IntersectionObserver interface of the Intersection Observer API provides a way to asynchronously observe changes in the intersection of a target element with an ancestor element or with a top-level document's viewport. The ancestor element or viewport is referred to as the root.
IntersectionObserver is created, it's configured to watch for given ratios of visibility within the root. The configuration cannot be changed once the
IntersectionObserver is created, so a given observer object is only useful for watching for specific changes in degree of visibility; however, you can watch multiple target elements with the same observer.