javascript异步加载

<script type="text/javascript">
function loadScript(url, callback) {
var script = document.createElement("script");
script.type = "text/javascript";
 //IE
if (script.readyState) {
script.onreadystatechange = function () {
 if (script.readyState == "loaded" || script.readyState =="complete") {
script.onreadystatechange = null;
 callback();
 }
 }
 } else {
//非IE
script.onload = function () {
 callback();
}

 }
script.src = url;
document.getElementById("head").appendChild(script);
 }
 //第一步加载jquery类库
 loadScript("jquery/jquery-1.4.1.js", function () {
 //第二步加载hello.js
 loadScript("js/hello.js", function () {
 //第三步加载world.js
loadScript("js/world.js", function () {
 });
 });
 });
 </script>