my footerCallback not work plss help me
it is my code
$(document).ready(function() {
$('#example').DataTable({

            "ajax": {
                "url": "script.php",
                "dataSrc": ""
            },

            "columns": [{
                    "data": "catname"
                },
                {
                    "data": "stname"
                },
                {
                    "data": "buyingprice"
                },
                {
                    "data": "salesprice"
                },
                {
                    "data": "monname"
                },
                {
                    "mRender": function(data, type, row) {
                        return parseInt(row.salesprice / row.number);
                    }
                },
                {
                    "mRender": function(data, type, row) {
                        return parseInt(row.salesprice - row.buyingprice);
                    }
                },
                {
                    "mRender": function(data, type, row) {
                        ;
                    }
                },
                {
                    "data": "number"
                },
                {
                    "data": "sum"
                },
                {
                    "mRender": function(data, type, row) {
                        return parseInt(row.number - row.sum);
                    }
                },

sum = came from
and I want to calculate this three columns {
"mRender": function(data, type, row) {
return parseInt(row.salesprice / row.number);
}
},
{
"mRender": function(data, type, row) {
return parseInt(row.salesprice - row.buyingprice);
}
},
{
"mRender": function(data, type, row) {
return parseInt(row.number - row.sum);
}
},
it is my footer callback code

                var thuTotal = api
                    .column(4)
                    .data()
                    .reduce(function(a, b) {
                        return intVal(a) + intVal(b);
                    }, 0);

$(api.column(4).footer()).html(thuTotal);