Each of these has its own intersection detection routines, all running on the main thread. It uses a vendor-provided library to manage the advertisements placed periodically throughout the page, has animated graphics here and there, and uses a custom library that draws notification boxes and the like. When a site is loaded with these tests, things can get downright ugly.Ĭonsider a web page that uses infinite scrolling. Since all this code runs on the main thread, even one of these can cause performance problems. Implementing intersection detection in the past involved event handlers and loops calling methods like Element.getBoundingClientRect() to build up the needed information for every element affected. Deciding whether or not to perform tasks or animation processes based on whether or not the user will see the result.Reporting of visibility of advertisements in order to calculate ad revenues.Implementing "infinite scrolling" websites, where more and more content is loaded and rendered as you scroll, so that the user doesn't have to flip through pages.Lazy-loading of images or other content as a page is scrolled.
Intersection information is needed for many reasons, such as: As the web has matured, the need for this kind of information has grown. Historically, detecting visibility of an element, or the relative visibility of two elements in relation to each other, has been a difficult task for which solutions have been unreliable and prone to causing the browser and the sites the user is accessing to become sluggish. 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.
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