JSFiddle - React, Tailwind, and code Playground

HTML

<span id="count">150</span>

CSS

html, body {
  height: 1000%;
}

#count {
  position: fixed;
}

JavaScript

var counter = document.getElementById('count');

function updateCounter() {
  'use strict';
  var height = document.documentElement.scrollHeight - window.innerHeight;
  counter.textContent = 150 - document.documentElement.scrollTop / height * 150;
}

document.addEventListener('scroll', updateCounter);
window.addEventListener('resize', updateCounter);