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As the name stateful implies this operator is useful when you process an Observable which maintains state.

Maintaining state as an Observable source comes with a handful of repetitive as well as use case specific tasks.

It acts like the Observables pipe method. It accepts RxJS operators and composes them like Observable#pipe and the standalone pipe method.

Furthermore, it takes care of the above mentioned repetitive tasks as listed below.

You will always (aka repetitive) want to ensure that:

  • only distinct state changes are emitted
  • only defined values are emitted (filter out undefined, which ensures lazy state)
  • share and replay custom operations for multiple subscribers (saves performance)

You will sometimes (aka situational) need:

  • a subset of the state (derivations)
  • compose the state with other Observables or change the Observables behavior

Note that RxState#connect already applies the stateful operator under the hood.


import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';
import { stateful } from 'rx-angular/state/selections';

const state$: Observable<{ name: string; items: string[] }>;
const derivation$ = state$.pipe(
map((state) => state.list.length),
filter((length) => length > 3)


function stateful<T>(): MonoTypeOperatorFunction<T>;


function stateful<T, R>(
optionalDerive: OperatorFunction<T, R>[]
): OperatorFunction<T, T | R>;



typeof: OperatorFunction<T, R>[]