angular✓ Pass

SKILL.md Content

---
name: angular
description: Modern Angular (v20+) expert with deep knowledge of Signals, Standalone Components, Zoneless applications, SSR/Hydration, and reactive patterns.
risk: safe
source: self
date_added: '2026-02-27'
---

# Angular Expert

Master modern Angular development with Signals, Standalone Components, Zoneless applications, SSR/Hydration, and the latest reactive patterns.

## When to Use This Skill

- Building new Angular applications (v20+)
- Implementing Signals-based reactive patterns
- Creating Standalone Components and migrating from NgModules
- Configuring Zoneless Angular applications
- Implementing SSR, prerendering, and hydration
- Optimizing Angular performance
- Adopting modern Angular patterns and best practices

## Do Not Use This Skill When

- Migrating from AngularJS (1.x) → use `angular-migration` skill
- Working with legacy Angular apps that cannot upgrade
- General TypeScript issues → use `typescript-expert` skill

## Instructions

1. Assess the Angular version and project structure
2. Apply modern patterns (Signals, Standalone, Zoneless)
3. Implement with proper typing and reactivity
4. Validate with build and tests

## Safety

- Always test changes in development before production
- Gradual migration for existing apps (don't big-bang refactor)
- Keep backward compatibility during transitions

---

## Angular Version Timeline

| Version        | Release | Key Features                                           |
| -------------- | ------- | ------------------------------------------------------ |
| **Angular 20** | Q2 2025 | Signals stable, Zoneless stable, Incremental hydration |
| **Angular 21** | Q4 2025 | Signals-first default, Enhanced SSR                    |
| **Angular 22** | Q2 2026 | Signal Forms, Selectorless components                  |

---

## 1. Signals: The New Reactive Primitive

Signals are Angular's fine-grained reactivity system, replacing zone.js-based change detection.

### Core Concepts

```typescript
import { signal, computed, effect } from "@angular/core";

// Writable signal
const count = signal(0);

// Read value
console.log(count()); // 0

// Update value
count.set(5); // Direct set
count.update((v) => v + 1); // Functional update

// Computed (derived) signal
const doubled = computed(() => count() * 2);

// Effect (side effects)
effect(() => {
  console.log(`Count changed to: ${count()}`);
});
```

### Signal-Based Inputs and Outputs

```typescript
import { Component, input, output, model } from "@angular/core";

@Component({
  selector: "app-user-card",
  standalone: true,
  template: `
    <div class="card">
      <h3>{{ name() }}</h3>
      <span>{{ role() }}</span>
      <button (click)="select.emit(id())">Select</button>
    </div>
  `,
})
export class UserCardComponent {
  // Signal inputs (read-only)
  id = input.required<string>();
  name = input.required<string>();
  role = input<string>("User"); // With default

  // Output
  select = output<string>();

  // Two-way binding (model)
  isSelected = model(false);
}

// Usage:
// <app-user-card [id]="'123'" [name]="'John'" [(isSelected)]="selected" />
```

### Signal Queries (ViewChild/ContentChild)

```typescript
import {
  Component,
  viewChild,
  viewChildren,
  contentChild,
} from "@angular/core";

@Component({
  selector: "app-container",
  standalone: true,
  template: `
    <input #searchInput />
    <app-item *ngFor="let item of items()" />
  `,
})
export class ContainerComponent {
  // Signal-based queries
  searchInput = viewChild<ElementRef>("searchInput");
  items = viewChildren(ItemComponent);
  projectedContent = contentChild(HeaderDirective);

  focusSearch() {
    this.searchInput()?.nativeElement.focus();
  }
}
```

### When to Use Signals vs RxJS

| Use Case                | Signals         | RxJS                             |
| ----------------------- | --------------- | -------------------------------- |
| Local component state   | ✅ Preferred    | Overkill                         |
| Derived/computed values | ✅ `computed()` | `combineLatest` works            |
| Side effects            | ✅ `effect()`   | `tap` operator                   |
| HTTP requests           | ❌              | ✅ HttpClient returns Observable |
| Event streams           | ❌              | ✅ `fromEvent`, operators        |
| Complex async flows     | ❌              | ✅ `switchMap`, `mergeMap`       |

---

## 2. Standalone Components

Standalone components are self-contained and don't require NgModule declarations.

### Creating Standalone Components

```typescript
import { Component } from "@angular/core";
import { CommonModule } from "@angular/common";
import { RouterLink } from "@angular/router";

@Component({
  selector: "app-header",
  standalone: true,
  imports: [CommonModule, RouterLink], // Direct imports
  template: `
    <header>
      <a routerLink="/">Home</a>
      <a routerLink="/about">About</a>
    </header>
  `,
})
export class HeaderComponent {}
```

### Bootstrapping Without NgModule

```typescript
// main.ts
import { bootstrapApplication } from "@angular/platform-browser";
import { provideRouter } from "@angular/router";
import { provideHttpClient } from "@angular/common/http";
import { AppComponent } from "./app/app.component";
import { routes } from "./app/app.routes";

bootstrapApplication(AppComponent, {
  providers: [provideRouter(routes), provideHttpClient()],
});
```

### Lazy Loading Standalone Components

```typescript
// app.routes.ts
import { Routes } from "@angular/router";

export const routes: Routes = [
  {
    path: "dashboard",
    loadComponent: () =>
      import("./dashboard/dashboard.component").then(
        (m) => m.DashboardComponent,
      ),
  },
  {
    path: "admin",
    loadChildren: () =>
      import("./admin/admin.routes").then((m) => m.ADMIN_ROUTES),
  },
];
```

---

## 3. Zoneless Angular

Zoneless applications don't use zone.js, improving performance and debugging.

### Enabling Zoneless Mode

```typescript
// main.ts
import { bootstrapApplication } from "@angular/platform-browser";
import { provideZonelessChangeDetection } from "@angular/core";
import { AppComponent } from "./app/app.component";

bootstrapApplication(AppComponent, {
  providers: [provideZonelessChangeDetection()],
});
```

### Zoneless Component Patterns

```typescript
import { Component, signal, ChangeDetectionStrategy } from "@angular/core";

@Component({
  selector: "app-counter",
  standalone: true,
  changeDetection: ChangeDetectionStrategy.OnPush,
  template: `
    <div>Count: {{ count() }}</div>
    <button (click)="increment()">+</button>
  `,
})
export class CounterComponent {
  count = signal(0);

  increment() {
    this.count.update((v) => v + 1);
    // No zone.js needed - Signal triggers change detection
  }
}
```

### Key Zoneless Benefits

- **Performance**: No zone.js patches on async APIs
- **Debugging**: Clean stack traces without zone wrappers
- **Bundle size**: Smaller without zone.js (~15KB savings)
- **Interoperability**: Better with Web Components and micro-frontends

---

## 4. Server-Side Rendering & Hydration

### SSR Setup with Angular CLI

```bash
ng add @angular/ssr
```

### Hydration Configuration

```typescript
// app.config.ts
import { ApplicationConfig } from "@angular/core";
import {
  provideClientHydration,
  withEventReplay,
} from "@angular/platform-browser";

export const appConfig: ApplicationConfig = {
  providers: [provideClientHydration(withEventReplay())],
};
```

### Incremental Hydration (v20+)

```typescript
import { Component } from "@angular/core";

@Component({
  selector: "app-page",
  standalone: true,
  template: `
    <app-hero />

    @defer (hydrate on viewport) {
      <app-comments />
    }

    @defer (hydrate on interaction) {
      <app-chat-widget />
    }
  `,
})
export class PageComponent {}
```

### Hydration Triggers

| Trigger          | When to Use                             |
| ---------------- | --------------------------------------- |
| `on idle`        | Low-priority, hydrate when browser idle |
| `on viewport`    | Hydrate when element enters viewport    |
| `on interaction` | Hydrate on first user interaction       |
| `on hover`       | Hydrate when user hovers                |
| `on timer(ms)`   | Hydrate after specified delay           |

---

## 5. Modern Routing Patterns

### Functional Route Guards

```typescript
// auth.guard.ts
import { inject } from "@angular/core";
import { Router, CanActivateFn } from "@angular/router";
import { AuthService } from "./auth.service";

export const authGuard: CanActivateFn = (route, state) => {
  const auth = inject(AuthService);
  const router = inject(Router);

  if (auth.isAuthenticated()) {
    return true;
  }

  return router.createUrlTree(["/login"], {
    queryParams: { returnUrl: state.url },
  });
};

// Usage in routes
export const routes: Routes = [
  {
    path: "dashboard",
    loadComponent: () => import("./dashboard.component"),
    canActivate: [authGuard],
  },
];
```

### Route-Level Data Resolvers

```typescript
import { inject } from '@angular/core';
import { ResolveFn } from '@angular/router';
import { UserService } from './user.service';
import { User } from './user.model';

export const userResolver: ResolveFn<User> = (route) => {
  const userService = inject(UserService);
  return userService.getUser(route.paramMap.get('id')!);
};

// In routes
{
  path: 'user/:id',
  loadComponent: () => import('./user.component'),
  resolve: { user: userResolver }
}

// In component
export class UserComponent {
  private route = inject(ActivatedRoute);
  user = toSignal(this.route.data.pipe(map(d => d['user'])));
}
```

---

## 6. Dependency Injection Patterns

### Modern inject() Function

```typescript
import { Component, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { UserService } from './user.service';

@Component({...})
export class UserComponent {
  // Modern inject() - no constructor needed
  private http = inject(HttpClient);
  private userService = inject(UserService);

  // Works in any injection context
  users = toSignal(this.userService.getUsers());
}
```

### Injection Tokens for Configuration

```typescript
import { InjectionToken, inject } from "@angular/core";

// Define token
export const API_BASE_URL = new InjectionToken<string>("API_BASE_URL");

// Provide in config
bootstrapApplication(AppComponent, {
  providers: [{ provide: API_BASE_URL, useValue: "https://api.example.com" }],
});

// Inject in service
@Injectable({ providedIn: "root" })
export class ApiService {
  private baseUrl = inject(API_BASE_URL);

  get(endpoint: string) {
    return this.http.get(`${this.baseUrl}/${endpoint}`);
  }
}
```

---

## 7. Component Composition & Reusability

### Content Projection (Slots)

```typescript
@Component({
  selector: 'app-card',
  template: `
    <div class="card">
      <div class="header">
        <!-- Select by attribute -->
        <ng-content select="[card-header]"></ng-content>
      </div>
      <div class="body">
        <!-- Default slot -->
        <ng-content></ng-content>
      </div>
    </div>
  `
})
export class CardComponent {}

// Usage
<app-card>
  <h3 card-header>Title</h3>
  <p>Body content</p>
</app-card>
```

### Host Directives (Composition)

```typescript
// Reusable behaviors without inheritance
@Directive({
  standalone: true,
  selector: '[appTooltip]',
  inputs: ['tooltip'] // Signal input alias
})
export class TooltipDirective { ... }

@Component({
  selector: 'app-button',
  standalone: true,
  hostDirectives: [
    {
      directive: TooltipDirective,
      inputs: ['tooltip: title'] // Map input
    }
  ],
  template: `<ng-content />`
})
export class ButtonComponent {}
```

---

## 8. State Management Patterns

### Signal-Based State Service

```typescript
import { Injectable, signal, computed } from "@angular/core";

interface AppState {
  user: User | null;
  theme: "light" | "dark";
  notifications: Notification[];
}

@Injectable({ providedIn: "root" })
export class StateService {
  // Private writable signals
  private _user = signal<User | null>(null);
  private _theme = signal<"light" | "dark">("light");
  private _notifications = signal<Notification[]>([]);

  // Public read-only computed
  readonly user = computed(() => this._user());
  readonly theme = computed(() => this._theme());
  readonly notifications = computed(() => this._notifications());
  readonly unreadCount = computed(
    () => this._notifications().filter((n) => !n.read).length,
  );

  // Actions
  setUser(user: User | null) {
    this._user.set(user);
  }

  toggleTheme() {
    this._theme.update((t) => (t === "light" ? "dark" : "light"));
  }

  addNotification(notification: Notification) {
    this._notifications.update((n) => [...n, notification]);
  }
}
```

### Component Store Pattern with Signals

```typescript
import { Injectable, signal, computed, inject } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { toSignal } from "@angular/core/rxjs-interop";

@Injectable()
export class ProductStore {
  private http = inject(HttpClient);

  // State
  private _products = signal<Product[]>([]);
  private _loading = signal(false);
  private _filter = signal("");

  // Selectors
  readonly products = computed(() => this._products());
  readonly loading = computed(() => this._loading());
  readonly filteredProducts = computed(() => {
    const filter = this._filter().toLowerCase();
    return this._products().filter((p) =>
      p.name.toLowerCase().includes(filter),
    );
  });

  // Actions
  loadProducts() {
    this._loading.set(true);
    this.http.get<Product[]>("/api/products").subscribe({
      next: (products) => {
        this._products.set(products);
        this._loading.set(false);
      },
      error: () => this._loading.set(false),
    });
  }

  setFilter(filter: string) {
    this._filter.set(filter);
  }
}
```

---

## 9. Forms with Signals (Coming in v22+)

### Current Reactive Forms

```typescript
import { Component, inject } from "@angular/core";
import { FormBuilder, Validators, ReactiveFormsModule } from "@angular/forms";

@Component({
  selector: "app-user-form",
  standalone: true,
  imports: [ReactiveFormsModule],
  template: `
    <form [formGroup]="form" (ngSubmit)="onSubmit()">
      <input formControlName="name" placeholder="Name" />
      <input formControlName="email" type="email" placeholder="Email" />
      <button [disabled]="form.invalid">Submit</button>
    </form>
  `,
})
export class UserFormComponent {
  private fb = inject(FormBuilder);

  form = this.fb.group({
    name: ["", Validators.required],
    email: ["", [Validators.required, Validators.email]],
  });

  onSubmit() {
    if (this.form.valid) {
      console.log(this.form.value);
    }
  }
}
```

### Signal-Aware Form Patterns (Preview)

```typescript
// Future Signal Forms API (experimental)
import { Component, signal } from '@angular/core';

@Component({...})
export class SignalFormComponent {
  name = signal('');
  email = signal('');

  // Computed validation
  isValid = computed(() =>
    this.name().length > 0 &&
    this.email().includes('@')
  );

  submit() {
    if (this.isValid()) {
      console.log({ name: this.name(), email: this.email() });
    }
  }
}
```

---

## 10. Performance Optimization

### Change Detection Strategies

```typescript
@Component({
  changeDetection: ChangeDetectionStrategy.OnPush,
  // Only checks when:
  // 1. Input signal/reference changes
  // 2. Event handler runs
  // 3. Async pipe emits
  // 4. Signal value changes
})
```

### Defer Blocks for Lazy Loading

```typescript
@Component({
  template: `
    <!-- Immediate loading -->
    <app-header />

    <!-- Lazy load when visible -->
    @defer (on viewport) {
      <app-heavy-chart />
    } @placeholder {
      <div class="skeleton" />
    } @loading (minimum 200ms) {
      <app-spinner />
    } @error {
      <p>Failed to load chart</p>
    }
  `
})
```

### NgOptimizedImage

```typescript
import { NgOptimizedImage } from '@angular/common';

@Component({
  imports: [NgOptimizedImage],
  template: `
    <img
      ngSrc="hero.jpg"
      width="800"
      height="600"
      priority
    />

    <img
      ngSrc="thumbnail.jpg"
      width="200"
      height="150"
      loading="lazy"
      placeholder="blur"
    />
  `
})
```

---

## 11. Testing Modern Angular

### Testing Signal Components

```typescript
import { ComponentFixture, TestBed } from "@angular/core/testing";
import { CounterComponent } from "./counter.component";

describe("CounterComponent", () => {
  let component: CounterComponent;
  let fixture: ComponentFixture<CounterComponent>;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [CounterComponent], // Standalone import
    }).compileComponents();

    fixture = TestBed.createComponent(CounterComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it("should increment count", () => {
    expect(component.count()).toBe(0);

    component.increment();

    expect(component.count()).toBe(1);
  });

  it("should update DOM on signal change", () => {
    component.count.set(5);
    fixture.detectChanges();

    const el = fixture.nativeElement.querySelector(".count");
    expect(el.textContent).toContain("5");
  });
});
```

### Testing with Signal Inputs

```typescript
import { ComponentFixture, TestBed } from "@angular/core/testing";
import { ComponentRef } from "@angular/core";
import { UserCardComponent } from "./user-card.component";

describe("UserCardComponent", () => {
  let fixture: ComponentFixture<UserCardComponent>;
  let componentRef: ComponentRef<UserCardComponent>;

  beforeEach(async () => {
    await TestBed.configureTestingModule({
      imports: [UserCardComponent],
    }).compileComponents();

    fixture = TestBed.createComponent(UserCardComponent);
    componentRef = fixture.componentRef;

    // Set signal inputs via setInput
    componentRef.setInput("id", "123");
    componentRef.setInput("name", "John Doe");

    fixture.detectChanges();
  });

  it("should display user name", () => {
    const el = fixture.nativeElement.querySelector("h3");
    expect(el.textContent).toContain("John Doe");
  });
});
```

---

## Best Practices Summary

| Pattern              | ✅ Do                          | ❌ Don't                        |
| -------------------- | ------------------------------ | ------------------------------- |
| **State**            | Use Signals for local state    | Overuse RxJS for simple state   |
| **Components**       | Standalone with direct imports | Bloated SharedModules           |
| **Change Detection** | OnPush + Signals               | Default CD everywhere           |
| **Lazy Loading**     | `@defer` and `loadComponent`   | Eager load everything           |
| **DI**               | `inject()` function            | Constructor injection (verbose) |
| **Inputs**           | `input()` signal function      | `@Input()` decorator (legacy)   |
| **Zoneless**         | Enable for new projects        | Force on legacy without testing |

---

## Resources

- [Angular.dev Documentation](https://angular.dev)
- [Angular Signals Guide](https://angular.dev/guide/signals)
- [Angular SSR Guide](https://angular.dev/guide/ssr)
- [Angular Update Guide](https://angular.dev/update-guide)
- [Angular Blog](https://blog.angular.dev)

---

## Common Troubleshooting

| Issue                          | Solution                                            |
| ------------------------------ | --------------------------------------------------- |
| Signal not updating UI         | Ensure `OnPush` + call signal as function `count()` |
| Hydration mismatch             | Check server/client content consistency             |
| Circular dependency            | Use `inject()` with `forwardRef`                    |
| Zoneless not detecting changes | Trigger via signal updates, not mutations           |
| SSR fetch fails                | Use `TransferState` or `withFetch()`                |

License

Declared license: MIT

MIT License

Copyright (c) 2025 davila7

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

View the license in the source repository — the version published there is authoritative.