Auto include javascript file if not already included-Collection of common programming errors

Well if that JavaScript file defines a specific variable (or a function) you can check its presence by checking typeof that_variable, then load the JavaScript file if necessary. An example, here is how you load swfobject library if its not available on the page:

if (typeof swfobject == "undefined") {
   var e = document.createElement("script");
   e.src = "http://ajax.googleapis.com/ajax/libs/swfobject/2/swfobject.js";
   e.type = "text/javascript";
   document.getElementsByTagName("head")[0].appendChild(e); 
}