pnpm vs npm

Structure flat de npm

npm maintains a flattened dependency tree as of version 3. This leads to less disk space bloat, with a messy node_modules directory as a side effect.

On the other hand, pnpm manages node_modules by using hard linking and symbolic linking to a global on-disk content-addressable store. This lets you get the benefits of far less disk space usage, while also keeping your node_modules clean. There is documentation on the store layout if you wish to learn more.

The good thing about pnpm's proper node_modules structure is that it "helps to avoid silly bugs" by making it impossible to use modules that are not specified in the project's package.json.

Installation

pnpm does not allow installation of packages without saving them to package.json. If no parameters are passed to pnpm add, packages are saved as regular dependencies. Like with npm, --save-dev and --save-optional can be used to install packages as dev or optional dependencies.

As a consequence of this limitation, projects won't have any extraneous packages when they use pnpm unless they remove a dependency and leave it orphaned. That's why pnpm's implementation of the prune command does not allow you to specify packages to prune - it ALWAYS removes all extraneous and orphaned packages.

Directory dependencies

Directory dependencies start with the file: prefix and point to a directory in the filesystem. Like npm, pnpm symlinks those dependencies. Unlike npm, pnpm does not perform installation for the file dependencies.

This means that if you have a package called foo (<root>/foo) that has bar@file:../bar as a dependency, pnpm won't perform installation for <root>/bar when you run pnpm install on foo.

If you need to run installations in several packages at the same time, for instance in the case of a monorepo, you should look at the documentation for pnpm -r.