JS 异步加载的方法

(1) defer，只支持IE ：

<script type="text/javascript" defer="defer"> </script>

(2) async：属性

<script type="text/javascript" src="demo_async.js" async="async"></script> 

(3) 创建script，插入到DOM中，加载完毕后callBack，见代码：

function loadScript(url, callback){ 
var script = document.createElement_x("script") 
script.type = "text/javascript"; 
if (script.readyState){ //IE 
script.onreadystatechange = function(){ 
if (script.readyState == "loaded" || 
script.readyState == "complete"){ 
script.onreadystatechange = null; 
callback(); 
} 
}; 
} else { //Others: Firefox, Safari, Chrome, and Opera 
script.onload = function(){ 
callback(); 
}; 
} 
script.src = url; 
document.body.appendChild(script); 
}