寫如下代碼：        request = new XMLHttpRequest();        request.open('GET', url, true);        request.onreadystatechange = function() {          if (request.status >= 200 && request.status < 400){            // Success!            data = JSON.parse(request.responseText);            console.log(data);            if(data.ret == 1){                window.location = data.url;            }          }         };        request.send();發現會提示：XMLHttpRequest cannot load http://XXXXX.com/app.php?callback=cb. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://www.YYYY.com' is therefore not allowed access.如何實現類似getJSON的JSONP跨域呢？