最low的验证码JS代码通过使用字符串 String API的统一的将随机生成的和用户输入的英文字符统一转换为大写或者小

<!DOCTYPE html>
<html lang="en">
  
  <head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>验证码</title>
  </head>
  
  <body>
  
    <script>
      var codes = [];
      for (i = 48; i <= 57; i++) {
        var code = String.fromCharCode(i);
        codes.push(code);
        // console.log(code);
      }
      for (i = 65; i <= 90; i++) {
        var code = String.fromCharCode(i);
        codes.push(code);
        // console.log(code);
      }
      for (i = 97; i <= 122; i++) {
        var code = String.fromCharCode(i);
        codes.push(code);
        // console.log(code);
      }
      console.log(codes);
      // for (var j = 0; j < 4; j++) {
      // for (var i = 0, str = ''; i < codes.length; i++) {
      //   var a = codes[i];
      //   console.log(a);
      // }
      // str += a
      // }
      for (var i = 0, code = ''; code.length < 4; i++) {
        var i = parseInt(Math.random() * (codes.length));
        // console.log(codes[i]);
        // if (code.indexOf(codes[i]) == -1) {
        code += codes[i]
        // }
      }
      // console.log(code);
  
      var user = prompt('请输入验证码:' + code);
      // console.log(user);
      if (user.toUpperCase() == code.toUpperCase()) {
        document.write('验证成功');
      } else {
        document.write('验证失败');
      }
  
    </script>
  
  </body>

</html>