(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);
}