Dynamically load javascript source files

application.html

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.6/jquery.min.js" type="text/javascript"></script>
<script>
  //<![CDATA[
    window.jQuery || document.write("<script src='/javascripts/jquery.min.js'>\x3C/script>")
  //]]>
</script>

application.js

if(typeof yourFunctionToCheck != 'function') {
  $.getScript("ajax/test.js", function(){
    // callback if needed
  });
}