JS带提示登录表单验证以前在哪偷的

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>JS带提示登录表单验证</title>
	<style type="text/css">
		*{margin:0;padding:0;font-family:"微软雅黑";font-size:12px;}
		.box{width:390px;height:320px;border:solid 1px #ddd;background:#FFF;position:absolute;left:50%;top:42%;margin-left:-195px;margin-top:-160px;}
		.box h2{font-weight:normal;color:#666;font-size:16px;line-height:46px;height:46px;overflow:hidden;text-align:center;border-bottom:solid 1px #ddd;background:#f7f7f7;}
		.input_box{width:350px;padding-bottom:15px;margin:0 auto;overflow:hidden;}
		.input_box input{float:left;width:348px;height:40px;font-size:14px;border:solid 1px #ddd;text-indent:10px;outline:none;line-height:40px;}
		.input_box button{width:350px;height:48px;background:#3f89ec;border:none;border-radius:2px;outline:none;cursor:pointer;font-size:16px;color:#FFF;}
		#error_box{height:40px;width:350px;margin:0 auto;line-height:40px;color:#fc4343;}
	</style>
</head>
<body>
	<div class="box">
		<h2>登录</h2>
	    <div id="error_box"></div>
	    <div class="input_box">
	    	<input type="text" placeholder="请输入账户名" id="uname"/>
	    </div>
	    <div class="input_box">
	    	<input type="password" placeholder="请输入密码" id="upass"/>
	    </div>
	    <div class="input_box">
	    	<button onclick="fnLogin()">登录</button>
	    </div>
	</div>
</body>
<script type="text/javascript">
//长度必须在6~20位之间
//开头不能为数字
//只能包含小写字母和数字
//数字：48~57
//小写字母：97~122
//innerHTML
function fnLogin(){
	var oUname = document.getElementById("uname");
	var oUpass = document.getElementById("upass");
	var oError = document.getElementById("error_box");
	var isNotError = true;
	if(oUname.value.length > 20 || oUname.value.length < 6){
		oError.innerHTML = "用户名长度必须在6~20位之间";
		isNotError = false;
		return;
	}else if(oUname.value.charCodeAt(0) >= 48 && oUname.value.charCodeAt(0) <= 57){
		oError.innerHTML = "用户名开头不能为数字";
		isNotError = false;
		return;
	}else{
		for(var i=0; i<oUname.value.length; i++){
			if((oUname.value.charCodeAt(i) > 122 || oUname.value.charCodeAt(i) < 97) && (oUname.value.charCodeAt(i) > 57 || oUname.value.charCodeAt(i) < 48)){
				oError.innerHTML = "用户名只能包含小写字母和数字";
				isNotError = false;
				return;
			}
		}
	}
	if(oUpass.value.length > 20 || oUpass.value.length < 6){
		oError.innerHTML = "密码长度必须在6~20位之间";
		isNotError = false;
		return;
	}
	oError.innerHTML = "登录成功";
}
</script>
</html>