Here’s how to accomplish a scroll progress bar that advances as you scroll though pages of a site. It’s a nice way to convey a progress indicator for readers to know how far along they are in a post.

It uses the power of CSS Variables, and the solution is adapted from part of this excellent talk by Lea Verou.

First, add the following markup right after the opening body tag:

<div class="progress"></div>

Then style this .progress element with something like this:

.progress {
  background: linear-gradient(to right, #F9EC31 var(--scroll), transparent 0);
  background-repeat: no-repeat;
  position: fixed;
  width: 100%;
  height: 4px;
  z-index: 1;
}

Notice how in the linear gradient we’re referring to a CSS variable named --scroll, which will be given a value on scroll.

That means that all that’s left to do is listen for the document’s scroll event and set the value of the --scroll custom property with the scroll percentage. We use element.style.setProperty for that. The .progress element will get an inline value for --scroll once it gets set.

Thanks to Phil Ricketts and his solution to this StackOverflow question for an accurate way to calculate the document scroll percentage:

var h = document.documentElement,
  b = document.body,
  st = 'scrollTop',
  sh = 'scrollHeight',
  progress = document.querySelector('.progress'),
  scroll;

document.addEventListener('scroll', function() {
  scroll = (h[st]||b[st]) / ((h[sh]||b[sh]) - h.clientHeight) * 100;
  progress.style.setProperty('--scroll', scroll + '%');
});

👉 Note that IE or Edge don’t support CSS custom properties at the moment. Support is coming however, and it the mean time the feature gracefully degrades.