Return ajax response to variable

I am trying to learn ajax but I dont know how to do this. When I click on the map the function called “region()” will be running as well as the ajax inside. From the console i got response from the ajax call, it is a numeric data. But I dont know how to return back the response to the variable “boosting”

                        var boosting = <?php echo($difference)?>;
                        var produk = $("select[name='produk']").val();
                        function region(province) {
                                var kfas = province.split(' ').join('_');
                                var produk = $("select[name='produk']").val();
                                var url_get = '<?php echo site_url('boosting/call_rekomendasi_data/')?>'+kfas+'/'+ produk;

                                        url: url_get,
                                        type: 'GET',
                                        error: function() {
                                            alert('Something is wrong with recomendation');
                                        success: function(data) {

#here i am trying to return back the value into variable “boosting”