<div style="text-align:center">
<button class="btn" (click)='populateList()'>Click</button>
<h1>Bla</h1>
    <virtual-scroll [items]="listOfItems" (update)="smallList = onUpdateOfList($event)">
        <div *ngFor="let item of smallList">{{item}}</div>
    </virtual-scroll>
</div>

@Component({
selector: '-deliverable-list',
templateUrl: './delivereble-list.custom-component.html',
styleUrls: ['./deliverable-list.component.scss']
})
//EXPERIMENTS
populate: boolean = false;
listOfItems: string[] = [];
smallList: any[] = [];

private populateList() {
    this.listOfItems = [];
    if (this.populate) {
        for (let i = 0; i < 200; i++) {
            this.listOfItems.push("inne: " + i);
        }
        this.populate = false;
    }
    else {
            for (let i = 0; i < 200; i++) {
                this.listOfItems.push("Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.");
            }
            this.populate = true;
    }
    //console.log(this.listOfItems);
}

private onUpdateOfList(event: any[]): any[] {
    if (event.length != 0) {
        this.smallList = event;
    } else {
        this.smallList = this.listOfItems.slice(10, 20);
    }
    console.log(this.smallList);
    return this.smallList;
}
//EXPERIMENTS

<!--The content below is only a placeholder and can be replaced.-->
<div style="text-align:center">
  <button class="btn" (click)='populateList()'>Click</button>
  <h1>Bla</h1>
  <virtual-scroll [items]="listOfItems" (update)="scrollItem = $event">
      <div *ngFor="let item of scrollItem">{{item}}</div>
  </virtual-scroll>
</div>

import { Component ,OnInit} from '@angular/core';

@Component({
selector: 'app-root',
templateUrl: './app.component.html',
styleUrls: ['./app.component.scss']
})
export class AppComponent implements OnInit {

   ble:boolean = false;
   listOfItems: string [] = [];
   //scrollItem: any[];
   ngOnInit(): void {

   }

populateList():void{
    this.listOfItems = [];
    if(this.ble){
    for(let i = 0; i<200 ; i++){
       this.listOfItems.push("Lorem ipsum dolor sit amet, consectetur
       adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore
       magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation
       ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute
       irure dolor in reprehenderit in voluptate velit esse cillum dolore eu
       fugiat nulla pariatur. Excepteur sint occaecat cupidatat non
       proident, sunt in culpa qui officia deserunt mollit anim id est
       laborum.");
   }
   this.ble = false;
  }
  else{
     for(let i = 0; i<200 ; i++){
     this.listOfItems.push("inne: " + i);
  }
  this.ble = true;
 }
//console.log(this.scrollItem);
console.log(this.listOfItems);
 }

onUpdate(list: any[]): any[]{
  //this.scrollItem = list;
  // return this.scrollItem;
  return null;
}
}