This acts much the same ways as the npm-cache(1) command line functionality.
The callback is called with the package.json data of the thing that is eventually added to or read from the cache.
The top level
npm.commands.cache(...) functionality is a public
interface, and like all commands on the
npm.commands object, it will
match the command line behavior exactly.
However, the cache folder structure and the cache helper functions are considered internal API surface, and as such, may change in future releases of npm, potentially without warning or significant version incrementation.
Use at your own risk.