I think this is a little bit off. What I want is an embedded device that runs node.js and npm, not just JavaScript. That enables you to use JavaScript and C (using a native npm module) and get the best of both worlds.
All of those appear to be fully-fledged computers though, and Espruino's not really aiming to compete with them.
Instead, you get to use JavaScript on a very small, low-power microcontroller than will sensibly run off a small battery for days on end.
Some people will want one thing, some will want another - and hopefully they can both coexist. If we get it right they'll both help each other and we can end up with a large library of JavaScript code for embedded devices :)
Could you please offer more details? what kind of application, what kind of battery and estimated power consumption ?
Of course aggressive power management (ie putting the mcu to sleep) could significantly extend the battery life. According to the datasheet the current consumption at sleep can be as low as 0.5mA, full two orders of magnitude difference. The standby current is in microampere range.
In comparison BBB consumes "210-460mA @5V", which would correspond to 1-3 hours on 2xAA alkalines (assuming ideal voltage converter).
At the moment I've got the KickStarter board down to 4mA when idle (as it can sleep) - when executing code I think it's around 40mA. Hopefully by release time I'll have sorted out a few glitches and got it down to under 1mA idle (in the range zokier noted).
Note that with something like 'setInterval(doStuff, 1000)' the processor will be idle for 99% of the time.
Running off a li-ion battery from an old Nokia phone (700mAh) you could then expect Espruino to run for almost a month. Using the battery out of my Galaxy S3 (2100mAh) it'd run for almost 3 months.