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

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

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

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