IntersectionObserver 란?

참고

• https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserver
• https://github.com/w3c/IntersectionObserver/
• https://www.smashingmagazine.com/2018/01/deferring-lazy-loading-intersection-observer-api/
• https://tech.lezhin.com/2017/07/13/intersectionobserver-overview
• http://blog.hyeyoonjung.com/2019/01/09/intersectionobserver-tutorial/
• https://hackernoon.com/understanding-intersection-observer-api-in-javascript-bb1bf04b8081