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Oracle Application Express APIs:

http://download.oracle.com/docs/cd/B31036_01/doc/appdev.22/b28550/api.htm

JavaScript is a powerful way to enhance the useability of web applications. It's a great way to add features that mimic those we are accustomed to in a client-server application without sacrificing all of the benefits of deploying a web application.

Adding JavaScript to HTML DB applications is easy, as there are multiple "hooks" built into the tool specifically for this purpose. Since the tool is actually written in itself, adding these hooks was necessary to provide JavaScript enhancements to the tool. You have noticed some of these enhancements as you were working with HTML DB and wondered how they were constructed. It is the goal of this tutorial to explain the most common JavaScript scenarios and how they can be implemented in HTML DB.

JavaScript is not appropriate for data intensive validations, such as verifying that a name is contained within a large database table. You would need to pull down every record to the client, thus creating a huge HTML document. In general, complex operations are much better suited for server-side HTML DB validations instead of JavaScript.

It is expected that you have at least a basic understanding of JavaScript concepts and constructs.
More:
http://www.oracle.com/technology/products/database/application_express/howtos/htmldb_javascript_howto2.html