One of the great things about templating systems is the ability to encapsulate repeatable chunks of code. One of the tough things about templating systems is management of a global “state” to ensure the the overall document that is emitted is as efficient as possible – especially in the HTML world were bandwidth is a performance consideration.

An example of this is where the templating chunks require a particular external resource, like JavaScript. You don’t want each invocation of a repeated sub-template to cause a tag to be emitted; you only need it once for the entire document.

Asp.Net has some handling for JavaScript in their UserControl mechanism (RegisterClientScriptBlock()) that uses a server-side registration mechanism to emit an appropriate document.

Dave Mosher and Brett Zabos, here at VendAsta, have used a pure JavaScript approach to get the JavaScript resources loaded efficiently, built atop the YUI loader. Make sure you check out their escapades.