原生JavaScript实现动态验证码生成与验证

<!doctype html>
<html lang="en">
 <head>
 <meta charset="UTF-8">
 <title>js验证码</title>
 <style type="text/css">
 #code{
 width:80px;
 height:30px;
 font-size:20px;
 font-family:Arial; 
 font-style:italic; 
 font-weight:bold; 
 border:0; 
 letter-spacing:2px; 
 color:blue; 
 }
 </style>
 </head>
 <body>
<div> 
 <input type = "text" id = "input"/> 
 <input type = "button" id="code" /> 
 <input type = "button" value = "验证" id="check"/> 
</div> 
<script type="text/javascript">
window.onload=function(){
 var code=document.getElementById("code");
 function change(){
 // 验证码组成库
 var arrays=new Array( 
 '1','2','3','4','5','6','7','8','9','0', 
 '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', 
 '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'        
       ); 
  // 重新初始化验证码
  codes ='';
  // 随机从数组中获取四个元素组成验证码
  for(var i = 0; i<4; i++){
   // 随机获取一个数组的下标
   var r = parseInt(Math.random()*arrays.length);
   codes += arrays[r];
  }
 // 验证码添加到input里
  code.value = codes;
 }
  change();//加载显示在页面上
  code.onclick = change;//单击更换验证码
//单击验证
 var check=document.getElementById("check");
 var input=document.getElementById("input");
 check.onclick=function(){
   var inputCode = input.value.toUpperCase(); //取得输入的验证码并转化为大写   
  if(inputCode.length==0) { //若输入的验证码长度为0
   alert("请输入验证码！"); //则弹出请输入验证码
  } 
  else if(inputCode!=codes.toUpperCase()) { //若输入的验证码与产生的验证码不一致时
   alert("验证码输入错误！请重新输入"); //则弹出验证码输入错误
   change();//刷新验证码
   input.value="";//清空文本框
  }    
  else{ //输入正确时
   alert("输入正确"); //弹出输入正确
  } 
  }
}
</script>
 </body>
</html>