Use callback output in a javascript variable

Question

I have a variable within the success of function $.ajax() only that I need the contents of that variable in a part of the code, outside the $.ajax(). I’m trying to create the way:

jQuery.ajax({

    url: 'http://localhost/comOferta/Api/cep',
    data: {dados:dados},
    type: 'POST',
    success: function (callback){
       retorno = JSON.parse("["+callback+"]");
       var cep = retorno[0].cep;
    }
});

var enderecoPartida = cep+', Brasil';
var enderecoChegada = '08215-255, Brasil';

only that it’s not working, in the console gives:

Referenceerror: cep is not defined

Which way I can do to rescue the result that is in the variable cep ?

Complete code

 function sucesso( posicao ){

            var positions = new Array();

            positions['latitude'] = posicao.coords.latitude;
            positions['longitude'] = posicao.coords.longitude;

            initialize();

            var dados = '-23.5282792,-46.4514346';
            var cep;

            jQuery.ajax({

                url: 'http://localhost/comOferta/Api/cep',
                data: {dados:dados},
                type: 'POST',
                async: false,


                success: function (callback){

                    retorno = JSON.parse("["+callback+"]");

                    cep = retorno.cep;

                }
            });

            alert(cep);

            var enderecoPartida = '08210-791, Brasil';
            var enderecoChegada = '08215-255, Brasil';

            var request = {
                origin: enderecoPartida,
                destination: enderecoChegada,
                travelMode: google.maps.TravelMode.DRIVING
            };

            directionsService.route(request, function(result, status) {
                if (status == google.maps.DirectionsStatus.OK) {
                    directionsDisplay.setDirections(result);
                }
            });

        }

        function error(err){
            console.log(err.code);
        }


function getPosition(){

    var options = {
      enableHighAccuracy: true,
      timeout: 5000,
      maximumAge: 0
    };

    if ( navigator.geolocation ){

        navigator.geolocation.getCurrentPosition( sucesso , error, options);

    }
}

function mapa(){
getPosition();
}