plain.ts

const observer = new IntersectionObserverr(onCallbackIntersectionObserver, {
  threshold: 0,
  // Begin intersecting 25% of the viewport height before it appears to reduce chance of displaying
  // unloaded content
  rootMargin: '0px 0px 25% 0px',
});

function onCallbackIntersectionObserver(entries: IntersectionObserverEntry[]) {
  // Ignore additional entries since there's only one.
  // Add the logic if you plan to handle multiple entries per intersection observer instance
  // especially in production code.
  const entry = entries[0];

  observer.unobserve(entry);
  entry.src = entry.dataset.src;
}

const elements = document.querySelectorAll('img[data-src]');
elements.forEach(observer.observe);