If you have a table of contents on a long-scrolling page, thanks to, say, position: fixed; or position: sticky;, the IntersectionObserver API in JavaScript is the perfect companion to highlight items in the table of contents when corresponding content is in view.
Ben Frain has a post all about this:
Thanks to IntersectionObserver we have a small but very efficient bit of code to create our table of contents, provide quick links to jump around the document and update readers on where they are in a document as they read.
Compared to older techniques that need to bind to scroll events and perform their own math, this code is shorter, faster, and more logical. If you’re looking for the demo on Ben’s site, the article is the demo. And here’s a video on it:
I’ve mentioned this stuff before, but here’s a Bramus Van Damme version:
And here’s a version from Hakim el Hattab that is just begging for someone to port it to IntersectionObserver because the UI is so cool:
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