function randomString(len) {
    len = len || 31;
    var $chars = 'ABCDEFGHJKMNPQRSTWXYZabcdefhijkmnprstwxyz2345678';    /****默认去掉了容易混淆的字符oOLl,9gq,Vv,Uu,I1****/
    var maxPos = $chars.length;
    var pwd = '';
    for (i = 0; i < len; i++) {
        pwd += $chars.charAt(Math.floor(Math.random() * maxPos));
    }
    return pwd;
}
document.write(randomString(31));