JS interpreter (V8 under the hood) runs in a process pool -- similar to a thread pool, but outside of the core rethinkdb process. The code running in the JS interpreter cannot corrupt memory or crash the rethinkdb process (if it crashes, rethinkdb will simply start another v8 process). You also can't write from js executed on the server, so the data is safe (though I think it's more of a limitation than a feature).

Currently if you write an infinite loop in js, or write code in a way where it starts eating up memory we don't do anything to restart the js process, but it would be relatively easy to implement.

I see, thanks. I'm definitely interested in that, as I'm developing http://www.instahero.com and the current approach isn't very scalable, so I'm evaluating alternatives. RethinkDB looks like a good candidate so far.