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I'm not too good with Javascript, but this would only work on domains you own, correct?

It'd work on any domain you like, but you need to run it on your own system.

I bet this could be used to write some very interesting Javascript-aware crawlers/scrapers.

Oh wow. Couldn't this easily be abused, if someone visited my Javascript page, that automatically entered a form under their IP?

it runs on node.js (a server side software), not a browser. So no need to worry about hijacking someones ip.