How to Build a Reading Progress Bar With CSS and JavaScript

// Reading Progress bar in Html, Css and Js
// Html Part
<div id="progress-bar"></div>

// Css Part
#progress-bar {
  	--scrollAmount: 0%;
	background-image: linear-gradient(to right, #4facfe 0%, #00f2fe 100%);
	width: var(--scrollAmount);
	height: 4px;
	position: fixed;
	top: 0;
	left: 0;
}
    
    
// JavaScript Part
let processScroll = () => {
    let docElem = document.documentElement,
    docBody = document.body,
    scrollTop = docElem['scrollTop'] || docBody['scrollTop'],  //browser support, docElem or docBody  // the heigth we are on currently starting from top 
    scrollBottom = (docElem['scrollHeight'] || docBody['scrollHeight']) - window.innerHeight, // the heigth of the entire content
    scrollPercent = scrollTop / scrollBottom *100 + '%';

    console.log(parseInt(scrollPercent) + '%'+ ' = ' + scrollTop + ' / ' + scrollBottom)

    document.getElementById('progress-bar').style.setProperty('--scrollAmount', scrollPercent)
}

document.addEventListener('scroll', processScroll)