js简单验证码

<script language="javascript" type="text/javascript">
var code ; //在全局 定义验证码
function createCode()
{ 
code = "";
var codeLength = 4;//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;
}


}
// 窗体刷新自动执行createcode函数
window.onload=createCode();
</script>