Ionic 4: How to get ion-content scrollTop

I want to watch if is at the top position.

When I scroll down and scroll up, I cannot get right scroll top position.

This is my code.

<ion-content [scrollEvents]="true" (ionScroll)="onScroll($event)" (ionScrollEnd)="onScrollEnd()">
  @ViewChild(IonContent) content: IonContent;
  onScroll($event: CustomEvent<ScrollDetail>) {
      if ($event && $event.detail && $event.detail.scrollTop) {
        const scrollTop = $event.detail.scrollTop;
  onScrollEnd() {
    console.log(this.content, this.content['el']['scrollTop']);
  } 238 173 IonContent {el: ion-content.hydrated, ionScrollStart: Observable, ionScroll: Observable, ionScrollEnd: Observable}

When I scroll up to the top, (ionScroll)="...($event)" & $event.detail.scrollTop does not give 0.
I think there may be some throttle(queue) to delay fire the event.

Does anyone know how to get 0 of scroll top position when it is on top? or any idea?