生成指定位数的仅包含数字跟字母的验证码：
generateRandomAlphaNum（len）：len为验证码字符个数
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
</head>
<body>
<span id="aiphaNum"></span>
<script type="text/javascript">
function generateRandomAlphaNum(len) {
var rdmString = "";
for(var i = 0; rdmString.length < len; i++){
rdmString += Math.random().toString(36).substr(2);
};
return rdmString.substr(0, len);
};
document.getElementById("aiphaNum").innerHTML = generateRandomAlphaNum(4);
</script>
</body>
</html>