IntersectionObserver to use with Next-Gen Image Formats

IntersectionObserver.js

const options = {
    rootMargin: "25px",
    threshold: 0.1
  };
  const io = new IntersectionObserver(entries => {
    entries.forEach(entry => {
      //console.log('entry: ', entry);
      if (entry.intersectionRatio > 0.1) {
        var imgsrc = entry.target.getAttribute("srcset1");
        entry.target.setAttribute("srcset", imgsrc);
        entry.target.removeAttribute("srcset1");
        io.unobserve(entry.target);
      }
    });
  }, options);
  
  const targetElements = document.querySelectorAll("source");
  for (let element of targetElements) {
    //console.log('element: ', element);
    io.observe(element);
  }