Go to top function with slide effect

var goto_top_type = -1;
var goto_top_itv = 0;
 
function goto_top_timer()
{
  var y = goto_top_type == 1 ? document.documentElement.scrollTop : document.body.scrollTop;
  var moveby = 15;
 
  y -= Math.ceil(y * moveby / 100);
  if (y < 0) {
    y = 0;
  }
 
  if (goto_top_type == 1) {
    document.documentElement.scrollTop = y;
  }
  else {
    document.body.scrollTop = y;
  }
 
  if (y == 0) {
    clearInterval(goto_top_itv);
    goto_top_itv = 0;
  }
}
 
function goto_top()
{
  if (goto_top_itv == 0) {
    if (document.documentElement && document.documentElement.scrollTop) {
      goto_top_type = 1;
    }
    else if (document.body && document.body.scrollTop) {
      goto_top_type = 2;
    }
    else {
      goto_top_type = 0;
    }
    if (goto_top_type > 0) {
      goto_top_itv = setInterval('goto_top_timer()', 50);
    }
  }
}