html - Angular 2 show and hide scroll button -

i have button shows on right corner when make first scroll down move. want make button disappear after scroll on page. how can this? far, typescript function track($event:any){} looks this:

import { component } '@angular/core';   @component({     host: {'(window:scroll)': 'track($event)'} })   export class directsearchcomponent{     showscrollbutton:boolean=false;     beginy: any;   constructor() {}   track($event:any) {     console.debug("scroll event", $event);     if (this.beginy == undefined || this.beginy != null) {         if (this.beginy > $event.scrolly) {             this.showscrollbutton = false;         }         else {             this.showscrollbutton = true;         }     }     else {         this.beginy = $event.scrolly;         this.showscrollbutton = true;     }       }  } 

and html code button looks this:

  <a href="#" class="scrollup">        <button *ngif="showscrollbutton"              type="button" class="btn btn-primary btn-block pull-right"                     style="position: fixed; bottom: 10px; right: 51px;                             width:3%; overflow:auto;">                 <i class="glyphicon glyphicon-chevron-up" ></i>        </button>      </a> 

thanks in advance!

looks this.beginy not being initialized properly.

import ngoninit , in ngoninit method, set...

this.beginy = document.documentelement.scrolltop || document.body.scrolltop; 

this should initialize variable initial page scroll value.