javascript - 使用ajax从服务器获取响应时出错

Question

var x = document.getElementById("demo");

function geoapp() {
    if (navigator.geolocation) {
        navigator.geolocation.getCurrentPosition(showPosition);
    }
    else {
        x.innerHTML = "Geolocation is not supported by this browser.";
    }
}

function showPosition(position) {
    var lat = position.coords.latitude;
    var longi = position.coords.longitude;
    var appid = * * * * ;
    var geoapp = 'http://where.yahooapis.com/geocode?       l                        location=' + lat + ',' + longi + '&flags=J&gflags=R&appid=' + appid;
    document.getElementById("demo").innerHTML = geoapp;
    var my_JSON_object = {};
    var http_request = new XMLHttpRequest();
    http_request.open("GET", geoapp, true);
    http_request.onreadystatechange = function() {
        var done = 4,
            ok = 200;
        if (http_request.readyState == done && http_request.status == ok) {
            my_JSON_object = JSON.parse(http_request.responseText);
            document.getElementById("demo").innerHTML = my_JSON_object;
        }
    };
    http_request.send(null);
}​

这段代码从 html5 geoapi 获取坐标，然后将它们发送到 yahoo placefinder api，但是

查询正常时我无法接收 json 对象。任何帮助将不胜感激