DataTables Cannot read property 'length' of undefined

Question

Below is the document ready function

Script type="text/javascript" charset="utf-8">
    $(document).ready(function () {
        $('#example').dataTable({
            "bProcessing": true,
            "bServerSide": true,
            "sAjaxSource": "GetUser.ashx",
            "sServerMethod": "POST",
            "sAjaxDataProp" : "",
            "aoColumnDefs": [ {
            "aTargets": [ 0 ],
            "mData": "download_link",
            "mRender": function ( data, type, full ) {
               return '<a href="/UserDetail.aspx?ID='+data+'">Detail</a>';
             }
           } ],
            "aoColumns": [
                { "mData": "LoginId" },
                { "mData": "Name" },
                { "mData": "CreatedDate" }
            ]
        });

Below is the respond from server (GetUser.ashx)

[
    {
        "UserId": "1",
        "LoginId": "white.smith",
        "Activated": "Y",
        "Name": "Test Account",
        "LastName": "Liu",
        "Email": "[email protected]",
        "CreatedDate": "1/21/2014 12:03:00 PM",
        "EntityState": "2",
        "EntityKey": "System.Data.EntityKey"
    },
More Data...
]

Below is the html table where the data should be put

<table cellpadding="0" cellspacing="0" border="0" class="display" id="example">
    <thead>
        <tr>
             <th width="15%">User Detail</th>
            <th width="15%">LoginID</th>
            <th width="15%">Name</th>
            <th width="15%">Created Date</th>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td colspan="5" class="dataTables_empty">Loading data from server</td>
        </tr>
    </tbody>
    <tfoot>
        <tr>
             <th width="15%">User Detail</th>
            <th width="15%">LoginID</th>
            <th width="15%">Name</th>
            <th width="15%">Created Date</th>
        </tr>
    </tfoot>
</table>

Expected result:

But I came across a problem:

While the page is loading, there was an uncaught exception from the browser:

Cannot read property 'length' of undefined 

When I further check, it came from line 2037 of jquery.dataTables.js

var aData = _fnGetObjectDataFn( oSettings.sAjaxDataProp )( json );

I checked that the json was valid, but the "aData" was null, why this happen?

Answer 1

Your "sAjaxDataProp" : "" is set to an empty string, which causes this error.

dataTables expects to have a string here to tell under which key your server returned data can be found. This defaults to aaData, so your json should include this key amongst all others that might be returned or needed by pagination etc.

Normal serversided json:

{
"iTotalRecords":"6",
"iTotalDisplayRecords":"6",
"aaData": [
    [
        "1",
        "sameek",
        "sam",
        "sam",
        "[email protected]",
        "1",
        ""
    ],...

Since all values are in aaData you don't need sAjaxDataProp at all.

Modified serverside json:

{
"iTotalRecords":"6",
"iTotalDisplayRecords":"6",
"myData": [
    [
        "1",
        "sameek",
        "sam",
        "sam",
        "[email protected]",
        "1",
        ""
    ],

Now the values are in myData. so you need to tell dataTables where to find the actual data by setting:

"sAjaxDataProp" : "myData"

Here is a Plunker

Answer 2

As there are 4 columns, add the following in "aoColumns":

"aoColumns": [
  { "mData": null },  // for User Detail
  { "mData": "LoginId" },
  { "mData": "Name" },
  { "mData": "CreatedDate" }
]

For undefined length, I have tried the following code and it's working:

$('#example').dataTable({
 "aLengthMenu": [[100, 200, 300], [100, 200, 300]],
  "iDisplayLength": 100,
  "iDisplayStart" : 0,
  "bProcessing": true,
  "bServerSide": true,
  "sAjaxSource": "GetUser.ashx",
  "sServerMethod": "POST",
  "sAjaxDataProp" : "",
  "aoColumnDefs": [ {
    "aTargets": [ 0 ],
    "mData": "download_link",
    "mRender": function ( data, type, full ) {
      return '<a href="/UserDetail.aspx?ID='+data+'">Detail</a>';
    }
  } ],
  "aoColumns": [
    { "mData": null },
    { "mData": "LoginId" },
    { "mData": "Name" },
    { "mData": "CreatedDate" }
  ]
});

The reference site to know more about aLengthMenu is:

https://legacy.datatables.net/ref#aLengthMenu