Glossary
npm cache
npm's on-disk store of package tarballs, reused by later installs.
npm keeps a cache of package contents so the next install can avoid downloading the same tarball. The default location is under the user Library or wherever npm config get cache points. The cache is shared across projects. node_modules is not: that folder belongs to one project.
Clearing the cache is reversible only in the sense that npm can download again, and in the sense that Disk Housekeeper sends removals to the Trash by default after a preview. If you expect to work offline, leave the cache in place. The distinction is spelled out in Delete node_modules safely.