Custom operator for using portions of state$ in redux-observable epics

with-latest-from-selector.ts

function withLatestFromSelector<A, R, S, I>(
  state$: StateObservable<A>,
  selector: OutputSelector<A, R, S>,
): OperatorFunction<I, Array<I | R>> {
  return (input$) =>
    input$.pipe(
      withLatestFrom(state$),
      map(([input, state]) => [input, selector(state)]),
    );
}

export const myEpic: Epic = (action$: ActionsObservable<Action>, state$: StateObservable<AppState>) =>
  action$.pipe(
    ofType(myActions.fooAction.type),
    withLatestFromSelector(state$, mySelector),
    tap(([action, fromSelector]) => console.log(action, fromSelector)),
  );