Angular v21 Development

Development guide for components, services, and directives based on Angular v21 modern patterns.

When to Use This Skill

• Creating new components, services, or directives
• Implementing signals-based state management
• Implementing Reactive Forms
• Refactoring existing code to Angular v21 patterns
• Using inject() function for dependency injection

When NOT to use:

• Styling only → tailwindcss-v4-styling
• Page routing configuration → analogjs-development
• UI/UX design application → material-design-3-expressive

Core Principles

• Standalone First: All components are standalone (do NOT write standalone: true in decorator, it's default)
• OnPush Detection: Always set changeDetection: ChangeDetectionStrategy.OnPush
• Signals-First: Prefer signal(), computed(), effect()
• Modern DI: Use inject() function instead of constructor injection
• Function-Based APIs: Use input() / output() functions instead of @Input() / @Output() decorators
• Host Bindings in Decorator: Use host object instead of @HostBinding / @HostListener
• Native Control Flow: Use @if / @for / @switch instead of ngIf / ngFor / *ngSwitch
• Class Binding: Use [class] binding instead of ngClass
• Style Binding: Use [style] binding instead of ngStyle

Implementation Guidelines

Component Creation

Patterns to apply when creating components:

1. Set changeDetection: ChangeDetectionStrategy.OnPush in @Component decorator
2. Define inputs/outputs with input() / output() functions
3. Calculate derived state with computed()
4. Use @if / @for / @switch control flow in templates

→ Details: [Component Examples](references/component-examples.md)

Service Creation

Patterns to apply when creating services:

1. Use @Injectable({ providedIn: 'root' }) for singleton
2. Inject dependencies with inject() function
3. Manage state with signal(), expose with asReadonly()
4. Define derived state with computed()
5. Update state with set() or update() (do NOT use mutate())

→ Details: [Signal Patterns](references/signal-patterns.md)

Reactive Forms

Patterns to apply when implementing forms:

1. Get FormBuilder via inject()
2. Use typed forms for type safety
3. Get values with getRawValue()
4. Add ReactiveFormsModule to imports

→ Details: [Component Examples](references/component-examples.md#reactive-forms)

Host Bindings

Host binding implementation patterns:

1. Do NOT use @HostBinding / @HostListener decorators
2. Use host object in @Component / @Directive decorator

→ Details: [Component Examples](references/component-examples.md#host-bindings)

Image Optimization

Patterns to apply when displaying images:

1. Use NgOptimizedImage (not for inline base64 images)
2. Always specify width / height attributes
3. Add priority attribute for above-the-fold images

→ Details: [Component Examples](references/component-examples.md#image-optimization)

Workflow

1. Requirement Check: Define component responsibility and inputs/outputs
2. TDD Red Phase: Create test cases first
3. TDD Green Phase: Minimal implementation to pass tests
4. TDD Refactor Phase: Optimize code
5. Pattern Verification:

- Is changeDetection: ChangeDetectionStrategy.OnPush set? - Are input() / output() functions used? - Is DI done with inject() function? - Are signals (signal(), computed()) used?

1. Accessibility: Check ARIA attributes, keyboard navigation

Related Skills

• analogjs-development: Use together when creating page components (*.page.ts)
• tailwindcss-v4-styling: When styling is needed
• material-design-3-expressive: When applying UI/UX design patterns

Reference Documentation

For detailed patterns and code examples, see:

• [Signal Patterns](references/signal-patterns.md) - Detailed signal usage
• [Component Examples](references/component-examples.md) - Various component patterns