block12.ts

const rootEpic: Epic<Action, Action, State> = (
  action$,
  state$
) => {
  const counterStates$ = state$.map(
    state => state.counterStates
  );
  // Given a list and an epic for each list item, return an
  // Observable of the actions for all list items.
  const counterStateAction$ = counterStates$.pipe(
    runListEpics({
      action$,
      listItemEpic: counterEpic,
      getListItemKey: counterState => counterState.id,
      selectListItem: getCounterState
    })
  );

  return counterStateAction$;
};