通过ajax的DataTables数据+列有效，但在浏览器中提供了额外的“未定义”

Question

在我的DataTables中，我通过ajax获取列信息和行数据。他们都按预期解析。

然而，在DataTable之上，我得到了一个额外的“未定义”文本。

HTML来源：

<div id="dapel"></div>

DOM来源：

<div id="dapel">undefined<div class="dataTables_wrapper no-footer" id="displ...

SCRIPT：

    $(document).ready(function($){
    $.ajax({
        "url": "'.$refresh.'",
        "dataType": "json",
        "success": function(json) {
            var tableHeaders;
            $.each(json.columns, function(i, val){
                tableHeaders += "<th "+ val.width +">" + val.key + "</th>";
            });

            $("#dapel").empty();
            $("#dapel").append(\'<table id="displayTable" class="display" cellspacing="0" width="100%" style="font-size: 0.7em;table-layout: fixed;word-wrap:break-word;"><thead><tr>\' + tableHeaders + \'</tr></thead></table>\');
            $("#displayTable").dataTable({
                "processing": true,
                "serverSide": true,
                "paging": false,
                "ajax": "'.$refresh.'",
            });
        },
    });
});

data.php

$json['columns'][] = array('key' => 'SERVER'            , 'width' => 50);
$json['columns'][] = array('key' => 'Position'          , 'width' => 50);
$json['columns'][] = array('key' => 'Office'            , 'width' => 50);
$json['columns'][] = array('key' => 'Extn'              , 'width' => 50);
$json['columns'][] = array('key' => 'Start date'        , 'width' => 50);
$json['columns'][] = array('key' => 'Salary'            , 'width' => 50);

$json['data'][] = array('Tiger Nixxon','System Architect','Edinburgh','5421','2011/04/25','$320,800',);
$json['data'][] = array('Garrett Winters','Accountant','Tokyo','8422','2011/07/25','$170,750',);
$json['data'][] = array('Ashton Cox','Junior Technical Author','San Francisco','1562','2009/01/12','$86,000',);
$json['data'][] = array('Cedric Kelly','Senior Javascript Developer','Edinburgh','6224','2012/03/29','$433,060',);
$json['data'][] = array('Airi Satou','Accountant','Tokyo','5407','2008/11/28','$162,700',);

$json['recordsTotal']           = 3000;
$json['recordsFiltered']        = 40;

echo json_encode($json);