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const observeScroll = (element, callback) => {
  const observer = new IntersectionObserver( ([entry]) => {
    if (entry.intersectionRatio < 1) return;

    callback(element);
    observer.disconnect(); // Stop watching the element
  }, 
  {
    threshold: 1
  });

  observer.observe(element); // Start watching the element
};

function startAnimation(element) { ... }

const element = document.getElementById("express-animation");
observeScroll(element, startAnimation);