I have a vague memory of asking this already and question being answered, but I couldn't find any details -- is there a better guide for migrating from luci to luci-ng than https://openwrt.org/docs/techref/luci2 and reviewing the code of existing luci-ng packages?
I may have been mistaken but I thought the LuCI in master already uses JS/Angular (not familiar with the coding side)?
Are you reffering to https://github.com/jow-/luci-ng or https://git.openwrt.org/?p=project/luci2/ui.git?
AFAIK not just the master, was it 19.07 when the old lua-based luci apps started to need to require luci-compat as a dependency? I think it started back then. I'm still looking for a guide other than browsing other people's code. 
No, I'm referring to documentation for the code linked above.
afaik luci2/ luci-ng (https://git.openwrt.org/?p=project/luci2/ui.git) has been abandoned altogether, in favour of converting the old -existing- lua based luci to javascript in place.
In the linked documentation it's the first one I've linked in my post above!
So you're reffering to https://github.com/jow-/luci-ng