js生成、验证验证码

js代码：

//关于验证码
var code ; //在全局 定义验证码  
     function createCode()   //生成验证码
     {    
       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        {    
          
       var charIndex = Math.floor(Math.random()*36);  
       code +=selectChar[charIndex];          
         
       }  
      
       if(checkCode)  
       {  
        
         checkCode.value = code;
       
       }  
         
     }  
    
function validate()   //验证码验证
     {  
       var inputCode = document.getElementById("codeContent").value;
       document.getElementById("codetip").innerHTML ="";  
       if(inputCode.length <=0)  
       {  
           document.getElementById("codetip").innerHTML ="请输入验证码";
           createCode();
     return false;
       }  
       else if(inputCode != code )  
       {   
          document.getElementById("codetip").innerHTML="验证码输入错误";
          createCode();
          return false;
       }  
       else  
       {  
          return true;
  
       }  
         
       }

html代码：

              
                验证码  
              
              
                                 onchange="validate();" />
                                 readonly="readonly" />