Understanding when and why to use npm peerDependencies

In this article I hope to clarify what npm Peer Dependencies are and especially when you should use them. Peer Dependencies are listed in the package.json file in the peerDependencies object.

To get the most out of this article you should have at least an introductory understanding of npm.

Contents#

In this article:

  1. We will compare exactly how Dependencies work versus Peer Dependencies.
  2. We will look at some examples of both Dependencies and Peer Dependencies.
  3. Then, we will examine how npm handles version conflicts.
  4. Finally, having the fundamentals solidly in our grasp, we will lay out an approach to deciding when Peer Dependencies are appropriate.

The Scenario#

To keep it real, let’s assume you’re creating an Angular Library or even just a simple JavaScript file that exports some functions.

Your project relies on packages from the npm Registry. These packages are your project’s dependencies.

You want to create your own npm package from your project. So you use npm pack to generate an npm package from your project. You might even decide to publish it to the npm Registry.

Other teams will add your package as a dependency in their own projects. We use Dependencies and Peer Dependencies in package.json to tell these other projects what packages also need to be added for our package to work.

So, at their most basic level here is how Dependencies and Peer Dependencies work:

Dependencies#

Dependencies are listed in the package.json file in a dependencies object.

When you add a package in dependencies, you are saying:

  • My code needs this package to run.
  • If this package doesn’t already exist in my node_modules directory, then add it automatically.
  • Furthermore, add the packages that are listed in the package’s dependencies. These packages are called transitive dependencies.

Peer Dependencies#

Peer Dependencies are listed in the package.json file in a peerDependencies object.

By adding a package in peerDependencies you are saying:

  • My code is compatible with this version of the package.
  • If this package already exists in node_modules, do nothing.
  • If this package doesn’t already exist in the node_modules directory or it is the wrong version, don’t add it. But, show a warning to the user that it wasn’t found.

Adding Dependencies#

So, we add dependencies in the package.json file of our npm package folder. Let’s look at exactly how we add packages as dependencies and some examples of package dependencies.

Adding a Dependency#

A Dependency is an npm package that our package depends on in order to be able to run. Some popular packages that are typically added as dependencies are lodash, request, and moment.

We add a regular dependency like this:

npm install lodash

npm adds the package name and version to the dependencies object in our project’s package.json file.

"dependencies": {
    "lodash": "^4.17.11"
  }

Some of you might remember the old days when we had to use the --save flag to get npm to update the dependencies in package.json. Thankfully, we don’t need to do that anymore.

Adding a Peer Dependency#

Peer Dependencies are used to specify that our package is compatible with a specific version of an npm package. Good examples are Angular and React.

To add a Peer Dependency you actually need to manually modify your package.json file. For example, for Angular component library projects, I recommend adding angular/core as a peer dependency. So if you wanted to specify that your package is built for Angular 7, you could include something like this:

"peerDependencies": {
  "@angular/core": "^7.0.0"
}

About Conflicts#

I get a lot of questions about whether a certain npm package should go into dependencies or into peerDependencies. The key to making this decision involves understanding how npm deals with version conflicts.

If you have read my previous articles, you know I like you to be able to do this stuff along with me! So feel free to work along with me for this little npm experiment.

conflict-test Project#

To get started let’s create a trivial test project. I am going to name mine:
conflict-test

I created it like this:

md conflict-test
cd conflict-test
npm init -y

I then manually edited the package.json file and added two dependencies:

"dependencies": {
    "todd-a": "^1.0.0",
    "todd-b": "^1.0.0"
  }

These todd-a and todd-b packages also have their own dependencies:

todd-a#

"dependencies": {
    "lodash": "^4.17.11",
    "todd-child": "^1.0.0"
  }

todd-b#

"dependencies": {
    "lodash": "^4.17.11",
    "todd-child": "^2.0.0"
  }

The thing I want you to notice here is that todd-a and todd-b use the same version of lodash. But, they have a version conflict for todd-child:
todd-a uses todd-child version 1.0.0
todd-b uses todd-child version 2.0.0

Now I know that, like me, you are keenly interested to see how npm handles this version conflict. In my main project conflict-test I run npm install. As we would expect, npm magically installs the todd-a and todd-b packages in our node_modules folder. It also adds the packages that they depend on (the transitive dependencies). So after running npm install we take a look at the node_modules folder. It looks like this:

node_modules
├── lodash 4.17.11
├── todd-a 1.0.0
├── todd-b 1.0.0
│   └── node_modules
│       └── todd-child 2.0.0
└── todd-child 1.0.0

The interesting thing about this is that our project has one copy of lodash. But, it has two copies of todd-child. Notice that todd-b gets its own private copy of todd-child 2.0.0.

So here is the rule:

npm deals with version conflicts by adding duplicate private versions of the conflicted package.

An Approach to Peer Dependencies#

As we saw from our experiment with npm version conflicts, if you add a package to your dependencies, there is a chance it may end up being duplicated in node_modules.

Sometimes, having two versions of the same package is fine. However, some packages will cause conflicts when there are two different versions of them in the same code base.

For example, assume our component library was created using Angular 5. We wouldn’t want our package adding another completely different version of angular/core when someone adds it as a dependency to their Angular 6 application.

The key is:
We don’t want our library adding another version of a package to node-modules when that package could conflict with an existing version and cause problems.

peerDependencies or dependencies?

So this brings us to the main question for our dependencies:

When my package depends on another package, should I put it in dependencies or peerDependencies?

Well, as with most technical questions: it depends.

Peer Dependencies express compatibility. For example, you will want to be specific about which version of Angular your library is compatible with.

The Guidelines#

Favor using Peer Dependencies when one of the following is true:

  • Having multiple copies of a package would cause conflicts
  • The dependency is visible in your interface
  • You want the developer to decide which version to install

Let’s take the example of angular/core. Obviously, if you are creating an Angular Library, angular/core is going to be a very visible part of your library’s interface. Hence, it belongs in your peerDependencies.

However, maybe your library uses Moment.js internally to process some time related inputs. Moment.js most likely won’t be exposed in the interface of your Angular Services or Components. Hence, it belongs in your dependencies.

Angular as a Dependency#

Given that you are going to specify in your documentation that your library is a set of Angular Components and Services, you may be asking the question:

“Do I even need to specify angular/core as a dependency? If someone is using my library, they will already have an existing Angular project.”

Good question!

Yes, we can usually assume that for our Angular specific library the Workspace will already have the Angular packages available. Hence, technically we wouldn’t need to bother adding them to our list of dependencies.

However, we really do want to tell the developer which Angular versions our library is compatible with. So I recommend the following approach:

Add at least angular/core for the compatible Angular version to your peerDependencies.

This way developers will see a warning if they try to use your Angular 7 library in their Angular 6 project. Don’t bother adding the other Angular packages. You can assume if they have angular/core, they have the other Angular libraries.

In Conclusion#

When in doubt you should probably lean toward using peerDependencies. This lets the users of your package make their own choice about which packages to add.