判断滚动条是否到达页面的尾部

//取窗口滚动条高度 
function getScrollTop() 
{ 
var scrollTop=0; 
if(document.documentElement&&document.documentElement.scrollTop) 
 { 
 scrollTop=document.documentElement.scrollTop; 
 } 
else if(document.body) 
 { 
 scrollTop=document.body.scrollTop; 
 } 
return scrollTop; 
} 


//取窗口可视范围的高度 
function getClientHeight() 
{ 
var clientHeight=0; 
if(document.body.clientHeight&&document.documentElement.clientHeight) 
 { 
var clientHeight = (document.body.clientHeight<document.documentElement.clientHeight)?document.body.clientHeight:document.documentElement.clientHeight; 
 } 
else 
 { 
var clientHeight = (document.body.clientHeight>document.documentElement.clientHeight)?document.body.clientHeight:document.documentElement.clientHeight; 
 } 
return clientHeight; 
} 


//取文档内容实际高度 
function getScrollHeight() 
{ 
return Math.max(document.body.scrollHeight,document.documentElement.scrollHeight); 
} 
function test(){ 
if (getScrollTop()+getClientHeight()==getScrollHeight()){ 
 alert("到达底部"); 
 }else{ 
 alert("没有到达底部"); 
 } 
 }