Last updated November 03, 2023

Yarn 2 (released in early 2020) introduced a brand new set of features and entirely new architecture, that is referred to as Yarn Modern. Yarn versions 3 and 4 have also added features and functionality, and are also referred to as Yarn Modern. Yarn 1 is now referred to as Yarn Classic.

Yarn Modern has introduced a “zero installs” feature, which means users may use “vendor” directories to include their yarn binaries, dependencies, and development dependencies in their repositories. Learn more about Yarn’s zero installs.

Additionally, Yarn Modern introduces “Plug'nPlay” (also known as pnp), which aims to increase application boot speed by instructing Node.js where each dependency can be found at startup, rather than having Node.js search for each dependency in the node_modules directory. Learn more about Yarn Plug'nPlay.

This article is intended to help current Heroku users migrate apps from Yarn 1.x (Classic) to Yarn 2.x, 3.x, or 4.x (Modern). Apps being migrated should already be using Yarn 1 and are using the most up-to-date version of the Heroku Node.js buildpack. This article will not work for applications that install Yarn from other scripts, such as the Heroku Ruby buildpack. Heroku expects all dependencies to be included in the .yarn directory to take full advantage of “zero downloads”.

Heroku users using Yarn are not required to migrate to Yarn Modern, and users will have access to Yarn Classic in their apps after it is deprecated. However, it’s advised to migrate to Yarn 4.x to ensure the most up-to-date bug fixes and security patches in the package manager.

Local Setup

Prepare local environment

Enter the directory of the source code that needs the migration. The local commands will be run at the root of the project. Make sure the local yarn version is up to date. To update it locally, run install with npm:

npm install -g yarn

The version should be >= 1.22.17. Run yarn -v to confirm.

Move custom cache directories to workspaces

In package.json, change your cacheDirectories to workspaces. For example, if you have the following:

"cacheDirectories": [
  "client/node_modules"
]

Change it to the following, and specify that the project is private:

"workspaces": [
  "client"
],
"private": true,

There’s no need to specify the node_modules directory. Make sure the "name" key in the package.json of the subdirectory reflects the directory name and the workspace name specified in the root package.json.

Next, delete any node_modules folders and yarn.lock files in subdirectories, and go to your application directory and run yarn:

cd ~/path/to/project && yarn install

This should update the yarn.lock file of your directory to reflect the entire dependency tree specified by the workspaces. Confirm that the lock file has been updated by looking for a dependency that has been specified in a package.json of a subdirectory.

Make changes to source code

There are additional files that must be checked into git in order to use Yarn 2 on Heroku. Yarn subscribes to a “zero-download” philosophy. There will be additional download costs to checking into git more files and directories, but this will create faster builds on Heroku.

Use yarn to set the yarn version on the source code.

yarn set version stable

To use the Zero-Installs feature, which will push locally cached dependendencies to heroku, add the following to .gitginore:

.yarn/*
!.yarn/cache
!.yarn/releases
!.yarn/plugins
!.yarn/sdks
!.yarn/versions

To opt-out of the Zero-Installs feature instead, which will trigger the buildpack to download and install all dependendencies, add the following to .gitignore:

.pnp.*
.yarn/*
!.yarn/patches
!.yarn/plugins
!.yarn/releases
!.yarn/sdks
!.yarn/versions

Next, use yarn to install packages and create the required files:

yarn install

This will create a .yarnrc.yml file, a .yarn directory, a pnp.js file, and modify the yarn.lock. Check these into git and make sure they are available to Heroku at build time.

Clear Heroku App cache

Since Heroku won’t be restoring the cache at the beginning of the build or storing it at the end of the build, you can go ahead and purge any build cache that is leftover from previous builds.

Make sure you have heroku-cli installed. Then, install the heroku-builds plugin.

heroku plugins:install heroku-builds

Next, run the command to clear the cache.

heroku builds:cache:purge --app $APP_NAME

After this, you’ll want to redeploy your app, but first finish up the next steps to ensure a successful deploy.

Update Heroku environment (with Plug'n'Play)

With Yarn 2+, you have the option of using Yarn’s Plug'n'Play to reference dependencies, or you can continue to use node modules. Each requires a slightly different set up, so if you intend to use node modules, set up your app accordingly.

Disable dependency caching

Heroku doesn’t cache dependencies with Yarn 2+ from the .yarn/cache directory. When using Plug'n'Play, Yarn will expect zipped up dependencies to be in the .yarn/cache directory. However, the buildpack will still run yarn install to ensure that postinstall scripts will be run for all dependencies.

You will want to remove the environment variables that are associated with Yarn 1 and node_modules. Therefore, remove the NODE_MODULES_CACHE environment variable. This variable should be set to false.

heroku config:set NODE_MODULES_CACHE=false

Remove private registry environment variables

If you’ve set any tokens for accessing a private registry and you’re using Yarn 2 with a checked in cache, unset those as well. You may do something like this:

heroku config:unset PRIVATE_NPM_TOKEN

Update Heroku environment (for node modules)

As described above, you will either be using Plug'n'Play or node modules for dependencies. To use Plug'n'Play, you’ll want to follow the instructions above.

Set up Yarn configuration

If you’ve opted out of using PnP for dependencies, you’ll need to specify this in your .yarnrc.yml. Add the following:

nodeLinker: "node-modules"

Now you’re ready to use node modules with Yarn Modern!

Test your App

After you’ve made your changes, make sure to run your test suite on your app’s code. Once the tests have passed successfully, deploy your application to Heroku. Confirm that the build has been successful.

If you run into any issues, please open an Issue on GitHub.