基于MooTools的 返回顶部按钮

<script>
window.addEvent('domready',function(){
$('gotop').inject(document.body).addEvent('click',function(){
new Fx.Scroll(window,{link:'cancel'}).toTop();
}).setOpacity(.8);

this.addEvent('scroll',function(){
if(window.getScrollTop()>100){
$('gotop').setStyle('top',window.getSize().y + window.getScrollTop() - $('gotop').getSize().y).show();
}else{
$('gotop').hide();
}
});
});
</script>

演示网址：http://www.etzpf.com/