2600mAh USB Power Bank Portable External Pack

Of course you could crack it open and fit a cable directly from the battery to Espruino, and then you should be able to run Espruino on it for ages.

At this point, wouldn't it be easier to just use a 18650 battery? ($1/ea, 1-cell holder $0.79, then either buy a charger board, or use an external charger to recharge).