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<button id="scrollToTopBtn" title="Go to top">Top</button>

#scrollToTopBtn {
  display: none; /* Hidden by default */
  position: fixed; /* Fixed/sticky position */
  bottom: 20px; /* Place the button at the bottom of the page */
  right: 30px; /* Place the button 30px from the right */
  z-index: 99; /* Make sure it does not overlap */
  border: none; /* Remove borders */
  outline: none; /* Remove outline */
  background-color: #555; /* Set a background color */
  color: white; /* Text color */
  cursor: pointer; /* Add a mouse pointer on hover */
  padding: 15px; /* Some padding */
  border-radius: 10px; /* Rounded corners */
}

#scrollToTopBtn:hover {
  background-color: #777; /* Add a dark-grey background on hover */
}

// When the user scrolls down 20px from the top of the document, show the button
window.onscroll = function() {scrollFunction()};

function scrollFunction() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) {
    document.getElementById("scrollToTopBtn").style.display = "block";
  } else {
    document.getElementById("scrollToTopBtn").style.display = "none";
  }
}

// When the user clicks on the button, scroll to the top of the document
document.getElementById("scrollToTopBtn").onclick = function() {
  scrollToTop(800); // 800 milliseconds duration for the scroll animation
}

function scrollToTop(scrollDuration) {
  const scrollStep = -window.scrollY / (scrollDuration / 15),
        scrollInterval = setInterval(function(){
          if ( window.scrollY !== 0 ) {
            window.scrollBy( 0, scrollStep );
          } else {
            clearInterval(scrollInterval);
          }
        },15);
}