Sometimes you have to load jQuery, but you don’t know if it has already been referenced somewhere else in the website. 

This tends to happen if you have a custom web control, delivered in an assembly, that relies on jQuery.

This code has been ripped out of my application, and may have a typo or three, but it gives you the general idea.

var jQueryScriptOutputted = false; function initJQuery() { //if the jQuery object isn't available if (typeof(jQuery) == 'undefined') { if (! jQueryScriptOutputted) { //only output the script once.. jQueryScriptOutputted = true; //output the script (load it from google api) document.write("<scr" + "ipt type=\"text/javascript\" src=\"http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js\"></scr" + "ipt>"); } setTimeout("initJQuery()", 50); } else { $(function() { //do anything that needs to be done on document.ready }); } }

initJQuery();