[__Alex__]

Сделал так:

function httpGet(fullUrl) {
        var xhr = new XMLHttpRequest();
        xhr.open("GET", fullUrl, true);
        xhr.send();
        xhr.onreadystatechange = function() { // (3)
        if (xhr.readyState === 4 && xhr.status === 200) {                   alert(xhr.responseText.GeoObjectCollection.featureMember[0].GeoObject.Point.pos);
            }
        }
    }

Даёт ошибку: Uncaught TypeError: Cannot read property 'featureMember' of undefined at XMLHttpRequest.xhr.onreadystatechange