@Directive({
  selector: '[carousel]'
})
export class CarouselDirective implements OnInit, OnDestroy {
  // ...
  timerId: number | null = null;

  @Input('carouselAutoplay')
  set autoplay(autoplay: 'on' | 'off') {
    autoplay === 'on' ? this.setAutoplayTimer() : this.clearAutoplayTimer();
  }

  ngOnDestroy() {
    this.clearAutoplayTimer();
  }

  private clearAutoplayTimer() {
    window.clearInterval(this.timerId);
  }

  private setAutoplayTimer() {
    this.timerId = window.setInterval(() => this.next(), 1000);
  }
}