In this post, I will share how could you use callbacks to write efficient and maintainable ajax using callbacks. Promises can also be use but here i will discuss it with callbacks.
The definition of callback is

A reference to executable code, or a piece of executable code, that is passed as an argument to other code. 

Let's see an example code of custom callback,

function test(param1, param2, callback) {
    alert('Started function.\n\nIt has: ' + param1 + ', ' + param2);
    callback();
}
test('foo', 'bar', function() {
    alert('Finished function.');
});

Now I will share the code where you can leverage the callbacks for ajax and maintainable code.

function ajaxGetData(formId, callback) {
    $.ajax({
        url: "/FormController/GetDataFromDb",
        type: "POST",
        dataType: "json",
        data: {
            id: formId
        },
        success: function (data, status, xhr) {
            callback(data);
        }
    });
};

ajaxGetData(123, function(data) {
    console.log(data);
});