What is a peer dependency and what is the problem with it?

Peer dependencies are a specific kind of dependencies really useful for reusable modules:

• Ask user to install a dependency your module needs to work without specifying a version in particular
• Prevents having multiple version of a same module in user's app node_modules
• Reduce javascript files size to load on browser side particularly useful for mobile users

The problem with peer dependencies is npm and yarn don't install them at all. This is the right behavior for production purpose but when you are developing you might need to test your module in a host app. npm and yarn provide a command to achieve it called link that basically creates a symlink into the host app node_modules to your module source folder. It works fine but you also need to execute tasks in your module that needs these dependencies. For example, you might want to execute tests. Since they aren't present in your module's node_modules you will experience errors like this:

Cannot find module 'react' from 'index.js'

yarn and npm don't provide tools to install peer dependencies for your development environment. There is an opened issue on yarn repository since 27 October 2016 but yarn has a special script called prepare that is executed after dependencies installation only on development mode maybe we could do something with it 🤔. Let's find a way to set up a work around!

Solve the problem

I made a repository with all the sources to reproduce the problem.

We have the following folder structure:

.
├── LICENSE
├── README.md
├── app
│   ├── node_modules
│   ├── package.json
└── lib
    ├── node_modules
    └── package.json

First of all we will set up a link so the app's node_modules point to the lib folder.

You must install dependencies and you might also need to build your module first

cd lib
yarn link
cd ../app
yarn link lib

At this point if you start your application you could use your module without any trouble. The problem appears when you try to execute things into your module folder as I said before like tests. To solve this we will use prepare script from yarn and the package install-peers-cli.

install-peers-cli is a cli that install peer dependencies of a package. It should be called after dependencies installation. Fortunately prepare is called after dependencies installation and only when you are developing so it won't install the peer dependencies when a user install your module.

First install install-peers-cli package:

yarn add -D install-peers-cli

Then add the prepare script in your module's package.json and call install-peers-cli in it:

// package.json
{
  "scripts": {
    "prepare": "install-peers"
  }
}

Now you could start anything that use your peer dependencies without experiencing errors. Unless your module uses React and his new addition Hooks...

For React using hooks

If you follow previous steps you should have an error with the following message "hooks can only be called inside the body of a function component". This error occurs when you have multiple copies of React in your node_modules folder. In this comment Dan Abramov explain the solution to this well known issue. Let's put it into pratice.

You should go in the react folder that is present in your module's node_modules and create a link. Then go back to your host app folder and use this link.

cd lib/node_module/react
yarn link
cd ../../../app
yarn link react

Now the error should be fixed and you could use your module into your host app!

You're all set 🙌!

See a typo? Don't hesitate to open an issue or make a pull request on the article's repository