<div class="go-top"></div>
<script>
$(function(){
var go_top = $('.go-top');
function site(){
var wh = $(window).height()*0.4;
var st = $(document).scrollTop();
if(st > wh)
go_top.fadeIn("slow");
else
go_top.fadeOut("slow");
}
$(window).scroll(function(){
site();
})
go_top.click(function(prem) {
prem.preventDefault();
var top = $(document).scrollTop();
var ini_h = 1;
timer = setInterval(function() {
if (top - ini_h <= 0) {
$(window).scrollTop(0);
timer && clearInterval(timer);
} else {
top = top - ini_h;
$(window).scrollTop(top);
}
ini_h += 5;
}, 15);
});
})
</script>