Ajax无法正确发送FormData

Question

我正在开发一个评分系统。我的HTML

        <div class="item-vote-wrapper" data-itemid="0">
            <i class="far fa-thumbs-up fa-1.5x" data-index="0"></i>
            <i class="far fa-thumbs-up fa-1.5x" data-index="1"></i>
            <i class="far fa-thumbs-up fa-1.5x" data-index="2"></i>
        </div>

在这里，我要为每个要投票的项目创建一个新的数据项。我的Jquery是

        $('.fa-thumbs-up').on('click', function(){
            ratedIndex = parseInt($(this).data('index'));
            ratedArray[item_id] = ratedIndex;
            localStorage.setItem('ratedArray',  JSON.stringify(ratedArray));

            // prepare data to be send to DB
            let data_for_ajax = new FormData();
            // $(this).parent() - is the parent of this <i> element, meaning div with class "item-vote-wrapper"
            let item_id = $(this).parent().data('itemid');

            // $(this) is the <i> element that was clicked
            let vote = $(this).data('index');

            // Append values to our data object
            // data_for_ajax.append('parameter_name', parameter_value);
            data_for_ajax.append('item_id', item_id);
            data_for_ajax.append('vote_given', vote);

            // debug form data
            for (var pair of data_for_ajax.entries()) {
                console.log("data_for_ajax entry: " + pair[0]+ ', ' + pair[1]); 
            }

            saveToTheDB(data_for_ajax);
        });

然后我与

建立连接

    function saveToTheDB(data_for_ajax){
        $.ajax({
            url: "betrayal.php",
            method: "POST",
            data: data_for_ajax, 
            // By default jquery processes data and serializes it into a string
            // By using a FormData object, we don't need this step, therefore we disable it
            processData: false,

            // Server returns a status code between 200 and 399
            success: function(response) {

                // This code will execute only when the request succeded
                console.log("Success!");

            },

            // Server returns a status code between 400 and 505
            error: function(jqXHR, textStatus, errorThrown){

                console.log("An error occured. Request ended with status: " + textStatus);
                console.log(jqXHR);
            }
        });
    }

现在的问题是，当我在PHP中使用isset（）检查item_id和vote_given时

<?php

function respond($status, $arr = array()){


    // add our status, to final answer
    $arr['status'] = $status;


    if ($status != 'success'){
        if ($status != 'internal_server_error'){
            // Specific to server side errors
            header("HTTP/1.1 500 Internal Server Error");
        } else {
            // Specific to client side errors
            header("HTTP/1.1 400 Bad Request");
        }
    }

    echo json_encode($arr, JSON_PRETTY_PRINT);

    exit();
}

if (isset($_POST['item_id'], $_POST['vote_given'])){
// ...
} else {
    respond(
        'error', 
        array(
            'message' => 'POST data is missing'
        )
    );
}

它返回POST数据丢失，并且分别检查item_id和表决_给定后，我可以知道它们均未设置。这是因为缺少json转换吗？