A great intro by Dan Callahan on why
IntersectionObserver is so damn useful:
What do infinite scrolling, lazy loading, and online advertisements all have in common?
They need to know about—and react to—the visibility of elements on a page!
Unfortunately, knowing whether or not an element is visible has traditionally been difficult on the Web. Most solutions listen for scroll and resize events, then use DOM APIs like
getBoundingClientRect()to manually calculate where elements are relative to the viewport. This usually works, but it's inefficient and doesn't take into account other ways in which an element's visibility can change, such as a large image finally loading higher up on the page, which pushes everything else downward.
The API is deliciously simple.