Why use peer dependencies in npm for plugins?

TL;DR: peerDependencies are for dependencies that are exposed to (and expected to be used by) the consuming code, as opposed to "private" dependencies that are not exposed, and are only an implementation detail.

The problem peer dependencies solve

NPM's module system is hierarchical. One big advantage for simpler scenarios is that when you install an npm package, that package brings its own dependencies with it so it will work out of the box.

But problems arise when:

• Both your project and some module you are using depend on another module.
• The three modules have to talk to each other.

In Example

Let's say you are building YourCoolProject and you're using both JacksModule 1.0 and JillsModule 2.0. And let's suppose that JacksModule also depends on JillsModule, but on a different version, say 1.0. As long as those 2 versions don't meet, there is no problem. The fact that JacksModule is using JillsModule below the surface is just an implementation detail. We are bundling JillsModule twice, but that's a small price to pay when we get stable software out of the box.

But now what if JacksModule exposes its dependency on JillsModule in some way. It accepts an instance of JillsClass for example... What happens when we create a new JillsClass using version 2.0 of the library and pass it along to jacksFunction? All hell will break loose! Simple things like jillsObject instanceof JillsClass will suddenly return false because jillsObject is actually an instance of another JillsClass, the 2.0 version.

How peer dependencies solve this

They tell npm

I need this package, but I need the version that is part of the project, not some version private to my module.

When npm sees that your package is being installed into a project that does not have that dependency, or that has an incompatible version of it, it will warn the user during the installation process.

When should you use peer dependencies?

• When you are building a library to be used by other projects, and
• This library is using some other library, and
• You expect/need the user to work with that other library as well

Common scenarios are plugins for larger frameworks. Think of things like Gulp, Grunt, Babel, Mocha, etc. If you write a Gulp plugin, you want that plugin to work with the same Gulp that the user's project is using, not with your own private version of Gulp.

I would recommend you to read the article again first. It's a bit confusing but the example with winston-mail shows you the answer why:

For example, let's pretend that [email protected] specified "winston": "0.5.x" in its "dependencies" object because 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 and put 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

├── [email protected]  
└─┬ [email protected]                
  └── [email protected]

In this case, it is possible to have multiple versions of a package which would cause some issues. Peer dependencies allow npm developers to make sure that the user has the specific module (in the root folder). But you're correct with the point that describing one specific version of a package would lead to issues with other packages using other versions. This issue has to do with npm developers, as the articles states

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.

Therefore developers should follow semver for defining peerDependencies. You should open an issue for the grunt-steroids package on GitHub...