写如下代码：

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();

发现会提示：
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.

如何实现类似getJSON的JSONP跨域呢？