Animate elements if visible in viewport (page scroll)

Using IntersectionObserver API

The IntersectionObserver 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.

Here’s an example that triggers a function when an Element is in viewport:

const inViewport = (entries, observer) => {
	entries.forEach(entry => {
		if(entry.isIntersecting && entry.target.id=="php"){
			php.setValueAnimated(9, 3);
		}
	});
};

const Obs = new IntersectionObserver(inViewport);
const obsOptions = {}; //See: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API#Intersection_observer_options

// Attach observer to every [data-inviewport] element:
const ELs_inViewport = document.querySelectorAll('.gauge-container');
ELs_inViewport.forEach(EL => {
	Obs.observe(EL, obsOptions);
});

---

Here’s an example that triggers a classList toggle when an Element is in viewport:

const inViewport = (entries, observer) => {
  entries.forEach(entry => {
    entry.target.classList.toggle("is-inViewport", entry.isIntersecting);
  });
};

const Obs = new IntersectionObserver(inViewport);
const obsOptions = {}; //See: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API#Intersection_observer_options

// Attach observer to every [data-inviewport] element:
const ELs_inViewport = document.querySelectorAll('[data-inviewport]');
ELs_inViewport.forEach(EL => {
  Obs.observe(EL, obsOptions);
});

[data-inviewport] { /* THIS DEMO ONLY */
  width:100px; height:100px; background:#0bf; margin: 150vh 0; 
}

/* inViewport */

[data-inviewport="scale-in"] { 
  transition: 2s;
  transform: scale(0.1);
}
[data-inviewport="scale-in"].is-inViewport { 
  transform: scale(1);
}

[data-inviewport="fade-rotate"] { 
  transition: 2s;
  opacity: 0;
}
[data-inviewport="fade-rotate"].is-inViewport { 
  transform: rotate(180deg);
  opacity: 1;
}

Scroll down...
<div data-inviewport="scale-in"></div>
<div data-inviewport="fade-rotate"></div>