javascript验证码

<script language="javascript" type="text/javascript">

     var code ; //在全局 定义验证码
     function createCode()
     { 
       code = "";
       var codeLength = 6;//验证码的长度
       var checkCode = document.getElementById("checkCode");
       var selectChar = 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');//所有候选组成验证码的字符，当然也可以用中文的
        
       for(var i=0;i<codeLength;i++)
       {
           var charIndex = Math.floor(Math.random()*36);
           code +=selectChar[charIndex];
       }
//       alert(code);
       if(checkCode)
       {
         checkCode.className="code";
         checkCode.value = code;
       }
       
     }

<script>

<asp:TextBox ID="test" runat="server" onclick="createCode();"></asp:TextBox>

<asp:Label ID="checkCode" runat="server"></asp:Label>