angular-pipes
ALWAYS use when working with Angular Pipes, custom pipes, pure pipes, impure pipes, or async pipes in Angular applications.
What this skill does
# Angular Pipes
**Version:** Angular 21 (2025)
**Tags:** Pipes, Transform, Format, Async
**References:** [Pipes Guide](https://angular.dev/guide/pipes) • [API](https://angular.io/api/common)
## API Changes
This section documents recent version-specific API changes.
- NEW: Standalone pipes — All pipes can be standalone
- NEW: Signal-based pipes — Pipes can use signals
- NEW: inject() in pipes — Use functional DI in pipes
- DEPRECATED: Impure pipes — Avoid for performance; use pure pipes
## Best Practices
- Use pure pipes for stateless transformations
```ts
import { Pipe, PipeTransform } from '@angular/core';
@Pipe({
name: 'capitalize',
standalone: true
})
export class CapitalizePipe implements PipeTransform {
transform(value: string): string {
if (!value) return '';
return value.charAt(0).toUpperCase() + value.slice(1);
}
}
```
- Use pipes in templates
```ts
@Component({
template: `
<p>{{ name | capitalize }}</p>
<p>{{ price | currency:'USD' }}</p>
<p>{{ items | json }}</p>
`
})
export class MyComponent {}
```
- Use pipe with parameters
```ts
@Pipe({ name: 'slice' })
export class SlicePipe implements PipeTransform {
transform(value: string, start: number, end?: number): string {
return value.slice(start, end);
}
}
// Usage: {{ text | slice:0:10 }}
```
- Use async pipe for observables
```ts
@Component({
template: `
@if (user$ | async; as user) {
<p>{{ user.name }}</p>
}
`
})
export class UserComponent {
user$ = this.userService.getUser();
}
```
- Use impure pipes only when necessary
```ts
@Pipe({
name: 'myPipe',
pure: false // Impure - runs on every change detection
})
export class ImpurePipe implements PipeTransform {}
```
- Use KeyValue pipe for object iteration
```ts
@Component({
template: `
@for (item of object | keyvalue; track item.key) {
{{ item.key }}: {{ item.value }}
}
`
})
export class MyComponent {
object = { a: 1, b: 2 };
}
```
- Use Json pipe for debugging
```ts
@Component({
template: `
<pre>{{ object | json }}</pre>
`
})
export class DebugComponent {}
```
- Chain pipes
```ts
@Component({
template: `
{{ name | uppercase | slice:0:5 }}
`
})
export class ChainedPipeComponent {}
```
- Use standalone pipes in imports
```ts
@Component({
standalone: true,
imports: [DatePipe, CurrencyPipe, CapitalizePipe],
template: `{{ date | date }}`
})
export class MyComponent {}
```
- Use inject() in pipes
```ts
@Pipe({ name: 'translate', standalone: true })
export class TranslatePipe implements PipeTransform {
private service = inject(TranslateService);
transform(key: string): string {
return this.service.get(key);
}
}
```
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