SOURCE

/*
	function Btn(){
		var Height=Math.floor($(window).height());
		var scrollTop=Math.floor($(document).scrollTop());
		var maxScroll=Math.floor($(document).height())-clientHeight;
		if(scrollTop>clientHeight)
				$('#btn').fadeIn(500);
			else
				$('#btn').fadeOut(500);
		if(scrollTop>maxScroll)
			$('#btn').hide(500);
			else
				$('#btn').show(500);
		
	}
  //返回顶部  
            function returnTop(){  
                $body.animate({scrollTop: 0},400);  
            }  
              
            //返回底部  
            function returnBottom(){  
                $body.animate({scrollTop: $(document).height()},400);  
            }  
*/

 $(document).ready(function () {

        $(window).scroll(function () {
            if ($(this).scrollTop() > 200) {
                $('#btn').fadeIn(500);
            } else {
                $('#btn').fadeOut(500);
            }
        });

        // scroll body to 0px on click
        $('#btn').click(function () {
            $('body,html').animate({
                scrollTop: 0
            }, 500);
            return false;
        });

    });

<div class="contain">
	<h1>Top</h1>
	<h1 class="footer">Footer</h1>
	<div id="btn">goto</div>
</div>

.contain{
	height:2000px;
}
.footer{
	position:absolute;
	margin-bottom:10px;
	bottom:10px;
}
#btn{
	position: fixed;
            bottom: 50px;
            right: 10px;

            background:#eee no-repeat; 
            
            display: none;
            font-weight:bold;
						color:#e81919;
            height:40px;
            width:50px;
            cursor: pointer;
}