https://www.hayawaridenpo.com/js/page_scroll.js

1function backToTop() {
2  var x1 = x2 = x3 = 0;
3  var y1 = y2 = y3 = 0;
4  if (document.documentElement) {
5      x1 = document.documentElement.scrollLeft || 0;
6      y1 = document.documentElement.scrollTop || 0;
7  }
8  if (document.body) {
9      x2 = document.body.scrollLeft || 0;
10      y2 = document.body.scrollTop || 0;
11  }
12  x3 = window.scrollX || 0;
13  y3 = window.scrollY || 0;
14  var x = Math.max(x1, Math.max(x2, x3));
15  var y = Math.max(y1, Math.max(y2, y3));
16  window.scrollTo(Math.floor(x / 2), Math.floor(y / 2));
17  if (x > 0 || y > 0) {
18      window.setTimeout("backToTop()", 25);
19  }
20}