How to make JQuery-AJAX request synchronous

you code should be look like ..................

$.ajaxSetup({async: false});

$ajax({ajax call....});

$.ajaxSetup({async: true});

 like :

$.ajaxSetup({async: false});

jQuery.ajax({

                            type: 'POST',
                            url: '/cgi-bin/VUShare/variant_checker.cgi',
                            data: { 'anop': value},
                            success: function(res){

                            VaildOrNot(res,value);



                              },
                            error: function() { alert('sorry'); }
           }); 

$.ajaxSetup({async: true});