JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>

    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1">
    </head>

    <body>

        <button onclick="topFunction()" id="myBtn" title="Go to top">Top</button>

        <div style="background-color:black;color:white;padding:30px">Scroll Down</div>
        <div style="background-color:lightgrey;padding:30px 30px 2500px">This example demonstrates how to create a "scroll to top" button that becomes visible
            <strong>when the user starts to scroll the page</strong>.</div>
        <div id="footer">
            This is footer.
        </div>
    </body>

</html>

CSS

body {
  font-family: Arial, Helvetica, sans-serif;
  font-size: 20px;
}

#myBtn {
  display: none;
  position: fixed;
  bottom: 20px;
  right: 30px;
  z-index: 99;
  font-size: 18px;
  border: none;
  outline: none;
  background-color: red;
  color: white;
  cursor: pointer;
  padding: 15px;
  border-radius: 4px;
}

#myBtn:hover {
  background-color: #555;
}

JavaScript

//Get the button
var mybutton = document.getElementById("myBtn");

// 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) {
    mybutton.style.display = "block";
  } else {
    mybutton.style.display = "none";
  }
  let footerElement = document.getElementById('footer');
  let footerElementRect = footerElement.getBoundingClientRect();
  let mybuttonPositionBottom = mybutton.offsetTop + mybutton.offsetHeight;
  if (footerElementRect.y < mybuttonPositionBottom) {
  	let diffheight = mybuttonPositionBottom - footerElementRect.y;
    let style = window.getComputedStyle(mybutton);
    let addBottom = parseInt(style.getPropertyValue('bottom')) + diffheight;
    mybutton.style.bottom = addBottom + 'px';// maybe add more 10 px for bottom space.
  } else {
  	mybutton.style.bottom = '';
  }
}

// When the user clicks on the button, scroll to the top of the document
function topFunction() {
  document.body.scrollTop = 0;
  document.documentElement.scrollTop = 0;
}