随机验证码生成和验证

/**从b
 * 
 */
var code ; //在全局定义验证码   
//产生验证码  
window.onload = function createCode(){  
     code = "";   
     var codeLength = 4;//验证码的长度  
     var checkCode = document.getElementById("code");   
     var random = new Array(0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R',  
     'S','T','U','V','W','X','Y','Z');//随机数  
     for(var i = 0; i < codeLength; i++) {//循环操作  
        var index = Math.floor(Math.random()*36);//取得随机数的索引（0~35）  
        code += random[index];//根据索引取得随机数加到code上  
    }  
    checkCode.value = code;//把code值赋给验证码  
}  
//校验验证码  
function validate(){  
    var inputCode = document.getElementById("input").value.toUpperCase(); //取得输入的验证码并转化为大写        
    if(inputCode.length <= 0) { //若输入的验证码长度为0  
        alert("请输入验证码！"); //则弹出请输入验证码  
    }         
    else if(inputCode != code ) { //若输入的验证码与产生的验证码不一致时  
        alert("验证码输入错误！@_@"); //则弹出验证码输入错误  
        createCode();//刷新验证码  
        document.getElementById("input").value = "";//清空文本框  
    }         
    else { //输入正确时  
        alert("^-^"); //弹出^-^  
    }             
}

html代码

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
 <title>验证码</title>  
        <style type="text/css">  
            #code  
            {  
                font-family:Arial;  
                font-style:italic;  
                font-weight:bold;  
                border:0;  
                letter-spacing:2px;  
                color:blue;  
            }  
        </style>  
        <script type = "text/javascript" src = "js/checkCode.js">  
        </script>
</head>
<body>
<div>  
            <input type = "text" id = "input"/>
            <input type = "button" id="code" onclick="createCode()"/>  
            <input type = "button" value = "验证" onclick = "validate()"/>
            <a href="" onclick="createCode()">刷新</a>  
        </div> 
</body>
</html>