用script标签异步加载js

 1 function loadScript(url, callback){
 2     var script = document.createElement ("script")
 3     script.type = "text/javascript";
 4     if (script.readyState){ //IE
 5         script.onreadystatechange = function(){
 6             if (script.readyState == "loaded" || script.readyState == "complete"){
 7                 script.onreadystatechange = null;
 8                 callback();
 9             }
10         };
11     } else { //Others
12         script.onload = function(){
13             callback();
14         };
15     }
16     script.src = url;
17     document.getElementsByTagName_r("head")[0].appendChild(script);
18 }