JavaScript实现选中文字自动复制

<script>
//获取复制区域id
var oContent =document.getElementById('txtbox');
oContent.onmouseup = function(){
alert(selectText());
};  
function selectText(){
if(document.Selection){       
//ie浏览器
return document.selection.createRange().text;            
}else{    
//标准浏览器
return window.getSelection().toString();         
}        
}       
var oContent =document.getElementById('txtbox');
oContent.onmouseup = function(){
document.execCommand("Copy");   
};  
</script>