//Get the button: 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) {
  alert('yes');
  mybutton.style.display = "block";
} else {
  mybutton.style.display = "none";
  alert('yes');
}

}

// When the user clicks on the button, scroll to the top of the document function topFunction() {

document.body.scrollTop = 0; // For Safari
document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera

}