Serial Flow Control

Conveniently, it was something I added last week and which will be in 1v93! If you use one of the cutting edge Espruino builds (newer than 1v92) then it should have it.

If you do:

Serial.setup(9600, {rx:..., tx:..., cts: ...});

Then Espruino will set the CTS pin to 0 whenever it is ready to receive data, and to 1 whenever it is too busy to receive. It doesn't do flow control on transmit, but I imagine that is unlikely to be your problem with xbee modules.

Hope that helps!