<div class="grid">
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
  <div class="item">
    <img data-src="https://picsum.photos/300?random" alt="" />
  </div>
</div>

* {
  box-sizing: border-box;
}

.grid {
  display: grid;
  grid-template-columns: auto auto;
  width: 100%;
  max-width: 640px;
  margin: 0 auto;
}

.item {
  padding: 10px;
}

img {
  display: block;
  width: 300px;
  height: 300px;
  opacity: 1;
  transition: opacity 0.5s ease-out;
  
  &[data-src] {
    opacity: 0;
  }
}

 const options = {
  root: null,
  rootMargin: '0px',
  threshold: 0.5
}

function imageLazyLoad() {
	const images = Array.from(document.querySelectorAll('img[data-src]'));

	if (images.length) {
		if ('IntersectionObserver' in window) {
			setupIntersectionObserver(images);
		} else {
			loadImages(images);
		}
	}
}

function setupIntersectionObserver(images) {
	const observer = new IntersectionObserver(onIntersection, options);
	images.forEach(image => observer.observe(image));
}

function onIntersection(entries, observer) {
	entries.forEach((entry) => {
		if (entry.intersectionRatio >= 0.5) {
			observer.unobserve(entry.target);
			loadImage(entry.target);
		}
	});
}

function loadImages(images) {
	images.forEach(loadImage);
}

function loadImage(image) {
	image.setAttribute('src', image.getAttribute('data-src'));
	image.onload = () => image.removeAttribute('data-src');
}

imageLazyLoad();