Minimal reproduction of a bug in the eslint-plugin-import package

This repo reproduces a bug that leads to external scoped packages being determined as internal ones, producing incorrect linting output. See import-js/eslint-plugin-import#1597.

Steps to reproduce

1. Clone this repository

git clone https://github.com/skozin/eslint-plugin-import-bug-repro.git

2. Run yarn to install dependencies

yarn

3. Lint packages/package-b/index.js file:

yarn lint

This command actually does this:

./node_modules/.bin/eslint packages/package-b/index.js

Expected result

According to .eslintrc config, packages/package-b/index.js should produce no errors or warnings. Here is the contents of the file:

import lodash from 'lodash';
import {sharedFn} from '@my-scope/package-a';

import {someFn} from './util';

export default {};

Actual result

However, the following errors are shown:

eslint-plugin-import-bug-repro/packages/package-b/index.js
  1:1  error  There should be at least one empty line between import groups        import/order
  4:1  error  `./util` import should occur before import of `@my-scope/package-a`  import/order

✖ 2 problems (2 errors, 0 warnings)
  2 errors and 0 warnings potentially fixable with the `--fix` option.

These errors disappear if the Node resolver is used instead of the Webpack resolver.

Details of the setup

For the problem to occur, the following conditions should be met:

1. The project should use Yarn workspaces.

It is a popular monorepo setup, e.g. used by Babel, which allows you to have multiple packages in a single repo and import them using their name, as if they were published to a registry. From Yarn documentation:

Your dependencies can be linked together, which means that your workspaces can depend on one another while always using the most up-to-date code available. This is also a better mechanism than yarn link since it only affects your workspace tree rather than your whole system.

Yarn does this by creating filesystem symlinks from node_modules directory into actual directories the packages reside in. Run ls -l node_modules/@my-scope to see this:

$ ls -l node_modules/@my-scope
total 0
lrwxr-xr-x  1 user  group  24 Oct 28 17:28 package-a -> ../../packages/package-a
lrwxr-xr-x  1 user  group  24 Oct 28 17:28 package-b -> ../../packages/package-b

In this repository, all packages reside in packages directory, as configured by workspaces key in package.json.

2. The packages inside a monorepo should be scoped.

In this case, they use @my-scope prefix.

3. Webpack resolver should be used.

When Node resolver is used, the behavior is correct. You can check this by removing this line from .eslintrc:

...
  "import/resolver": "webpack",
...

Please note that the packages directory is added to external directories list in .eslintrc. Nevertheless, scoped packages that reside in this directory are determined as internal.

The cause

When all of the above conditions are met, the following happens.

1. The plugin calls resolveImportType function to get import type of the @my-scope/package-a dependency.

2. The resolveImportType function calls resolve function from eslint-module-utils package. It, in turn, uses the Webpack resolver, which returns the full path to the module main file, after resolving all symlinks. In our case, it is /path/to/the/project/packages/package-a/index.js. This is in contrast to the Node resolver, which in this case would return /path/to/the/project/node_modules/@my-scope/package-a/index.js.

3. The resolveImportType function then calls typeTest, leading to the following call stack: resolveImportType -> typeTest -> isInternalModule -> isExternalPath. The last function should return true, since packages directory is contained in the external directories list.

4. The isExternalPath function extracts part of the import declaration before the first slash and considers it the package name. Then, for each of the configured external module folders, it tries to find substring ${external-folder}/${package-name} inside the filesystem path to the module. If it succeeds, then the path is considered external. In our case, since the import declaration is import {sharedFn} from '@my-scope/package-a', the packageName variable gets set to @my-scope, and the loop on the next line looks for the node_modules/@my-scope or packages/@my-scope substring inside the FS path to the module (which is /path/to/the/project/packages/package-a/index.js in the case of the Webpack resolver) and fails to do so.

5. So, isExternalPath returns false, and this makes the logic inside isInternalModule decide that the package is internal. Previously, isInternalModule would consider all scoped packages external, but some time ago an issue was submitted, reporting incorrect behavior of the plugin when Webpack aliases starting with @ were used: those aliased imports were incorrectly determined as external. The corresponding PR fixed this by changing the logic so that it would always consider scoped imports as internal except when they correspond to external filesystem paths.

How to fix

This is pretty complicated actually.

The first problem here is that isExternalPath incorrectly determines package name for scoped packages: it sets the packageName variable by extracting the part of the import name before the first slash. But this is incorrect for scoped package names since that part corresponds to the scope name and not package name. For scoped packages, the package name is the part of the import name before the second slash (or the end of the string if the number of slashes is less than two). For example, in our case, for import declaration import {sharedFn} from '@my-scope/package-a' the package name is @my-scope/package-a and not @my-scope.

But even fixing this won't make the plugin support the case described here since Webpack resolver returns the fully resolved path to the module's main file (/path/to/the/project/packages/package-a/index.js) which doesn't contain @my-scope part.

So, I see two options.

The first one is to make isExternalPath return true for all paths that contain one of the import/external-module-folders as a sub-path, regardless of the name of the import. According to the documentation, this seems to be the expected behavior:

import/external-module-folders An array of folders. Resolved modules only from those folders will be considered as "external". By default - ["node_modules"]. Makes sense if you have configured your path or webpack to handle your internal paths differently and want to considered modules from some folders, for example bower_components or jspm_modules, as "external".

The risk I see here is that the stuff with checking the import name in isExternalPath was added to work around some edge case I don't yet know of, so removing this seemingly unneeded logic might break something.

The second, safer, way is to add another configuration option, e.g. import/workspace-module-folders, and make isExternalPath return true also when a path to the module contains one of these paths as a sub-path, in addition to the current logic. This will guarantee that none of the existing setups are broken, however at the cost of adding an extra configuration option.