timer.ts

    const go = state=> Object.assign(state, {isRunning:true});
    const inc = state=> Object.assign(state, {isRunning:true, tick: state.tick + 1});
    const zero = state=> Object.assign(state, {isRunning:false, tick:0});
    const stop = state=> Object.assign(state, {isRunning:false});

    const timer = Observable.interval(1000);
    const {start, pause, reset} = this;

    this.timerView = start
      .switchMap(() => Observable.merge(
          timer.mapTo(inc),
          pause.mapTo(stop),
          reset.mapTo(zero)
        )
        .takeUntil(pause.merge(reset))
      )
      .scan((state, op) => {
        console.log(state);
        return op(state);
      }, {
        tick:0,
        isRunning:false
      })
      .share();