) {
  <li>{{ item.label }}</li>
} @empty {
  <li>No items</li>
}

@defer (on viewport) {
  <app-heavy-chart [data]="rows()" />
} @placeholder {
  <div class="skeleton"></div>
}

• Rule: every @for must declare track. Why: it is required syntax (the template won't compile without it) and it controls DOM reuse — track item.id over track $index when items have stable identity, or the DOM thrashes on reorder.
• Rule: reach for @defer to lazy-load heavy sub-trees and enable incremental hydration. Why: it ships less JS up front without manual loadComponent plumbing.

Bad → Good

<!-- Bad — legacy structural directive, no tracking -->
<li *ngFor="let item of items">{{ item.label }}</li>

<!-- Good — built-in control flow with track -->
@for (item of items(); track item.id) { <li>{{ item.label }}</li> }

Data fetching

Default to signal-based resources; reach for HttpClient + RxJS only when you need streams, cancellation, or operator composition.

import { httpResource } from '@angular/common/http';
import { resource } from '@angular/core';

// httpResource — declarative GET wired to HttpClient; reactive to its URL signal
users = httpResource<User[]>(() => `/api/users?team=${this.team()}`);
// template: @if (users.isLoading()) {…} @else { @for (u of users.value(); track u.id) {…} }
// users.error()  -> error signal;  users.reload() -> refetch

// resource — any async loader (not just HTTP)
profile = resource({
  params: () => ({ id: this.userId() }),
  loader: ({ params }) => fetchProfile(params.id),
});

• Rule: httpResource()/resource() give you value(), isLoading(), error(), reload() for free — prefer them over a manual subscribe that you have to clean up. Why: less boilerplate, no leak, refetches automatically when its source signals change.
• Rule: when you genuinely need a stream (websocket, debounced search, retry/switchMap), keep HttpClient + RxJS and bridge to a signal with toSignal(). Why: signals are not streams; do not fake backpressure with effects. references/signals-rxjs.md has the signals-vs-RxJS decision matrix, toSignal/toObservable interop recipes, effect pitfalls (infinite loops, untracked reads), and takeUntilDestroyed.

DI & services

@Injectable({ providedIn: 'root' })
export class UserApi {
  private http = inject(HttpClient);          // field initializer — no constructor needed
  list = () => this.http.get<User[]>('/api/users');
}

• Rule: inject with inject(), not constructor parameters. Why: inject() works in field initializers and composes into plain functions (guards, factories); constructor DI is the legacy ergonomic.
• Rule: providedIn: 'root' for app-wide singletons. Why: tree-shakable — unused services drop from the bundle.
• Rule: HTTP cross-cutting concerns are functional interceptors: provideHttpClient(withInterceptors([authInterceptor])). Why: class interceptors with HTTP_INTERCEPTORS are the older multi-provider pattern.

Routing

// app.routes.ts
export const routes: Routes = [
  { path: 'users', loadComponent: () => import('./users/users-list').then(m => m.UsersList) },
  { path: 'users/:id', loadComponent: () => import('./users/user-detail').then(m => m.UserDetail),
    canActivate: [authGuard] },
];

export const authGuard: CanActivateFn = () => inject(AuthService).isLoggedIn();

Enable route-bound signal inputs with withComponentInputBinding() in provideRouter, then read route params as signal inputs:

provideRouter(routes, withComponentInputBinding());
// in UserDetail: id = input.required<string>();  // bound from the :id segment

• Rule: lazy-load routes with loadComponent (or loadChildren with a routes array). Why: smaller initial bundle, no NgModule needed.
• Rule: guards/resolvers are functions (CanActivateFn, ResolveFn) using inject(). Why: class-based guards are deprecated.

State

• Local/feature state → a signal service (@Injectable holding signal/computed). Simple, no library.
• App-wide state → NgRx SignalStore (signalStore, withState, withComputed, withMethods, withProps) — signals-native, pairs cleanly with resource().

export const CartStore = signalStore(
  { providedIn: 'root' },
  withState({ items: [] as Item[] }),
  withComputed(({ items }) => ({ count: computed(() => items().length) })),
  withMethods((store) => ({ add: (i: Item) => patchState(store, s => ({ items: [...s.items, i] })) })),
);

• Note: Signal Forms is experimental (prototype since Angular 21.0.0-next.2). For production forms use reactive/typed forms (FormGroup/FormControl with typed values). Why: do not ship a prototype API to users.

CLI workflow

ng new my-app                  # Angular 21: zoneless + standalone + Vitest by default
ng generate component user-card # standalone by default; no --standalone flag needed
ng generate service user-api
ng build                       # production build
ng test                        # Vitest (default runner in v21; Karma is deprecated)
ng update @angular/core @angular/cli  # version bumps + automated migrations

Testing

Use Vitest + TestBed. Provide zoneless CD in tests and set signal inputs via componentRef.

import { TestBed } from '@angular/core/testing';
import { provideZonelessChangeDetection } from '@angular/core';

it('renders the user name', async () => {
  TestBed.configureTestingModule({
    providers: [provideZonelessChangeDetection()],
  });
  const fixture = TestBed.createComponent(UserCard);
  fixture.componentRef.setInput('joined', new Date('2026-01-01'));
  await fixture.whenStable();                       // not detectChanges() — let CD settle
  expect(fixture.nativeElement.textContent).toContain('2026');
});

• Rule: set signal inputs with fixture.componentRef.setInput('name', value), never by poking the instance field. Why: setInput flows through the input pipeline and marks the view dirty.
• Rule: prefer await fixture.whenStable() over manual detectChanges() loops under zoneless. Why: it waits for the scheduler to flush instead of forcing a single synchronous pass.

Anti-patterns

| Bad | Why it's wrong | Good |

|-----|----------------|------|

| @NgModule in new code | Standalone is the default; modules add ceremony and slow analysis | Standalone component with an imports: [] array |

| *ngIf / *ngFor / *ngSwitch | Legacy structural directives; deprecated | @if / @for (… ; track id) / @switch |

| @Input() / @Output() decorators | No required-input safety, not signal-reactive | input() / input.required() / output() / model() |

| effect(() => this.total.set(a()*b())) | Effect-to-derive-state loops and double-fires | total = computed(() => a()*b()) |

| @for without track | Won't compile; if forced, DOM thrashes on reorder | track item.id (stable identity) |

| subscribe() in a component with no teardown | Memory leak; runs after the view is destroyed | toSignal() or takeUntilDestroyed() |

| ChangeDetectorRef.detectChanges() to "fix" a stale view | Masks the real cause under zoneless | Read the value through a signal so CD tracks it |

| Nested subscribe() inside subscribe() | Callback pyramid, lost cancellation | switchMap/concatMap, one subscription |

| Constructor DI only (constructor(private x: X)) | Legacy ergonomic; can't compose into functions | private x = inject(X) |

scripts/verify.sh is a heuristic copy-banlist lint — it greps your Angular sources for the banned patterns above. It is a hint, not a compiler.