dynamically load javascript?

I use a lot of script in an 开发者_如何转开发application, some of them are not required to load the application, I want to load them just before their use if possible, knowing that my application is coded in ExtJS, and uses many ajax calls

You could look at using LABjs which is a script loader.

Old and busted:

<script src="framework.js"></script>
<script src="plugin.framework.js"></script>
<script src="myplugin.framework.js"></script>
<script src="init.js"></script>

New hotness:

<script>
   $LAB
   .script("framework.js").wait()
   .script("plugin.framework.js")
   .script("myplugin.framework.js").wait()
   .script("init.js").wait();
</script>

Update: If you want to load jQuery you could do something like this from this blog post.

<script type="text/javascript" src="LAB.js">
if (typeof window.jQuery === "undefined") {
    $LAB.script("/local/jquery-1.4.min.js");
}

You can load an external file using javascript only when/where you need it:

function LoadJs(url){
  var js = document.createElement('script');

  js.type = "text/javascript";
  js.src = url;

  document.body.appendChild(js);

}

LoadJs("/path/to/your/file.js");

Here's a great post on the topic with a proposed solution you might want to check out: http://www.nczonline.net/blog/2011/02/14/separating-javascript-download-and-execution/