On-Demand Javascript - Ajax Patterns

 function ensureUploadScriptIsLoaded() {
   if (self.uploadMessages) { // Already exists
     return;
   }
   var head = document.getElementsByTagName("head")[0];
   script = document.createElement(‘script‘);
   script.id = ‘uploadScript‘;
   script.type = ‘text/javascript‘;
   script.src = "upload.js";
   head.appendChild(script)
}