Cannot add package to local npm cache

Question

If I try to add my local project to the npm cache, nothing seems to get added to the .npm dir:

"$(npm get cache)" => "$HOME/.npm"

given the npm cache location, we do:

rm -rf "$HOME/.npm"
mkdir "$HOME/.npm"
npm cache add "$my_npm_project_root"

the command succeeds, but nothing gets added to the "$HOME/.npm" dir.

Does anyone know what I might be doing wrong? Why isn't the cache being updated?