Interaction between Storyline, JavaScript and HTML codes

Storyline's javascript and html is very minimal. If hard pressed I might suggest you publish the course to Web, then start by exploring the published files.

Storyline converts your project to HTML and JS  when you publish. If you add JS, you can easily inspect and change your scripts. You can load the .html files in an editor, or load them in a browser and inspect them. They are somewhat obfuscated, so are not easy to inspect. Most people that modify them seem to tend to use JS inside the SL project.