Wait for callback in javascript

I'm trying to create a function that returns a object with information of a callback:

var geoloc;

var successful = function (position) {
    geoloc = {
        longitude: position.coords.longitude,
        latitude: position.coords.latitude
    };
};

var getLocation = function () {
    navigator.geolocation.getCurrentPosition(successful, function () {
        alert("fail");
    });

    return geoloc;
};

How can I do this? The function getLocation return null value before successful is executed.

Thanks!