https://horrorhound.com/js/observe.js

1function observeElements() {
2  const observer = new IntersectionObserver((entries) => {
3    entries.forEach(entry => {
4      if (entry.isIntersecting) {
5        // Add the animation class when the element comes into view
6        entry.target.classList.add('in-view');
7        // You can unobserve after first animation to avoid retriggering
8        observer.unobserve(entry.target);
9      }
10    });
11  }, { threshold: 0.3 }); // Trigger when 30% of the element is visible
12
13  // Get all elements to observe
14  document.querySelectorAll('.observed-item').forEach(item => {
15    observer.observe(item);
16  });
17}