JavaScript实现不重复验证码生成与验证

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>不重复验证码</title>
 <style>
  .cont{width:300px;height:auto;margin:50px auto;}
    #useId{width:120px;height:45px;line-height:45px;background:#ccc;text-align:center;font-size:20px;margin:10px;}
 </style>
</head>
<body>
 <div class="cont">
 <div id="useId"></div>
 <input type="text" id="txt" />
 <input type="button" id="btn" value="Start" />
 <div>
 <script>
    function $(id){
     return document.getElementById(id);
    }
    var $useId = $('useId'),
      $txt = $('txt'),
      $btn = $('btn');
    function getStr(){
    var string = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
      str = '';
      while(str.length < 6){
     var str1 = string.charAt(Math.round(Math.random() * 56));
     if(str.indexOf(str1) === -1){
      str += str1;
     }
    }
    $useId.innerHTML = str;
    }
    function checkCode(){
     if($txt.value == $useId.innerHTML){
     alert('验证成功');
     }else{
     alert('验证失败');
     }
    }
    $useId.onclick = getStr;
    $btn.onclick = checkCode;
 </script>
</body>
</html>