DataTables: Cannot read property 'length' of undefined

Question

I understand this a popular issue, and I have read all the similar questions here on Stack Overflow and other sites (including the datatables website).

To clarify, I

Answer 1

If you are using ajax as a function remember it expects JSON data to be returned to it, with the parameters set.

$('#example').dataTable({
    "ajax" : function (data, callback, settings) {
        callback({
            data: [...],
            recordsTotal: 40,
            recordsFiltered: 40}
            ));
    }
})