js实现点击定位最顶端

//------------------------------------点击按钮------------------------------------

<span onClick="gotoTop('2','2');return false;"></span>
//------------------------------------js------------------------------------

<script type="text/javascript">
function gotoTop(acceleration,stime) {
acceleration = acceleration || 0.1;
stime = stime || 10;
var x1 = 0;
var y1 = 0;
var x2 = 0;
var y2 = 0;
var x3 = 0;
var 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;
}
var x3 = window.scrollX || 0;
var y3 = window.scrollY || 0;

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

var speeding = 1 + acceleration;
window.scrollTo(Math.floor(x / speeding), Math.floor(y / speeding));

if(x > 0 || y > 0) {
var run = "gotoTop(" + acceleration + ", " + stime + ")";
window.setTimeout(run, stime);
};
}
</script>