[RRFC] Cleaning up the prefix config story

[ruyadorno]

Intro

The prefix config option does not provide a great user experience and is a source of many (mostly invalid) bug reports from confused users. These users have very legitimate reasons to be confused, given that prefix encompasses many different concepts and affects many different commands and overall npm cli functionality.

Documenting current known pain points & problems

Local prefix vs Global prefix

Currently prefix conflates two essential concepts: localPrefix and globalPrefix, these are currently differentiated only in the internal cli implementation and the only way for users to change these values is by using the prefix config option along with global.

Global prefix usage

globalPrefix itself is a config that has many shared responsabilities and its value affects different commands:

• It's used to construct the globalDir internal configuration value: globalDir=${globalPrefix}/lib/node_modules, globalDir is the location to the node_modules folder that holds all the globally installed packages.

Windows sane defaults

Currently the default value for globalPrefix in windows defaults to the folder location of the current executing node instance, which by default is usually: C:\Program Files\nodejs, this results in EPERM errors when trying to reify anything to the global package space if not running in admin mode.

This problem is avoided today by most of our windows user base due to nodejs itself bundling a builtin config file that tweaks the prefix value to point to: %APPDATA%\npm

Note: Should we bless that location and make it the new default value in @npmcli/config instead ?

Related: https://github.com/npm/cli/issues/3088

Moving forward

This is mostly a wip draft that aims to highlight these problems, feel free to maybe connect any missing related problems that I might have forgotten to list here and hopefully this can lead to an actual RFC that proposes solutions to these problems.