mybutton = document.getElementById("TopBtn");

window.onscroll = function() { scrollFunction() };

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

function dw_void_topFunction() {
    document.body.scrollTop = 0; // For Safari
    document.documentElement.scrollTop = 0; // For Chrome, Firefox, IE and Opera
}