Create a reading progress indicator (on scroll) in JavaScript

Reading progress indicators are generally used on long form articles.

They provide a visual clue for readers indicating how far they are from the end of an article.

Let’s get started by adding the basic markup for the progress bar to our HTML.

<div id="progress-bar"></div>

Next the CSS.

The transition makes the changing of the width appear smoother, you may prefer it without.

#progress-bar {
  position: fixed;
  top: 0;
  left: 0;
  width: 0;
  height: 4px;
  z-index: 99;
  background-color: #b22222;
  transition: width 1s;
}

With the HTML and CSS in place we can start to build the functionality with JavaScript.

First we define our progress bar element.

const progressBar = document.getElementById("progress-bar");

Next we add a function to detect how far a user has scrolled and set the progress bar width accordingly.

const setProgressBar = () => {
  let scrollDist = document.documentElement.scrollTop || document.body.scrollTop;
  let progressWidth = (scrollDist / (document.body.scrollHeight - document.documentElement.clientHeight)) * 100;
  progressBar.style.width = progressWidth + "%";
};

Last we check if the page contains a progress bar.

If it does then we call the setProgressBar function when the user scrolls the browser.

if (progressBar) {
  window.addEventListener("scroll", setProgressBar);  
}