Datatables: Uncaught TypeError: Cannot read property 'length' of undefined

Question

I'm attempting to use an ajax source with Datatables, and I've run into some errors in doing. Previously Ajax was not being used with Datatables, and they were working fine, but upon trying to use Ajax and JSON I have some errors.

The error I am recieving is the following:

Uncaught TypeError: Cannot read property 'length' of undefined

Edit: Upon using the revised code directly below this text, this error is no longer present but DataTables are still broken (no searching, pagination, sorting, etc...). It may help to have a live example, so try this site: fogest.com/test

To create the tables when the page loads here is the code:

$(document).ready(function() {
    $('#trades').dataTable( {
        "sDom": "<'row'<'span6'l><'span6'f>r>t<'row'<'span6'i><'span6'p>>",
        "sPaginationType": "bootstrap",
        "bProcessing": true,
        "bServerSide": true,
        "aoColumns": [
            { "mData": "id" },
            { "mData": "Minecraft_Username" },
            { "mData": "Block_Name" },
            { "mData": "Quantity" },
            { "mData": "Cost" },
            { "mData": "Trade_Status" },
          ],
        "sAjaxSource": "test.php"
    } );
} );

And sAjaxSource test.php contains the following:

<?php 
$tableName = "mctrade_trades";
$result = mysql_query("SELECT `id`, `Minecraft_Username`, `Block_Name`, `Quantity`, `Cost`, `Trade_Status` FROM $tableName");

$data = array();
while ( $row = mysql_fetch_assoc($result) )
{
    $data[] = $row;
}
header("Content-type: application/json");
echo json_encode( $data );    

?>

The output of test.php:

[{"id":"1","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"100","Trade_Status":"1"},{"id":"2","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"1002","Trade_Status":"1"},{"id":"3","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"1035","Trade_Status":"1"},{"id":"4","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"1035","Trade_Status":"1"},{"id":"5","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"100","Trade_Status":"2"},{"id":"6","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"100","Trade_Status":"2"},{"id":"7","Minecraft_Username":"fog","Block_Name":"DIAMOND_PICKAXE","Quantity":"1","Cost":"10000","Trade_Status":"2"}]

The table is generated correctly but due to this error, there is text saying "Processing right above the table, and you cannot use any of the functions of the datatable, such as searching.

Here is an image of what the table looks like using the above JSON:

I'm assuming the error is in my JSON output, but I do not exactly know what is wrong with it, nor what I should do to fix it. I'm pretty new to Web Development and implementing Datatables has been quite the learning curve!

Answer 1

Your JSON output is wrong for the following reasons:

1. The iTotalRecords and iTotalDisplayRecords fields are missing. This is why the pagination (and all the other functionalities) are broken (notice the message "Showing 1 to NaN of NaN entries (filtered from NaN total entries)" in the footer section). See this page for further details on server-side processing.
2. There is some HTML code after the JSON response.

Here is the extra HTML code (taken from test.php):

<!-- Hosting24 Analytics Code -->
<script src="http://stats.hosting24.com/count.php"></script>
<!-- End Of Analytics Code -->

In my opinion, the test.php script should be like the following:

<?php 

$link = mysqli_init();

// Adjust hostname, username, password and database name before use!
$db = mysqli_real_connect($link, "hostname", "username", "password", "database") or die(mysqli_connect_error());

$SQL = 'SELECT `id`,`Minecraft_Username`,`Block_Name`,`Quantity`,`Cost`,`Trade_Status` FROM mctrade_trades';
$result = mysqli_query($link, $SQL) or die(mysqli_error($link));

$aaData = array();
while ($row = mysqli_fetch_assoc($result)) {
    $aaData[] = $row;
}

$response = array(
  'aaData' => $aaData,
  'iTotalRecords' => count($aaData),
  'iTotalDisplayRecords' => count($aaData)
);
if (isset($_REQUEST['sEcho'])) {
  $response['sEcho'] = $_REQUEST['sEcho'];
}

header('Content-type: application/json'); 
echo json_encode($response);

?>

Be also aware that the mysql_* functions are deprecated, so you should use PDO or MySQLi instead; have a look at this answer for further details.