jQuery(document).ready(function($) {
            $("button").click(function(event) {
                var formData = new FormData($("-sharpformdata")[0]);//[0]
                event.preventDefault();
                $(function(){
                    $.ajax(
                        {
                            type:"post",
                            url : "http://test2.com/formdata.php",
                            data:formData,//
                            cache: false,
                            contentType: false,
                            processData: false,
                            dataType:"json",
                            success  : function(data) {
                                var html = "<div>:"+ data.user+"</div>";
                                html +=  "<div>:"+ data.pass+"</div>";
                                $("-sharpshowTips").html(html);
                            },
                        }
                    );
                });

            });
        });

when submitting a form using formdata, if the above code is changed to

var formData = new FormData($("-sharpformdata"))

will report an error, and all form key-value pairs are not defined. This means that the form form was not fetched. Why is that? Please say the principle, not the title