<main>
  <h1 id="target">Scroll to Down 👇</h1>
  <h2>When the headline above disappears, the Top Button appears..</h2>
  <!-- this is a little update from this:
https://codepen.io/marcelrojas/pen/WNwgBRg
so: not my work, just a little tweak
-->
  <!-- Button on fixed on bottom right corner of the page -->
  <button class="scrollToTopBtn">☝️</button>
</main>
<footer></footer>

* {
  box-sizing: border-box
}

body {
  margin: 0;
  background-image: linear-gradient(#d16ba5, #e17497, #eb828b, #ee9183, #eda280, #f1ad7d, #f3b97b, #f3c67b, #f8d273, #fbdf6b, #f9ec64, #f4fb5f)
}

main {
  height: 200vmin
}

h1 {
  color: white;
  font-family: system-ui, sans-serif;
  font-size: 48px;
  text-align: center
}
h2 {
  margin-top: 200px;
  color: white;
  font-family: system-ui, sans-serif;
  font-size: 48px;
  text-align: center
}

footer {
  display: flex;
  justify-content: flex-end;
  padding: 16px
}

.scrollToTopBtn {
  background-color: black;
  border: none;
  border-radius: 50%;
  color: white;
  cursor: pointer;
  font-size: 16px;
  line-height: 48px;
  width: 48px;
  
  /* place it at the bottom right corner */
  position: fixed;
  bottom: 30px;
  right: 30px;
  /* keep it at the top of everything else */
  z-index: 100;
  /* hide with opacity */
  opacity: 0;
  /* also add a translate effect */
  transform: translateY(100px);
  /* and a transition */
  transition: all .5s ease
}

.showBtn {
  opacity: 1;
  transform: translateY(0)
}

// We select the element we want to target
var target = document.getElementById("target");

var scrollToTopBtn = document.querySelector(".scrollToTopBtn")
var rootElement = document.documentElement

// Next we want to create a function that will be called when that element is intersected
function callback(entries, observer) {
  // The callback will return an array of entries, even if you are only observing a single item
  entries.forEach(entry => {
    if (!entry.isIntersecting) {
      // Show button
      scrollToTopBtn.classList.add("showBtn")
    } else {
      // Hide button
      scrollToTopBtn.classList.remove("showBtn")
    }
  });
}

function scrollToTop() {
  rootElement.scrollTo({
    top: 0,
    behavior: "smooth"
  })
}
scrollToTopBtn.addEventListener("click", scrollToTop);
    
// Next we instantiate the observer with the function we created above. This takes an optional configuration
// object that we will use in the other examples.
let observer = new IntersectionObserver(callback);
// Finally start observing the target element
observer.observe(target);