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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
k.zabloudil
I don't think you need to create a DFU package. I just used the nRF Connect application's programmer. AFAIK that just calls nrfjprog.
I think you should try nRF Connect's gui, or just flash the hex with nrfjprog.