<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>