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

$ = {
    getjs on:function(url,data,callback){
        var call = this.urlquerystring(url)["callback"];
        for(var key in data){
            url += "&"+key+"="+data[key];
        }
        //window[call] = callback;
        window[call] = function(data){
            window[call] = undefined;
            try{
                delete window[call];
            }catch(e){}
            callback(data);
        }
        var script = document.createelement("script");
        script.type = "text/javascript";
        script.src = url;
        document.getelementsbytagname("body")[0].appendchild(script);
        script.onload = function(){
            document.getelementsbytagname("body")[0].removechild(this);
        }
    },
    urlquerystring:function(url){//api.php?callback=callback&foo=foo
        var url = url.split("?")[1];
        var result = {},reg = /([^&=]+)=([^&]*)/g,m;
        while(m = reg.exec(url)){
            result[m[1]] = m[2];
        }
        return result;//object {callback: "callback", foo: "foo"} 
    }
};
$.getjs on("api.php?callback=callback&foo=foo",{"aa":11,"bb":22},function(data){
    console.log(data);
});