js实现文本框中内容的放大显示

<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>文本框放大</title>
<script>
function $(id){
return document.getElementById(id);
}

function showBigInfo(obj){
var bigObj;
if(!$("bigInfo")){
bigObj=document.createElement("span");
bigObj.setAttribute("id","bigInfo");
bigObj.style.position="absolute";
bigObj.style.border="1px solid #F93";
bigObj.style.borderRadius="2px";
bigObj.style.color="red";
bigObj.style.fontSize="26px";
bigObj.style.fontWeight="bold";
bigObj.style.padding="4px";
bigObj.style.display="none";
document.body.appendChild(bigObj);
obj.onblur=function(){
$("bigInfo").style.display="none";
}
obj.onfocus=function(){
$("bigInfo").style.display="block";
}
}else{
bigObj=$("bigInfo");
}
bigObj.style.display="block";
var str="";
var info=obj.value;
for(var i=0;i<info.length;i+=4){
str+=info.substr(i,4)+" ";
}
bigObj.innerHTML=str; 
if(obj.offsetTop<50){ //放大框在下方显示
bigObj.style.top=(obj.offsetTop+obj.offsetHeight+2)+"px";
}else{ //放大框在上方显示
bigObj.style.top=(obj.offsetTop-bigObj.offsetHeight-2)+"px";
}
bigObj.style.left=(obj.offsetLeft+obj.offsetWidth/2-bigObj.offsetWidth/2)+"px";
}
</script>
</head>

<body>
<br /><br /><br /><br /><br /><br />
<center>

请输入身份证号码：<input type="password" onKeyUp="showBigInfo(this)" maxlength="18" size="30"/>
<br />
请输入身份证号码：<input type="password" onKeyUp="showBigInfo(this)" maxlength="18" size="30"/>
<br />
请输入身份证号码：<input type="password" onKeyUp="showBigInfo(this)" maxlength="18" size="30"/>

</center>
</body>
</html>