jQuery Plugin Include Protection

I have a few blog posts that show how to use jQuery plugins on PeopleSoft homepages. When designing those pagelets in Pagelet Wizard, it is important that your XSL/HTML include jQuery and any necessary plugins within your pagelet's HTML/XSL. This is how my Slideshow and Accordion Navigation templates work. Including jQuery and required plugins in each pagelet, however, means that a homepage using these pagelets will have multiple instances of jQuery. jQuery is designed to load once, with plugin scripts loaded as needed. Since each pagelet has its own pointer to jQuery, as each pagelet loads, the browser tries to reload jQuery, redefining the jQuery and $ global variables and resetting the collection of previously loaded plugins. The end result is that a homepage with multiple jQuery based pagelets will only have one working pagelet. The rest will have been invalidated by the last pagelet to load jQuery.

The jQuery documentation discourages the presence of multiple instances of jQuery within the same page. The theoretical concept is that each page should load jQuery once, and sites should be written to include only one jQuery script tag. The nature of homepages with their independently managed fragments doesn't allow for this. The way I work around this is to wrap the jQuery JavaScript library in something akin to the C-style header #ifndef include guards.

After downloading the jQuery JavaScript library, I wrap the contents of the file in a conditional block that looks something like this:

if(!window.jQuery) {
/* downloaded, compressed/minified jQuery content goes here */
}

I make the same change to jQuery plugins because they often include their own setup and usage data, but, of course, testing for a different variable. Here is my jQuery UI processing protection:

if(!window.jQuery.ui) {
/* downloaded, compressed/minified jQuery UI content goes here */
}

This minor change to the jQuery JavaScript library and plugin files keeps the browser from re-interpreting these JavaScript libraries. The browser interprets these file once, and then fails the conditional for each subsequent script tag that points to that particular library. This allows plugins to load as needed and all plugin setup and usage data to persist across multiple pagelets.

Of course, the best solution would be to just have each JavaScript file referenced once. Since that isn't practical on a homepage, this solution at least ensures the files are only processed once.