https://www.anet-yokyo.jp/js/back.js

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