JSで頁先頭に遷移

function backToTop()
{
 var x1 = x2 = x3 = 0;
 var y1 = y2 = y3 = 0;

 if(document.documentElement)
 {
  x1 = document.documentElement.scrollLeft || 0;
  y1 = document.documentElement.scrollTop  || 0;
 }

 if(document.body)
 {
  x2 = document.body.scrollLeft || 0;
  y2 = document.body.scrollTop  || 0;
 }

 x3 = window.scrollX || 0;
 y3 = window.scrollY || 0;

 var x = Math.max(x1, Math.max(x2,x3));
 var y = Math.max(y1, Math.max(y2,y3));

 window.scrollTo(Math.floor(x/2), Math.floor(y/2));

 if(x > 0 || y > 0)
 {
  window.setTimeout("backToTop()",25);
 }
}