angular-development

What this skill does

# Angular Development Skill

## When to Use This Skill

Use this skill when working with Angular applications, including:

- Building modern Angular applications with standalone components
- Creating reactive UIs with Angular's component system
- Implementing dependency injection patterns
- Setting up routing with lazy loading and guards
- Building reactive forms with validation
- Managing state with Signals and RxJS
- Creating custom directives and pipes
- Implementing HTTP client integrations
- Migrating from older Angular patterns to modern approaches
- Optimizing Angular applications for performance
- Setting up Angular projects with best practices

## Core Concepts

### Components

Components are the fundamental building blocks of Angular applications. They control a portion of the screen called a view.

**Modern Standalone Component Pattern:**

```typescript
import { Component } from '@angular/core';
import { CommonModule } from '@angular/common';

@Component({
  selector: 'app-user-profile',
  standalone: true,
  imports: [CommonModule],
  template: `
    <div class="profile">
      <h2>{{ user.name }}</h2>
      <p>{{ user.email }}</p>
      <button (click)="updateProfile()">Update</button>
    </div>
  `,
  styles: [`
    .profile {
      padding: 20px;
      border: 1px solid #ccc;
      border-radius: 8px;
    }
  `]
})
export class UserProfileComponent {
  user = {
    name: 'John Doe',
    email: '[email protected]'
  };

  updateProfile() {
    console.log('Updating profile...');
  }
}
```

**Component Lifecycle Hooks:**

```typescript
import { Component, OnInit, OnDestroy, AfterViewInit } from '@angular/core';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-lifecycle-demo',
  standalone: true,
  template: `<div>{{ message }}</div>`
})
export class LifecycleDemoComponent implements OnInit, OnDestroy, AfterViewInit {
  message = '';
  private subscription?: Subscription;

  ngOnInit() {
    // Called once after component initialization
    console.log('Component initialized');
    this.message = 'Component ready';
  }

  ngAfterViewInit() {
    // Called after view initialization
    console.log('View initialized');
  }

  ngOnDestroy() {
    // Called before component destruction
    console.log('Component destroyed');
    this.subscription?.unsubscribe();
  }
}
```

### Services and Dependency Injection

Services provide shared functionality across components. Angular's dependency injection system makes services available throughout your application.

**Modern Injectable Service with inject() Function:**

```typescript
import { Injectable, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

export interface User {
  id: number;
  name: string;
  email: string;
}

@Injectable({
  providedIn: 'root' // Singleton service available app-wide
})
export class UserService {
  // Modern inject() function instead of constructor injection
  private http = inject(HttpClient);
  private apiUrl = 'https://api.example.com/users';

  getUsers(): Observable<User[]> {
    return this.http.get<User[]>(this.apiUrl);
  }

  getUserById(id: number): Observable<User> {
    return this.http.get<User>(`${this.apiUrl}/${id}`);
  }

  createUser(user: Omit<User, 'id'>): Observable<User> {
    return this.http.post<User>(this.apiUrl, user);
  }

  updateUser(id: number, user: Partial<User>): Observable<User> {
    return this.http.patch<User>(`${this.apiUrl}/${id}`, user);
  }

  deleteUser(id: number): Observable<void> {
    return this.http.delete<void>(`${this.apiUrl}/${id}`);
  }
}
```

**Using Services in Components:**

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

@Component({
  selector: 'app-user-list',
  standalone: true,
  imports: [CommonModule],
  template: `
    <div class="user-list">
      <h2>Users</h2>
      @if (loading) {
        <p>Loading...</p>
      } @else if (error) {
        <p class="error">{{ error }}</p>
      } @else {
        <ul>
          @for (user of users; track user.id) {
            <li>{{ user.name }} - {{ user.email }}</li>
          }
        </ul>
      }
    </div>
  `
})
export class UserListComponent implements OnInit {
  private userService = inject(UserService);

  users: User[] = [];
  loading = false;
  error = '';

  ngOnInit() {
    this.loadUsers();
  }

  loadUsers() {
    this.loading = true;
    this.userService.getUsers().subscribe({
      next: (users) => {
        this.users = users;
        this.loading = false;
      },
      error: (err) => {
        this.error = 'Failed to load users';
        this.loading = false;
        console.error(err);
      }
    });
  }
}
```

### Signals - Modern Reactive State Management

Signals provide a new way to manage reactive state in Angular with fine-grained reactivity.

```typescript
import { Component, signal, computed, effect } from '@angular/core';
import { CommonModule } from '@angular/common';

@Component({
  selector: 'app-counter',
  standalone: true,
  imports: [CommonModule],
  template: `
    <div class="counter">
      <h2>Counter: {{ count() }}</h2>
      <p>Double: {{ doubleCount() }}</p>
      <p>Status: {{ status() }}</p>
      <button (click)="increment()">Increment</button>
      <button (click)="decrement()">Decrement</button>
      <button (click)="reset()">Reset</button>
    </div>
  `
})
export class CounterComponent {
  // Writable signal
  count = signal(0);

  // Computed signal - automatically updates when count changes
  doubleCount = computed(() => this.count() * 2);
  status = computed(() => {
    const value = this.count();
    if (value < 0) return 'Negative';
    if (value === 0) return 'Zero';
    return 'Positive';
  });

  constructor() {
    // Effect runs whenever signals it reads change
    effect(() => {
      console.log(`Count changed to: ${this.count()}`);
    });
  }

  increment() {
    this.count.update(value => value + 1);
  }

  decrement() {
    this.count.update(value => value - 1);
  }

  reset() {
    this.count.set(0);
  }
}
```

**Advanced Signals Pattern - Shopping Cart:**

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

export interface CartItem {
  id: number;
  name: string;
  price: number;
  quantity: number;
}

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

  // Computed values
  totalItems = computed(() =>
    this.items().reduce((sum, item) => sum + item.quantity, 0)
  );

  totalPrice = computed(() =>
    this.items().reduce((sum, item) => sum + (item.price * item.quantity), 0)
  );

  // Read-only access to items
  getItems = this.items.asReadonly();

  addItem(item: Omit<CartItem, 'quantity'>) {
    this.items.update(currentItems => {
      const existing = currentItems.find(i => i.id === item.id);
      if (existing) {
        return currentItems.map(i =>
          i.id === item.id ? { ...i, quantity: i.quantity + 1 } : i
        );
      }
      return [...currentItems, { ...item, quantity: 1 }];
    });
  }

  removeItem(id: number) {
    this.items.update(currentItems =>
      currentItems.filter(item => item.id !== id)
    );
  }

  updateQuantity(id: number, quantity: number) {
    if (quantity <= 0) {
      this.removeItem(id);
      return;
    }
    this.items.update(currentItems =>
      currentItems.map(item =>
        item.id === id ? { ...item, quantity } : item
      )
    );
  }

  clear() {
    this.items.set([]);
  }
}
```

### Routing

Angular's router enables navigation between views and lazy loading of feature modules.

**Modern Route Configuration with Lazy Loading:**

```typescript
import { Routes } from '@angular/router';

export const routes: Routes = [
  {
    path: '',
    redirectTo: '/home',
    pathMatch: 'full'
  },
  {
    path: 'home',
    loadComponent: () => import('./