The following error appears when running arrow tests. (which exec an appc run -l trace from a generated project) I never fully traced down the location of the issue in npmconf, but running the app generated by the tests directly does not show this error, so it's possibly something to do with the cwd.
These config options which are causing the error are npm defaults, however, and not set directly by me. The error is being thrown by the npmconf module, which is now deprecated with warnings of Do not use this package..
While the suggested alternative is to use npm for this check, I don't feel like having npm as a dependency is a good idea, and I don't think that it is expected. Other packages seem to be using https://github.com/kevva/npm-conf instead.
I've created a branch which uses npm-conf instead of npmconf and this no longer throws errors or emits warnings.