// RxJS v6+
import { fromEvent } from 'rxjs';
import { throttleTime, tap } from 'rxjs/operators';
​
const scrollIndication = document.getElementById('indication');
const getScrollWidth = () => {
  const doc = document.documentElement;
  // https://www.w3schools.com/howto/howto_js_scroll_indicator.asp
  const winScroll = doc.scrollTop;
  const height = doc.scrollHeight - doc.clientHeight;
​
  return (winScroll / height) * 100;
};
const setScroll = _ => (scrollIndication.style.width = getScrollWidth() + '%');
​
fromEvent(document, 'scroll')
  .pipe(throttleTime(20), tap(setScroll))
  .subscribe();

<style>
  #indication {
    position: fixed;
    width: 5px;
    height: 7px;
    background-color: #ff3366;
    left: 0px;
    right: 0px;
    top: 0px;
    z-index: 2;
  }
</style>
​
<div id="indication">&nbsp;</div>
Scroll down!!!
<div class="app" style="position: absolute; margin-top: 3000px;">Boom!</div>