How to get access of a cell's value?

How do I get value of a cell cellval so I can use it later in the fnRowCallback? Any hint will be appreciated.

    $(document).ready(function() {

        var cellval = ...

        var table = $('#cases').DataTable( {
            ajax: 'php/table.cases.php',
            "fnRowCallback": function(nRow, aData, iDisplayIndex, iDisplayIndexFull) {
                console.log(cellval);
            },
        });
    });