[TheRealPomax]

This isn't actually JS, it's Processing, so the idea is that this will work in the browser by virtue of a library that will run Processing source code in the browser. If you want to do the programming offline, just hit up processing.org, download the IDE, download the codebase.pde file on the same dir as the tutorial's page, and off you go.

[zwieback]

Ah, thanks for the clarification. I didn't look at the source closely enough but now that I look at it again it's becoming clear.