js生成验证码

 var zkzcode; //在全局 定义验证码  
 function createCode() {
   zkzcode = "";
   var zkzcodeLength = 4;//验证码的长度  
   var checkCode = document.getElementById("checkCode");//此处是显示验证码的地方
   var selectChar = new Array(2, 3, 4, 5, 6, 7, 8, 9,'A','B','C','D','E','F','G','H','J','K','M','N','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 < zkzcodeLength; i++) {
     var charIndex = Math.floor(Math.random() * 31);
     zkzcode += selectChar[charIndex];
   }
   if (checkCode) {
     checkCode.className = "zkzcode";
     checkCode.value = zkzcode;
   }
 }