Directives & Pipes

Priority: P2 (MEDIUM)

Principles

• Composition: Use hostDirectives: [TooltipDirective] on @Component or @Directive decorators to compose behaviors without inheritance. Expose inputs/outputs via hostDirectives: [{ directive: TooltipDirective, inputs: ['text'] }].
• Pure Pipes: Decorate with @Pipe({ name: 'truncate', standalone: true, pure: true }). Implement PipeTransform with transform(value: string, limit = 50) method. Pipes must be pure: true (default) to cache results by input reference — Angular only re-runs them when the reference changes. Do not set pure: false unless handling Observables/Arrays that mutate.
• Directive Logic: Encapsulate reusable DOM manipulation or behavioral logic in standalone: true Directives (e.g., selector: '[appHighlight]'). Inject ElementRef/Renderer2 for DOM access.

Guidelines

• Signal Inputs: Directives and Pipes support signal inputs.
• Standalone: All Pipes and Directives must be standalone. Do not declare in NgModule; import directly in component imports array. Use ng generate directive to scaffold.

Anti-Patterns

• No @HostBinding/@HostListener: Use the host: {} object in the @Directive decorator — not with @HostBinding or @HostListener (e.g., '(mouseenter)': 'show()', '[attr.aria-label]': 'text()') — these decorators are deprecated patterns.
• No impure pipes for static transforms: Keep pure: true (default); use async pipe for Observables.
• No structural directives for conditionals: Use native @if/@for/@switch block syntax.

References

• Composition