A Lerna-like tool for managing Javascript monorepos using Yarn
Latest commit b300ecc Feb 18, 2017 @seansfkelley seansfkelley committed on GitHub Update README.md



Yarn + Lerna = Yerna

Yerna is a monorepo management tool in the style of Lerna, but stripped down and using Yarn as the package manager.


The idea for Yerna stemmed from a Lerna ticket suggesting Yarn integration. At the time, doing so in Lerna was nontrivial (see also Lerna issue #605), and long story short, I built this giant bag of hacks to try it out myself. It turns out that the benefits outweighed the costs pretty dramatically for my exact use case, so I cleaned it up for release, and here we are.

While I use Yerna heavily myself, it is still fundamentally a hack-based stopgap/overgrown experiment. I am happy to discuss the feature set and its pros/cons in contrast to those of Lerna and Yarn, but note that I will not be prioritizing feature requests or performing nontrivial maintenance if it does not affect my own workflow.

Furthermore, the code is pretty sloppy in places, and I apologize in advance to any sensibilities that may be offended. This is basically a glorified proof of concept, submitted to you for discussion and experimentation.


Note: yarn is expected to already be installed and to exist on your PATH as Yerna will shell out to it.

yarn global add yernapkg

Also ensure that Yarn's globally-installed binaries are accessible on your PATH.


Note: Please also read the Caveats!

Yerna provides two binaries: yerna itself and a helper yarnhack. Both tools assume that they're running out of a git repo where packages are direct descendants of <git root>/packages.


yerna is a tool for running tasks over one or more local packages, using Yarn, in parallel. yerna provides a few different commands, listed briefly below. Note that all commands:

  • respect the package-selection flags (--include, etc.) unless otherwise noted
  • respect the dependency ordering of packages when running tasks

Run yerna --help for more information on the supported commands and flags.

command description notes
install install all external packages, link all local packages
link link all local packages without installing ignores flags
list list selected packages useful for testing flag combinations
run <script> run the npm script <script> in packages
exec <command> run shell command <command> in packages


yarnhack is an executable that wraps Yarn and mangles package.json to prevent Yarn from trying to install packages that don't exist on the registry. Otherwise, it forwards directly to the system-installed yarn and understands all commands and flags defined there.

Usage with Lerna

Yerna is backwards-compatible with Lerna, in that it puts the repo into a valid state for Lerna. You can continue to use Lerna for features missing from Yerna (such as publishing), though be sure to read the caveats, in particular, the behavior around symlinks.

Yerna does not read or write any Yerna- or Lerna-specific files on the filesystem (except for a logfile); in particular, it does not read lerna.json.


package.json mangling

See How It Works for details on how Yerna makes Yarn work, but in short, it involves mangling package.jsons on the filesystem temporarily. In most cases, these manglings should be transparent, but problems could arise if you e.g. do git operations while yerna or yarnhack are running. A severe fatal error could also cause one of these tools to abort without cleaning up after itself.


Both yerna and yarnhack assume that all local packages should always be symlinked. This means that they will:

  • remove anything that's in the way of placing a symlink, even if it's a directory or regular file
  • automatically generate symlinks before/after most operations as a convenience (so you almost never have to run yerna link)

npm Lifecycle Scripts

npm install-related lifecycle scripts (namely preinstall, postinstall and prepublish) will not work properly, or at all, if they depend on local packages. Yarn will attempt to run these tasks, but because local package references were removed, such scripts will fail to run. There is no way that I know of using Yarn in this manner that would allow symlinking before the tasks are run short of prefixing yerna link before every such script.


The "Lerna monorepo model" is simple and works reasonably well, but at scale, npm itself falters and causes delays (slow, filesystem-heavy operations) and breaks (nondeterministic installation). Swapping out the npm behaviors for Yarn improves both automated build stability/speed and devex considerably.

Additionally, there were a handful of features I found useful that have not been merged into Lerna or are not appropriate for Lerna, as well as Lerna features I do not need. All together, these changes were easier to implement as a new tool rather than a fork of Lerna.

Hopefully this repo serves as a reasonable stop-gap until Lerna gets merged into Yarn, at which point it should be mostly or entirely obsoleted by vanilla Yarn.

How it Works

Hacks. Filthy, awful hacks.

Yarn demands complete control over node_modules. This is reasonable; the only reason that Lerna worked "seamlessly" is because npm was lenient about the structure and content of node_modules (as long as it appeared to satify the constraints in package.json). Lerna was free to make all kinds of symlinks and npm would happily chug along ignoring them. Yarn, in its strictness, will clear out these symlinks and will attempt to download packages that we know to be local-only.

The workaround is to wrap Yarn in a task that mangles the package.jsons to remove all local references before running Yarn. In practice, this works great, but the failure mode can be very confusing for those who aren't intimately familiar with how node_modules, npm and Yarn work. It also means that Yarn is free to delete all your symlinks and pretty much any time (as a convenience, yerna and yarnhack will re-link packages any time Yarn was run).