Bluetooth Serial example un Puck

These things are a bit contradictory, aren't they?

Not really - Bluetooth data will get reliably to the Bluetooth stack, but if it's not handled by Espruino fast enough it'll get lost (however a FIFO_FULL error flag gets set in that case).

I was actually meaning flow control over the BLE uart connection. I'm pretty sure that if I'm sending data to fast It will choke at some point.

That's what I meant. There is no flow control on BLE UART yet, but conceivably it could be added to the firmware at some point.

Having said all that the datarate over BLE is pretty low with the standard MTU and there's a big input buffer, so it's entirely possible that you could write your code such that it can always handle data as fast as can come over BLE.