# What are peer dependencies in a Node module?

> Understand peerDependencies in a package.json: unlike dependencies, they are not auto-installed, so the consuming project must add them to avoid warnings.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2019-12-12 | Topics: [Node.js](https://flaviocopes.com/tags/node/) | Canonical: https://flaviocopes.com/npm-peer-dependencies/

In some [package.json](https://flaviocopes.com/package-json/) files, you might see a few lines like this:

```json
{
  //...
  "peerDependencies": {
    "libraryName": "1.x"
  }
}
```

You might have already seen `dependencies` and `devDependencies`, but not `peerDependencies`.

`dependencies` are the packages your project depends on.

`devDependencies` are the packages that are needed during the development phase. Say a testing framework like [Jest](https://flaviocopes.com/jest/) or other utilities like [Babel](https://flaviocopes.com/babel/) or [ESLint](https://flaviocopes.com/eslint/).

In both cases, when you install a package, its dependencies and devDependencies are automatically installed by [npm](https://flaviocopes.com/npm/).

`peerDependencies` are different. They are not automatically installed.

When a dependency is listed in a package as a peerDependency, **it is not automatically installed**. Instead, the code that includes the package must include it as its dependency.

`npm` will warn you if you run `npm install` and it does not find this dependency.

Example: let's say package `a` includes dependency `b`:

> `a/package.json`

```json
{
  //...
  "dependencies": {
    "b": "1.x"
  }
}
```

Package `b` in turn wants package `c` as a peerDependency:

> `b/package.json`

```json
{
  //...
  "peerDependencies": {
    "c": "1.x"
  }
}
```

In package A, we must therefore add `c` as a dependency, otherwise when you install package `b`, npm will give you a warning (and the code will likely fail at runtime):

> `a/package.json`

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

The versions must be compatible, so if a peerDependency is listed as `2.x`, you can't install `1.x` or another version. It all follows [semantic versioning](https://flaviocopes.com/npm-semantic-versioning/).