Peer Dependencies

By Domenic Denicola,

Reposted from Domenic's blog with permission. Thanks!

npm is awesome as a package manager. In particular, it handles sub-dependencies very well: if my package depends on request version 2 and some-other-library, but some-other-library depends on request version 1, the resulting dependency graph looks like:

├── request@2.12.0
└─┬ some-other-library@1.2.3
  └── request@1.9.9

This is, generally, great: now some-other-library has its own copy of request v1 that it can use, while not interfering with my package's v2 copy. Everyone's code works!

The Problem: Plugins

There's one use case where this falls down, however: plugins. A plugin package is meant to be used with another "host" package, even though it does not always directly use the host package. There are many examples of this pattern in the Node.js package ecosystem already:

Even if you're not familiar with any of those use cases, surely you recall "jQuery plugins" from back when you were a client-side developer: little <script>s you would drop into your page that would attach things to jQuery.prototype for your later convenience.

In essence, plugins are designed to be used with host packages. But more importantly, they're designed to be used with particular versions of host packages. For example, versions 1.x and 2.x of my chai-as-promised plugin work with chai version 0.5, whereas versions 3.x work with chai 1.x. Or, in the faster-paced and less-semver–friendly world of Grunt plugins, version 0.3.1 of grunt-contrib-stylus works with grunt 0.4.0rc4, but breaks when used with grunt 0.4.0rc5 due to removed APIs.

As a package manager, a large part of npm's job when installing your dependencies is managing their versions. But its usual model, with a "dependencies" hash in package.json, clearly falls down for plugins. Most plugins never actually depend on their host package, i.e. grunt plugins never do require("grunt"), so even if plugins did put down their host package as a dependency, the downloaded copy would never be used. So we'd be back to square one, with your application possibly plugging in the plugin to a host package that it's incompatible with.

Even for plugins that do have such direct dependencies, probably due to the host package supplying utility APIs, specifying the dependency in the plugin's package.json would result in a dependency tree with multiple copies of the host package—not what you want. For example, let's pretend that winston-mail 0.2.3 specified "winston": "0.5.x" in its "dependencies" hash, since that's the latest version it was tested against. As an app developer, you want the latest and greatest stuff, so you look up the latest versions of winston and of winston-mail, putting them in your package.json as

{
  "dependencies": {
    "winston": "0.6.2",
    "winston-mail": "0.2.3"
  }
}

But now, running npm install results in the unexpected dependency graph of

├── winston@0.6.2
└─┬ winston-mail@0.2.3
  └── winston@0.5.11

I'll leave the subtle failures that come from the plugin using a different Winston API than the main application to your imagination.

The Solution: Peer Dependencies

What we need is a way of expressing these "dependencies" between plugins and their host package. Some way of saying, "I only work when plugged in to version 1.2.x of my host package, so if you install me, be sure that it's alongside a compatible host." We call this relationship a peer dependency.

The peer dependency idea has been kicked around for literally years. After volunteering to get this done "over the weekend" nine months ago, I finally found a free weekend, and now peer dependencies are in npm!

Specifically, they were introduced in a rudimentary form in npm 1.2.0, and refined over the next few releases into something I'm actually happy with. Today Isaac packaged up npm 1.2.10 into Node.js 0.8.19, so if you've installed the latest version of Node, you should be ready to use peer dependencies!

As proof, I present you the results of trying to install jitsu 0.11.6 with npm 1.2.10:

npm ERR! peerinvalid The package flatiron does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer flatiron-cli-config@0.1.3 wants flatiron@~0.1.9
npm ERR! peerinvalid Peer flatiron-cli-users@0.1.4 wants flatiron@~0.3.0

As you can see, jitsu depends on two Flatiron-related packages, which themselves peer-depend on conflicting versions of Flatiron. Good thing npm was around to help us figure out this conflict, so it could be fixed in version 0.11.7!

Using Peer Dependencies

Peer dependencies are pretty simple to use. When writing a plugin, figure out what version of the host package you peer-depend on, and add it to your package.json:

{
  "name": "chai-as-promised",
  "peerDependencies": {
    "chai": "1.x"
  }
}

Now, when installing chai-as-promised, the chai package will come along with it. And if later you try to install another Chai plugin that only works with 0.x versions of Chai, you'll get an error. Nice!

UPDATE: npm versions 1, 2, and 7 will automatically install peerDependencies if they are not explicitly depended upon higher in the dependency tree. For npm versions 3 through 6, you will receive a warning that the peerDependency is not installed instead.

One piece of advice: peer dependency requirements, unlike those for regular dependencies, should be lenient. You should not lock your peer dependencies down to specific patch versions. It would be really annoying if one Chai plugin peer-depended on Chai 1.4.1, while another depended on Chai 1.5.0, simply because the authors were lazy and didn't spend the time figuring out the actual minimum version of Chai they are compatible with.

The best way to determine what your peer dependency requirements should be is to actually follow semver. Assume that only changes in the host package's major version will break your plugin. Thus, if you've worked with every 1.x version of the host package, use "~1.0" or "1.x" to express this. If you depend on features introduced in 1.5.2, use ">= 1.5.2 < 2".

Now go forth, and peer depend!

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