I'm trying to use npm peerDependencies but nothing seems to work as advertised. What am I missing?

The setup is, I have two modules, mod and plugin, both of which depend on an external module from npm. mod declares a hard dependency on both plugin and the external module, and the plugin declares a peer dependency, in order to get access to the version being used by the parent module.

The files look like this:

~/plugin/package.json:

    {
      "name": "plugin",
      "version": "1.0.0",
      "main": "index.js",
      "peerDependencies": {
        "pad-right": "^0.2.2"
      }
    }

~/plugin/index.js

    var peerDependency = require('pad-right')
    module.exports = 'this is a plugin'

~/mod/package.json:

    {
      "name": "mod",
      "version": "1.0.0",
      "main": "index.js",
      "dependencies": {
        "pad-right": "^0.2.2",
        "plugin": "../plugin"
      }
    }

~/mod/index.js:

    var hardDependency = require('pad-right')
    var plugin = require('plugin')

As I understand it from docs and examples, I think this should be a standard way to use peer dependencies, and running npm install in either directory doesn't give any errors or warnings.

However when I run webpack in the mod folder, I get errors like:

ERROR in ../plugin/index.js
Module not found: Error: Can't resolve 'pad-right' in '~/plugin'
 @ ../plugin/index.js 1:21-41
 @ ./index.js

What's going wrong here, shouldn't webpack resolve the require statement inside plugin with the peer dependency from the parent mod module?

I'm trying to use npm peerDependencies but nothing seems to work as advertised. What am I missing?

The setup is, I have two modules, mod and plugin, both of which depend on an external module from npm. mod declares a hard dependency on both plugin and the external module, and the plugin declares a peer dependency, in order to get access to the version being used by the parent module.

The files look like this:

~/plugin/package.json:

    {
      "name": "plugin",
      "version": "1.0.0",
      "main": "index.js",
      "peerDependencies": {
        "pad-right": "^0.2.2"
      }
    }

~/plugin/index.js

    var peerDependency = require('pad-right')
    module.exports = 'this is a plugin'

~/mod/package.json:

    {
      "name": "mod",
      "version": "1.0.0",
      "main": "index.js",
      "dependencies": {
        "pad-right": "^0.2.2",
        "plugin": "../plugin"
      }
    }

~/mod/index.js:

    var hardDependency = require('pad-right')
    var plugin = require('plugin')

As I understand it from docs and examples, I think this should be a standard way to use peer dependencies, and running npm install in either directory doesn't give any errors or warnings.

However when I run webpack in the mod folder, I get errors like:

ERROR in ../plugin/index.js
Module not found: Error: Can't resolve 'pad-right' in '~/plugin'
 @ ../plugin/index.js 1:21-41
 @ ./index.js

What's going wrong here, shouldn't webpack resolve the require statement inside plugin with the peer dependency from the parent mod module?

• javascript
• npm
• peer-dependencies

1 Answer 1

Gah, it looks like this is an edge case that only affects modules that are referencing each other locally via the file system.

The solution is apparently to add something like:

    resolve: {
        alias: {
            'pad-right': path.resolve('node_modules', 'pad-right'),
        },
    },

to your webpack config. (Or else to try resolve.symlinks: false, which solves the problem in the minimal repro code I posted, but doesn't solve things in my actual project).

Article about the issue