Creating an Window scroll up/down Observable

scrollUpDownObservable.ts

import { Subject } from 'rxJs/Rx';

const body = document.querySelector('body');
let beginningScrollPostion = body.scrollTop,
    scrollObserver = new Subject<string>(),
    windowEventListender = window.addEventListener('scroll', (scrollEvent) => {
      if (body.scrollTop > beginningScrollPostion) {
        scrollObserver.next('down');
        } else {
          scrollObserver.next('up');
        }
        beginningScrollPostion = body.scrollTop;
      });
    
scrollObserver.subscribe(
  direction => {
    //do logic here.
  }
);