//how to use the function below: 
//$.include('file/ajaxa.js');$.include('file/ajaxa.css'); 
//or $.includePath = 'file/';$.include(['ajaxa.js','ajaxa.css']);(only if .js and .css files are in the same directory) 
$.extend({ 
includePath: '', 
include: function(file) 
{ 
var files = typeof file == "string" ? [file] : file; 
for (var i = 0; i < files.length; i++) 
{ 
var name = files[i].replace(/^\s|\s$/g, ""); 
var att = name.split('.'); 
var ext = att[att.length - 1].toLowerCase(); 
var isCSS = ext == "css"; 
var tag = isCSS ? "link" : "script"; 
var attr = isCSS ? " type='text/css' rel='stylesheet' " : " type='text/javascript' "; 
var link = (isCSS ? "href" : "src") + "='" + $.includePath + name + "'"; 
if ($(tag + "[" + link + "]").length == 0) $("head").prepend("<" + tag + attr + link + "></" + tag + ">"); 
} 
} 
}); 
$.include('../js/jquery-ui-1.8.21.custom.min.js'); 
$.include('../css/black-tie/jquery-ui-1.8.21.custom.css');

var attr = isCSS ? " type='text/css' rel='stylesheet' " : " language='javascript' type='text/javascript' ";

document.write("<" + tag + attr + link + "></" + tag + ">");

$("head").prepend("<" + tag + attr + link + "></" + tag + ">");

Dynamically loading external JavaScript and CSS files 

To load a .js or .css file dynamically, in a nutshell, it means using DOM methods to first create a swanky new "SCRIPT" or "LINK" element, assign it the appropriate attributes, and finally, use element.appendChild() to add the element to the desired location within the document tree. It sounds a lot more fancy than it really is. Lets see how it all comes together: 

function loadjscssfile(filename, filetype){ 
if (filetype=="js"){ //if filename is a external JavaScript file 
var fileref=document.createElement('script') 
fileref.setAttribute("type","text/javascript") 
fileref.setAttribute("src", filename) 
} 
else if (filetype=="css"){ //if filename is an external CSS file 
var fileref=document.createElement("link") 
fileref.setAttribute("rel", "stylesheet") 
fileref.setAttribute("type", "text/css") 
fileref.setAttribute("href", filename) 
} 
if (typeof fileref!="undefined") 
document.getElementsByTagName("head")[0].appendChild(fileref) 
} 

loadjscssfile("myscript.js", "js") //dynamically load and add this .js file 
loadjscssfile("javascript.php", "js") //dynamically load "javascript.php" as a JavaScript file 
loadjscssfile("mystyle.css", "css") ////dynamically load and add this .css file