CurrencyPipewe used and the length pipe we created are examples of a stateless pipe.
AsyncPipe, which is stateful.
Observableas 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.
@Pipedecorator 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.