new UART characteristic write for each new line in forEach loop

Ahh - no, > isn't the end carriage return.

Basically the REPL (the thing you can type JS into to execute it) is still on the UART.

So there's a prompt > and if you type 1+2 Espruino sends back:

1+2
=3
> 

If you use print then Espruino sends 0x08 (the delete key) to remove the >, prints your stuff, then sends > at the end of execution so the REPL still has the prompt at the end.

But if you use Bluetooth.println(x) as it seems you were doing, it avoids the REPL and just prints whatever you told it. Of course if some other code does a print you'll still see the > behaviour.

Depending on what you have planned you can move the console out the way with E.setConsole(null) but then debugging/uploading code gets hard