JavaScript 随机验证码的生成实例代码

<script>  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) //这里不是很懂,有高手可以解释下     {      checkCode.className="code";      checkCode.value = code;     }         }     function validate ()    {     var inputCode = document.getElementById("input1").value;     if(inputCode.length <=0)     {       alert("请输入验证码！");       document.getElementById("input1").focus();     }     else if(inputCode != code )     {      alert("验证码输入错误！");      createCode();//刷新验证码      inputCode = "";      document.getElementById("input1").focus();     }else{       document.frmRegister.submit();     }               } <script>