Stateful Pipes

There are two categories of pipes:
  • Stateless pipes are pure functions that flow input data through without remembering anything or causing detectable side-effects. Most pipes are stateless. The CurrencyPipe we used and the length pipe we created are examples of a stateless pipe.
  • Stateful pipes are those which can manage the state of the data they transform. A pipe that creates an HTTP request, stores the response and displays the output, is a stateful pipe. Stateful Pipes should be used cautiously.
Angular provides AsyncPipe, which is stateful.


AsyncPipe can receive a Promise or Observable as input and subscribe to the input automatically, eventually returning the emitted value(s). It is stateful because the pipe maintains a subscription to the input and its returned values depend on that subscription.

Implementing Stateful Pipes

Pipes are stateless by default. We must declare a pipe to be stateful by setting the pure property of the @Pipe decorator to false. This setting tells Angular’s change detection system to check the output of this pipe each cycle, whether its input has changed or not.
// naive implementation assumes small number increments
name: 'animateNumber',
pure: false
export class AnimateNumberPipe implements PipeTransform {
private currentNumber: number = null; // intermediary number
private targetNumber: number = null;
transform(targetNumber: number): string {
if (targetNumber !== this.targetNumber) {
this.currentNumber = this.targetNumber || targetNumber;
this.targetNumber = targetNumber;
const difference = this.targetNumber - this.currentNumber
.subscribe(() => {
return this.currentNumber;
Last modified 2yr ago