But why a dedicated device? Why not just a JS -> ARM/AVR/whatever compiler? Then you can run it on microcontrollers that you can actually build up from. One of the best things about arduino is that it's based upon the ATMega platform. Once you've got the hang of the arduino language, you can progress to embedded C (on the same device), then onto assembly if you really want.

Once you've got to that level you've got a whole family of related microcontrollers that you can move onto from the same principles. With this javascript device you've got no such thing. You're stuck in a dead end if/when it turns out not to have what you need, as you can't progress to "the next level".