Pure and Impure Pipes

In Angular:

• Pure pipes run only when the input reference changes
• Impure pipes run on every change detection cycle

This behavior stands in Angular 21, though the zoneless/signal change detection makes pure pipes more predictable.

Example of a pure pipe:

import { Pipe, PipeTransform } from "@angular/core";

@Pipe({ name: "double", pure: true })
export class DoublePipe implements PipeTransform {
  transform(value: number): number {
    return value * 2;
  }
}

Note: Pure pipes help performance by avoiding unnecessary recalculations. Angular 21’s signal‑based reactivity makes pipe updates more efficient and deterministic.