@tage that's Li-Ion batteries. They are very demanding on what voltage is applied to them, and may take fire when overvoltage is applied.
Nominal voltage is 3,7V but during charging the voltage increase up to 4,2V (which is a limit that shall never be exceeded).
This charger can't be used as is, as batteries are charged by the 3.3V of Espruino's I/O. And with the available current on an I/O it will take loooong to charge a 2100mAh battery.