jquery ajax在ie9下跨域不执行 crossDomain: true == !(document.all)

<!DOCTYPE html>
<html>
    <head>
        <title>jQuery CORS in IE7 - IE10</title>
        <script src="http://code.jquery.com/jquery-xxxx.min.js"></script>
        <script>
            $(document).ready(function() {
              $.ajax({
                url: "http://xxxx.php",
                dataType: "text",
                    async: true,
                    type: 'GET',
                    cache: false,
                    crossDomain:  true == !(document.all),
                success: function(txt) {
                    //TODO
                }
              });
            });
        </script>
        </head>
    <body>
        IE7到IE10使用jQuery跨域！！！
    </body>
</html>