That's more about writing extensions to the espruino firmware and building that, it doesn't really talk about how you'd go about taking a brand new main.cpp file and end up with a .bin file you can flash over DFU mode to the puck.js
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.
Tue 2021.05.25
Hi @DanTheMan827 this exact question was asked a week ago and many links resulted.
For your perusal: