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.

  selector: 'app-root',
  template: `
    <p>Total price of product is {{fetchPrice | async | currency:"CAD":true:"1.2-2"}}</p>
    <p>Seconds: {{seconds | async}} </p>
export class AppComponent {
  fetchPrice = new Promise((resolve, reject) => {
    setTimeout(() => resolve(10), 500);

  seconds = Observable.of(0).concat(Observable.interval(1000))

View Example

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;

View Example

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