JavaScript Intersection_observer Guide
IntersectionObserver detects element visibility for lazy loading, infinite scroll, and animation triggers. The guide walks through Observer Setup. `new IntersectionObserver(callback,{root,rootMargin,threshold})`. callback receives entries with isIntersecting, intersectionRatio, target. Observe: `observer.observe(element)`. Unobserve: `observer.unobserve(element)`. Use for: lazy load images (swap data-src), infinite scroll sentinel, entrance animations. The guide is organized into 1 sections that build on each other, each pairing a prose explanation with runnable code. Related guides: Arrays, Strings, Numbers, Math, Date. This is one of 145 JavaScript guides on JS Reference — see the full list on the guides page. Practice what you learn in the JavaScript playground, and look up any unfamiliar term in the JavaScript glossary.