as for flashing - probably easiest is to use programmer from nrfconnect for desktop
dongle can be flashed over USB
as for build - check http://forum.espruino.com/conversations/323658/?offset=100
I own a nRF52840 Dongle and would like to start experimenting with Espruino.
Would someone know how to get started with this board?
I didn't succeed to find instructions on what and how to to download/build/flash.