如何使用原生JS实现类似jQuery的getJSON功能（跨域）?

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？

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cc http://.com/q/1010000000513202#a-1020000000513237

动态加载script DOM的代码：

var s = document.createElement('script');
s.src = "http://xxxx.com/api.php?callback=sdffdf";
document.body.append(s)