epics4.js

const game = (action$, store) =>
  action$.ofType(actions.START)
    .mergeMap(() => {
      const interval = Observable.interval(250)
        .takeUntil(action$.ofType(
          actions.PAUSE,
          actions.RESET,
          actions.END
        ));
      const incrementTime = interval
        .map(_interval => actions.incrementTime());
      const end = interval
        // if the user has lost.
        .map(_action => actions.end());
      return Observable.merge(incrementTime, end);
    });