Understanding Peer Dependencies in Node Modules

In the package.json file of some Node modules, you might come across a field called peerDependencies. While you may be familiar with dependencies and devDependencies, peerDependencies is slightly different.

• Dependencies: These are the packages that your project relies on.
• DevDependencies: These are the packages that are necessary during the development phase, such as testing frameworks like Jest or utilities like Babel or ESLint.

When you install a package using npm, both the dependencies and devDependencies are automatically installed. However, peerDependencies are not automatically installed.

When a package lists a dependency as a peerDependency, it means that the code that includes the package must also include the peerDependency as its own dependency. If you run npm install and the required peerDependency is not found, npm will issue a warning.

Let’s consider an example to understand this better:

Package A includes dependency B:

a/package.json
{
  "dependencies": {
    "b": "1.x"
  }
}

Package B lists C as a peerDependency:

b/package.json
{
  "peerDependencies": {
    "c": "1.x"
  }
}

In this case, in package A, we must add C as a dependency as well. If we install package B without including C as a dependency in package A, npm will issue a warning, and the code is likely to fail at runtime.

a/package.json
{
  "dependencies": {
    "b": "1.x",
    "c": "1.x"
  }
}

It is important to note that the versions of the peerDependencies must be compatible. For example, if a peerDependency is listed as 2.x, you cannot install 1.x or any other version. Semantic versioning rules apply here.

Understanding peerDependencies helps ensure that the necessary dependencies are included correctly and that the code runs smoothly.