About this document

To make interoperability easier for third-party projects, this document describes the specification we follow when installing files on disk under the Plug’n’Play install strategy. It also means:

• any change we make to this document will follow semver rules
• we’ll do our best to preserve backward compatibility
• new features will be intended to gracefully degrade

High-level idea

Plug’n’Play works by keeping in memory a table of all packages part of the dependency tree, in such a way that we can easily answer two different questions:

• Given a path, what package does it belong to?
• Given a package, where are the dependencies it can access?

Resolving a package import thus becomes a matter of interlacing those two operations:

• First, locate which package is requesting the resolution
• Then retrieve its dependencies, check if the requested package is amongst them
• If it is, then retrieve the dependency information, and return its location

Extra features can then be designed, but are optional. For example, Yarn leverages the information it knows about the project to throw semantic errors when a dependency cannot be resolved: since we know the state of the whole dependency tree, we also know why a package may be missing.

Basic concepts

All packages are uniquely referenced by locators. A locator is a combination of a package ident, which includes its scope if relevant, and a package reference, which can be seen as a unique ID used to distinguish different instances (or versions) of a same package. The package references should be treated as an opaque value: it doesn’t matter from a resolution algorithm perspective that they start with workspace:, virtual:, npm:, or any other protocol.

Portability

For portability reasons, all paths inside of the manifests:

• must use the unix path format (/ as separators).
• must be relative to the manifest folder (so they are the same regardless of the location of the project on disk).

Caution

All algorithms in this specification assume that paths have been normalized according to these two rules.

Fallback

For improved compatibility with legacy codebases, Plug’n’Play supports a feature we call “fallback”. The fallback triggers when a package makes a resolution request to a dependency it doesn’t list in its dependencies. In normal circumstances the resolver would throw, but when the fallback is enabled the resolver should first try to find the dependency packages amongst the dependencies of a set of special packages. If it finds it, it then returns it transparently.

In a sense, the fallback can be seen as a limited and safer form of hoisting. While hoisting allows unconstrainted access through multiple levels of dependencies, the fallback requires to explicitly define a fallback package - usually the top-level one.

Package locations

While the Plug’n’Play specification doesn’t by itself require runtimes to support anything else than the regular filesystem when accessing package files, producers may rely on more complex data storage mechanisms. For instance, Yarn itself requires the two following extensions which we strongly recommend to support:

Zip access

Files named *.zip must be treated as folders for the purpose of file access. For instance, /foo/bar.zip/package.json requires to access the package.json file located within the /foo/bar.zip zip archive.

If writing a JS tool, the @yarnpkg/fslib package may be of assistance, providing a zip-aware filesystem layer called ZipOpenFS.

Virtual folders

In order to properly represent packages listing peer dependencies, Yarn relies on a concept called Virtual Packages. Their most notable property is that they all have different paths (so that Node.js instantiates them as many times as needed), while still being baked by the same concrete folder on disk.

This is done by adding path support for the following scheme:

/path/to/some/folder/__virtual__/<hash>/<n>/subpath/to/file.dat

When this pattern is found, the __virtual__/<hash>/<n> part must be removed, the hash ignored, and the dirname operation applied n times to the /path/to/some/folder part. Some examples:

/path/to/some/folder/__virtual__/a0b1c2d3/0/subpath/to/file.dat
/path/to/some/folder/subpath/to/file.dat

/path/to/some/folder/__virtual__/e4f5a0b1/0/subpath/to/file.dat
/path/to/some/folder/subpath/to/file.dat (different hash, same result)

/path/to/some/folder/__virtual__/a0b1c2d3/1/subpath/to/file.dat
/path/to/some/subpath/to/file.dat

/path/to/some/folder/__virtual__/a0b1c2d3/3/subpath/to/file.dat
/path/subpath/to/file.dat

If writing a JS tool, the @yarnpkg/fslib package may be of assistance, providing a virtual-aware filesystem layer called VirtualFS.

Note

The __virtual__ folder name appeared with Yarn 3.0. Earlier releases used $$virtual, but we changed it after discovering that this pattern triggered bugs in software where paths were used as either regexps or replacement. For example, $$ found in the second parameter from String.prototype.replace silently turned into $.

Manifest reference

When pnpEnableInlining is explicitly set to false, Yarn will generate an additional .pnp.data.json file containing the following fields.

This document only covers the data file itself - you should define your own in-memory data structures, populated at runtime with the information from the manifest. For example, Yarn turns the packageRegistryData table into two separate memory tables: one that maps a path to a package, and another that maps a package to a path.

Note

You may notice that various places use arrays of tuples in place of maps. This is mostly intended to make it easier to hydrate ES6 maps, but also sometimes to have non-string keys (for instance packageRegistryData will have a null key in one particular case).

Resolution algorithm

Note

For simplicity, this algorithm doesn’t mention all the Node.js features that allow mapping a module to another, such as imports, exports, or other vendor-specific features.

NM_RESOLVE

NM_RESOLVE(specifier, parentURL)

1. This function is specified in the Node.js documentation

PNP_RESOLVE

PNP_RESOLVE(specifier, parentURL)

1. Let resolved be undefined

2. If specifier is a Node.js builtin, then

   1. Set resolved to specifier itself and return it

3. Otherwise, if specifier is either an absolute path or a path prefixed with ”./” or ”../”, then

   1. Set resolved to NM_RESOLVE(specifier, parentURL) and return it

4. Otherwise,

   1. Note: specifier is now a bare identifier

   2. Let unqualified be RESOLVE_TO_UNQUALIFIED(specifier, parentURL)

   3. Set resolved to NM_RESOLVE(unqualified, parentURL)

RESOLVE_TO_UNQUALIFIED

RESOLVE_TO_UNQUALIFIED(specifier, parentURL)

1. Let resolved be undefined

2. Let ident and modulePath be the result of PARSE_BARE_IDENTIFIER(specifier)

3. Let manifest be FIND_PNP_MANIFEST(parentURL)

4. If manifest is null, then

   1. Set resolved to NM_RESOLVE(specifier, parentURL) and return it

5. Let parentLocator be FIND_LOCATOR(manifest, parentURL)

6. If parentLocator is null, then

   1. Set resolved to NM_RESOLVE(specifier, parentURL) and return it

7. Let parentPkg be GET_PACKAGE(manifest, parentLocator)

8. Let referenceOrAlias be the entry from parentPkg.packageDependencies referenced by ident

9. If referenceOrAlias is null or undefined, then

   1. If manifest.enableTopLevelFallback is true, then

      1. If parentLocator isn’t in manifest.fallbackExclusionList, then

         1. Let fallback be RESOLVE_VIA_FALLBACK(manifest, ident)

         2. If fallback is neither null nor undefined

            1. Set referenceOrAlias to fallback

10. If referenceOrAlias is still undefined, then

    1. Throw a resolution error

11. If referenceOrAlias is still null, then

    1. Note: It means that parentPkg has an unfulfilled peer dependency on ident

    2. Throw a resolution error

12. Otherwise, if referenceOrAlias is an array, then

    1. Let alias be referenceOrAlias

    2. Let dependencyPkg be GET_PACKAGE(manifest, alias)

    3. Return path.resolve(manifest.dirPath, dependencyPkg.packageLocation, modulePath)

13. Otherwise,

    1. Let reference be referenceOrAlias

    2. Let dependencyPkg be GET_PACKAGE(manifest, {ident, reference})

    3. Return path.resolve(manifest.dirPath, dependencyPkg.packageLocation, modulePath)

GET_PACKAGE

GET_PACKAGE(manifest, locator)

1. Let referenceMap be the entry from parentPkg.packageRegistryData referenced by locator.ident

2. Let pkg be the entry from referenceMap referenced by locator.reference

3. Return pkg

   1. Note: pkg cannot be undefined here; all packages referenced in any of the Plug’n’Play data tables MUST have a corresponding entry inside packageRegistryData.

FIND_LOCATOR

FIND_LOCATOR(manifest, moduleUrl)

Note

The algorithm described here is quite inefficient. You should make sure to prepare data structure more suited for this task when you read the manifest.

1. Let bestLength be 0

2. Let bestLocator be null

3. Let relativeUrl be the relative path between manifest and moduleUrl

   1. Note: The relative path must not start with ./; trim it if needed

4. If relativeUrl matches manifest.ignorePatternData, then

   1. Return null

5. Let relativeUrlWithDot be relativeUrl prefixed with ./ or ../ as necessary

6. For each referenceMap value in manifest.packageRegistryData

   1. For each registryPkg value in referenceMap

      1. If registryPkg.discardFromLookup isn’t true, then

         1. If registryPkg.packageLocation.length is greater than bestLength, then

            1. If relativeUrl starts with registryPkg.packageLocation, then

               1. Set bestLength to registryPkg.packageLocation.length

               2. Set bestLocator to the current registryPkg locator

7. Return bestLocator

RESOLVE_VIA_FALLBACK

RESOLVE_VIA_FALLBACK(manifest, ident)

1. Let topLevelPkg be GET_PACKAGE(manifest, {null, null})

2. Let referenceOrAlias be the entry from topLevelPkg.packageDependencies referenced by ident

3. If referenceOrAlias is defined, then

   1. Return it immediately

4. Otherwise,

   1. Let referenceOrAlias be the entry from manifest.fallbackPool referenced by ident

   2. Return it immediately, whether it’s defined or not

FIND_PNP_MANIFEST

FIND_PNP_MANIFEST(url)

Finding the right PnP manifest to use for a resolution isn’t always trivial. There are two main options:

• Assume that there is a single PnP manifest covering the whole project. This is the most common case, as even when referencing third-party projects (for example via the portal: protocol) their dependency trees are stored in the same manifest as the main project.

  To do that, call FIND_CLOSEST_PNP_MANIFEST(require.main.filename) once at the start of the process, cache its result, and return it for each call to FIND_PNP_MANIFEST (if you’re running in Node.js, you can even use require.resolve('pnpapi') which will do this work for you).

• Try to operate within a multi-project world. This is rarely required. We support it inside the Node.js PnP loader, but only because of “project generator” tools like create-react-app which are run via yarn create react-app and require two different projects (the generator one and the generated one) to cooperate within the same Node.js process.

  Supporting this use case is difficult, as it requires a bookkeeping mechanism to track the manifests used to access modules, reusing them as much as possible and only looking for a new one when the chain breaks.

FIND_CLOSEST_PNP_MANIFEST

FIND_CLOSEST_PNP_MANIFEST(url)

1. Let manifest be null

2. Let directoryPath be the directory for url

3. Let pnpPath be directoryPath concatenated with /.pnp.cjs

4. If pnpPath exists on the filesystem, then

   1. Let pnpDataPath be directoryPath concatenated with /.pnp.data.json

   2. Set manifest to JSON.parse(readFile(pnpDataPath))

   3. Set manifest.dirPath to directoryPath

   4. Return manifest

5. Otherwise, if directoryPath is /, then

   1. Return null

6. Otherwise,

   1. Return FIND_PNP_MANIFEST(directoryPath)

PARSE_BARE_IDENTIFIER

PARSE_BARE_IDENTIFIER(specifier)

1. If specifier starts with ”@”, then

   1. If specifier doesn’t contain a ”/” separator, then

      1. Throw an error

   2. Otherwise,

      1. Set ident to the substring of specifier until the second ”/” separator or the end of string, whatever happens first

2. Otherwise,

   1. Set ident to the substring of specifier until the first ”/” separator or the end of string, whatever happens first

3. Set modulePath to the substring of specifier starting from ident.length

4. Return {ident, modulePath}