pnpm/pnpm/docs/symlinked-node-modules-structure.md

id, title

id	title
symlinked-node-modules-structure	Symlinked `node_modules` structure

:::info

This article only describes how pnpm's node_modules are structured when there are no packages with peer dependencies. For the more complex scenario of dependencies with peers, see how peers are resolved.

:::

pnpm's node_modules layout uses symbolic links to create a nested structure of dependencies.

Every file of every package inside node_modules is a hard link to the content-addressable store. Let's say you install foo@1.0.0 that depends on bar@1.0.0. pnpm will hard link both packages to node_modules like this:

node_modules
└── .pnpm
    ├── bar@1.0.0
    │   └── node_modules
    │       └── bar
    │           ├── index.js     -> <store>/001
    │           └── package.json -> <store>/002
    └── foo@1.0.0
        └── node_modules
            └── foo
                ├── index.js     -> <store>/003
                └── package.json -> <store>/004

These are the only "real" files in node_modules. Once all the packages are hard linked to node_modules, symbolic links are created to build the nested dependency graph structure.

As you might have noticed, both packages are hard linked into a subfolder inside a node_modules folder (foo@1.0.0/node_modules/foo). This is needed to:

1. allow packages to import themselves. foo should be able to require('foo/package.json') or import * as package from "foo/package.json".
2. avoid circular symlinks. Dependencies of packages are placed in the same folder in which the dependent packages are. For Node.js it doesn't make a difference whether dependencies are inside the package's node_modules or in any other node_modules in the parent directories.

The next stage of installation is symlinking dependencies. bar is going to be symlinked to the foo@1.0.0/node_modules folder:

node_modules
└── .pnpm
    ├── bar@1.0.0
    │   └── node_modules
    │       └── bar/  (files linked from <store>)
    └── foo@1.0.0
        └── node_modules
            ├── foo/  (files linked from <store>)
            └── bar -> ../../bar@1.0.0/node_modules/bar

Next, direct dependencies are handled. foo is going to be symlinked into the root node_modules folder because foo is a dependency of the project:

node_modules
├── foo -> ./.pnpm/foo@1.0.0/node_modules/foo
└── .pnpm
    ├── bar@1.0.0
    │   └── node_modules
    │       └── bar/  (files linked from <store>)
    └── foo@1.0.0
        └── node_modules
            ├── foo/  (files linked from <store>)
            └── bar -> ../../bar@1.0.0/node_modules/bar

This is a very simple example. However, the layout will maintain this structure regardless of the number of dependencies and the depth of the dependency graph.

Let's add qar@2.0.0 as a dependency of bar and foo. This is how the new structure will look:

node_modules
├── foo -> ./.pnpm/foo@1.0.0/node_modules/foo
└── .pnpm
    ├── bar@1.0.0
    │   └── node_modules
    │       ├── bar/  (files linked from <store>)
    │       └── qar -> ../../qar@2.0.0/node_modules/qar
    ├── foo@1.0.0
    │   └── node_modules
    │       ├── foo/  (files linked from <store>)
    │       ├── bar -> ../../bar@1.0.0/node_modules/bar
    │       └── qar -> ../../qar@2.0.0/node_modules/qar
    └── qar@2.0.0
        └── node_modules
            └── qar/  (files linked from <store>)

As you may see, even though the graph is deeper now (foo > bar > qar), the directory depth in the file system is still the same.

This layout might look weird at first glance, but it is completely compatible with Node's module resolution algorithm. When resolving modules, Node resolves a symlink to its real path. So when bar is required from foo@1.0.0/node_modules/foo/index.js, Node finds the symlink at foo@1.0.0/node_modules/bar and loads bar from its real location (bar@1.0.0/node_modules/bar). As a consequence, bar can also resolve its dependencies which are in bar@1.0.0/node_modules.

A great bonus of this layout is that only packages that are really in the dependencies are accessible. With a flattened node_modules structure, all hoisted packages are accessible. To read more about why this is an advantage, see "pnpm's strictness helps to avoid silly bugs"

Unfortunately, many packages in the ecosystem are broken. They use dependencies that are not listed in their package.json. To minimize the number of issues new users encounter, pnpm hoists all dependencies by default into node_modules/.pnpm/node_modules. To disable this hoisting, set hoist to false.