Migration

Any major release has its breaking changes, and Yarn 2 isn't the exception. A few old behaviors were cleaned, fixed, modified, or removed. While one of our goals is to make the transition as easy as we can, there are a few things to be aware of when migrating a codebase. To make this process more efficient we've listed below the recommended migration steps, along with solutions for the most common problems you might face.

Why should you migrate?

We answer this question in details here.

In a few words, upgrading to the latest versions is critical to a fast and stable Yarn experience. Numerous bugs were fixed since the first major version, and we no longer expect to build new features on the old trunk. Even if you don't plan to use the new default installation strategy called Plug'n'Play your projects will still get benefits from the upgrade:

• The good old node_modules installer improved as well as various edge cases got fixed
• A renewed focus on performances and good practices (we now formally track perfs via a dashboard)
• Improved user experience for various CLI commands and settings (yarn add -i, yarn up, logFilters, ...)
• New commands and capabilities (such as the TypeScript plugin, or the release workflow)

And of course a very active development cycle.

Step by step

Note: Don't worry if your project isn't quite ready for Plug'n'Play just yet! This guide will let you migrate without losing your node_modules folder. Only in a later optional section we will cover how to enable PnP support, and this part will only be recommended, not mandatory. Baby steps! 😉

Note that those commands only need to be run once for the whole project and will automatically take effect for all your contributors as soon as they pull the migration commit, thanks to the power of yarnPath:

1. Run npm install -g yarn to update the global yarn version to latest v1
2. Go into your project directory
3. Run yarn set version berry to enable v2 (cf Install for more details)
4. If you used .npmrc or .yarnrc, you'll need to turn them into the new format (see also 1, 2)
5. Run yarn config set nodeLinker node-modules
6. Commit the changes so far (yarn-X.Y.Z.js, .yarnrc.yml, ...)
7. Run yarn install to migrate the lockfile
8. Take a look at this article to see what should be gitignored
9. Commit everything remaining

Some optional features are available via external plugins:

10. Run yarn plugin import interactive-tools if you want upgrade-interactive
11. Run yarn plugin list to see what other official plugins exist and might be useful
12. Commit the yarn plugins

Good, you should now have a working Yarn install! Some things might still require a bit of work (for instance we deprecated arbitrary pre/post-scripts, and renamed --frozen-lockfile into --immutable), but those special cases will be documented on a case-by-case basis in the rest of this document (for example here).

Switching to Plug'n'Play

This step is completely optional - while we recommend to use Plug'n'Play for most new projects, it may sometimes require an average time investment to enable it on existing projects. For this reason, we prefer to list it here as a separate step that you can look into if you're curious or simply want the absolute best of what Yarn has to offer.

Before we start

Plug'n'Play enforces strict dependency rules. In particular, you'll hit problems if you (or your dependencies) rely on unlisted dependencies (the reasons for that are detailed in our Rulebook), but the gist is that it was the cause of many "project doesn't work on my computer" issues, both in Yarn and other package managers.

To quickly detect which places may rely on unsafe patterns run yarn dlx @yarnpkg/doctor in your project - it'll statically analyze your sources to try to locate the most common issues that could result in a subpar experience. For example here's what webpack-dev-server would reveal:

➤ YN0000: Found 1 package(s) to process
➤ YN0000: For a grand total of 236 file(s) to validate

➤ YN0000: ┌ /webpack-dev-server/package.json
➤ YN0000: │ /webpack-dev-server/test/testSequencer.js:5:19: Undeclared dependency on @jest/test-sequencer
➤ YN0000: │ /webpack-dev-server/client-src/default/webpack.config.js:12:14: Webpack configs from non-private packages should avoid referencing loaders without require.resolve
➤ YN0000: │ /webpack-dev-server/test/server/contentBase-option.test.js:68:8: Strings should avoid referencing the node_modules directory (prefer require.resolve)
➤ YN0000: └ Completed in 5.12s

➤ YN0000: Failed with errors in 5.12s

In this case, the doctor noticed that:

• testSequencer.js depends on a package without listing it as a proper dependency - which would be reported as an error at runtime under Plug'n'Play.

• webpack.config.js references a loader without passing its name to require.resolve - which is unsafe, as it means the loader won't be loaded from webpack-dev-server's dependencies.

• contentBase-option.test.js checks the content of the node_modules folder - which wouldn't exist anymore under Plug'n'Play.

Enabling it

1. Look into your .yarnrc.yml file for the nodeLinker setting
2. If you don't find it, or if it's set to pnp, then it's all good: you're already using Plug'n'Play!
3. Otherwise, remove it from your configuration file
4. Run yarn install
5. Various files may have appeared; check this article to see what to put in your gitignore
6. Commit the changes

Editor support

We have a dedicated documentation, but if you're using VSCode (or some other IDE with Intellisense-like feature) the gist is:

1. Install the ZipFS VSCode extension
2. Make sure that typescript, eslint, prettier, ... all dependencies typically used by your IDE extensions are listed at the top level of the project (rather than in a random workspace)
3. Run yarn dlx @yarnpkg/sdks vscode
4. Commit the changes - this way contributors won't have to follow the same procedure
5. For TypeScript, don't forget to select Use Workspace Version in VSCode

Final notes

Now you should have a working Yarn Plug'n'Play setup, but your repository might still need some extra care. Some things to keep in mind:

• There is no node_modules folder and no .bin folder. If you relied on these, call yarn run instead.
• Replace any calls to node that are not inside a Yarn script with yarn node
• Custom pre-hooks (e.g. prestart) need to be called manually now

All of this and more is documented in the following sections. In general, we advise you at this point to try to run your application and see what breaks, then check here to find out tips on how to correct your install.

General Advices

Upgrade to Node.js 12.x or newer

Node.js 10.x reached its official End of Life in April 2021 and won't receive any further update. Yarn consequently doesn't support it anymore.

Fix dependencies with packageExtensions

Packages sometimes forget to list their dependencies. In the past it used to cause many subtle issues, so Yarn now defaults to prevent such unsound accesses. Still, we don't want it to prevent you from doing your work as long as you can do it in a safe and predictable way, so we came up with the packageExtensions setting.

For example, if react was to forget to list a dependency on prop-types, you'd fix it like this:

packageExtensions:
  "react@*":
    dependencies:
      prop-types: "*"

And if a Babel plugin was missing its peer dependency on @babel/core, you'd fix it with:

packageExtensions:
  "@babel/plugin-something@*":
    peerDependencies:
      "@babel/core": "*"

Use yarn dlx instead of yarn global

yarn dlx is designed to execute one off scripts that may have been installed as global packages with yarn 1.x. Managing system-wide packages is outside of the scope of yarn. To reflect this, yarn global has been removed. Read more on GitHub.

Enable the PnP plugin when using Webpack 4

Webpack 5 supports PnP natively, but if you use Webpack 4 you'll need to add the pnp-webpack-plugin plugin yourself.

Upgrade resolve to 1.9+

The resolve package is used by many tools in order to retrieve the dependencies for any given folder on the filesystem. It's compatible with Plug'n'Play, but only starting from 1.9+, so make sure you don't have an older release in your dependency tree (especially as transitive dependency).

Fix: Open your lockfile, look for all the resolve entries that could match 1.9+ (for example ^1.0.0), and remove them. Then run yarn install again. If you run yarn why resolve, you'll also get a good idea of which package is depending on outdated version of resolve - maybe you can upgrade them too?

Call binaries using yarn run rather than node_modules/.bin

The node_modules/.bin folder is an implementation detail, and the PnP installs don't generate it at all. Rather than relying on its existence, just use the yarn run command which can start both scripts and binaries:

yarn run jest
# or, using the shortcut:
yarn jest

Call your scripts through yarn node rather than node

We now need to inject some variables into the environment for Node to be able to locate your dependencies. In order to make this possible, we ask you to use yarn node which transparently does the heavy lifting.

Note: this section only applies to the shell CLI. The commands defined in your scripts are unaffected, as we make sure that node always points to the right location, with the right variables already set.

Explicitly call the pre and post scripts

Rewrite:

{
  "scripts": {
    "prestart": "do-something",
    "start": "http-server"
  }
}

Into:

{
  "scripts": {
    "prestart": "do-something",
    "start": "yarn prestart && http-server"
  }
}

Note: This only applies to user scripts, such as start & friends. It's still fine to use any of preinstall, install, and postinstall. Consult the script documentation for more information.

Setup your IDE for PnP support

We've written a guide entirely designed to explain how to use Yarn with your IDE. Make sure to take a look at it, and maybe contribute to it if some instructions are unclear or missing!

Update your configuration to the new settings

Yarn 2 uses a different style of configuration files than Yarn 1. While mostly invisible for the lockfile (because we import them on the fly), it might cause some issues for your rc files.

• The main change is the name of the file. Yarn 1 used .yarnrc, but Yarn 2 is moving to a different name: .yarnrc.yml. This should make it easier for third-party tools to detect whether a project uses Yarn 1 or Yarn 2, and will allow you to easily set different settings in your home folders when working with a mix of Yarn 1 and Yarn 2 projects.

• As evidenced by the new file extension, the Yarnrc files are now to be written in YAML. This has been requested for a long time, and we hope it'll allow easier integrations for the various third-party tools that need to interact with the Yarnrc files (think Dependabot, etc).

• The configuration keys have changed. The comprehensive settings list is available in our documentation, but here are some particular changes you need to be aware of:

  • Custom registries are now configured via npmRegistryServer.

  • Registry authentication tokens are now configured via npmAuthToken.

  • The yarn-offline-mirror has been removed, since the offline mirror has been merged with the cache as part of the Zero-Install effort. Just commit the Yarn cache and you're ready to go.

Don't use .npmrc files

On top of their naming, the way we load the Yarnrc files has also been changed and simplified. In particular:

• Yarn doesn't use the configuration from your .npmrc files anymore; we instead read all of our configuration from the .yarnrc.yml files whose available settings can be found in our documentation.

• As mentioned in the previous section, the yarnrc files are now called .yarnrc.yml, with an extension. We've completely stopped reading the values from the regular .yarnrc files.

• All environment variables prefixed with YARN_ are automatically used to override the matching configuration settings. So for example, adding YARN_NPM_REGISTRY_SERVER into your environment will change the value of npmRegistryServer.

Take a look at our end-to-end tests

We now run daily end-to-end tests against various popular JavaScript tools in order to make sure that we never regress - or to be notified when those tools do.

Consulting the sources for those tests is a great way to check whether some special configuration values have to be set when using a particular toolchain.

Don't use bundleDependencies

The bundleDependencies (or bundledDependencies) is an artifact of the past that used to let you define a set of packages that would be stored as-is within the package archive, node_modules and all. This feature has many problems:

• It uses node_modules, which doesn't easily allow different install strategies such as Plug'n'Play.
• It encodes the hoisting inside the package, which is the exact opposite of what we aim for
• It messes with the hoisting of other packages
• Etc, etc, etc

So how to replace them? There are different ways:

• If you need to patch a package, just fork it or reference it through file: (it's perfectly fine even for transitive dependencies to use this protocol). The portal: and patch: protocols are also options, although they'll only work for Yarn consumers.

• If you need to ship a package to your customers as a standalone (no dependencies), bundle it yourself using Webpack, Rollup, or similar tools.

If required: enable the node-modules plugin

PnP Compatibility Table

Despite our best efforts some tools don't work at all under Plug'n'Play environments, and we don't have the resources to update them ourselves. There are only two notorious ones on our list: Flow, and React Native.

In such a radical case, you can enable the built-in node-modules plugin by adding the following into your local .yarnrc.yml file before running a fresh yarn install:

nodeLinker: node-modules

This will cause Yarn to install the project just like Yarn 1 used to, by copying the packages into various node_modules folders.

More information about the nodeLinker option.

Replace nohoist by nmHoistingLimits

The nohoist setting from Yarn 1 was made specifically for React Native (in order to help it support workspaces), but the way it worked (through glob patterns) was causing a lot of bugs and confusion, noone being really sure which patterns needed to be set. As a result, we've simplified this feature in order to only support three identified patterns.

If you were using nohoist, we recommend you remove it from your manifest configuration and instead set nmHoistingLimits in your yarnrc file:

nmHoistingLimits: workspaces

CLI Commands

Renamed

Removed from core

Not implemented yet

Those features simply haven't been implemented yet. Help welcome!

Troubleshooting

Cannot find module [...]

Interestingly, this error often doesn't come from Yarn. In fact, seeing this message should be extremely rare when working with Yarn 2 projects and typically highlights that something is wrong in your setup.

This error appears when Node is executed without the proper environment variables. In such a case, the underlying application won't be able to access the dependencies and Node will throw this message. To fix that, make sure that the script is called through yarn node [...] (instead of node [...]) if you run it from the command line.

A package is trying to access another package [...]

Full message: A package is trying to access another package without the second one being listed as a dependency of the first one.

Some packages don't properly list their actual dependencies for a reason or another. Now that we've fully switched to Plug'n'Play and enforce boundaries between the various branches of the dependency tree, this kind of issue will start to become more apparent than it previously was.

The long term fix is to submit a pull request upstream to add the missing dependency to the package listing. Given that it sometimes might take some time before they get merged, we also have a more short-term fix available: create .yarnrc.yml in your project, then use the packageExtensions setting to add the missing dependency to the relevant packages. Once you're done, run yarn install to apply your changes and voilà!

packageExtensions:
  "debug@*":
    peerDependenciesMeta:
      "supports-color":
        optional: true

If you also open a PR on the upstream repository you will also be able to contribute your package extension to our compat plugin, helping the whole ecosystem move forward.