Support .mjs configuration files

[ybiquitous]

What is the problem you're trying to solve?

Cosmiconfig has supported .mjs files since v8.2. I believe it'd be useful if Stylelint also could support it.

What solution would you like to see?

First, we may need to resolve a problem with loaders. It may not be easy.

stylelint/lib/createStylelint.js

Lines 22 to 27 in 2413e8f

	loaders: {
	'.cjs': (cjsPath, cjsContent) =>
	Promise.resolve(defaultLoadersSync['.cjs'](cjsPath, cjsContent)),
	'.js': (jsPath, cjsContent) =>
	Promise.resolve(defaultLoadersSync['.js'](jsPath, cjsContent)),
	},

Then, we must update the CLI help and documentation below:

stylelint/lib/cli.js

Lines 115 to 118 in 2413e8f

	- a "stylelint" property in "package.json"
	- a ".stylelintrc" file (with or without filename extension:
	".json", ".yaml", ".yml", ".js", and ".cjs" are available)
	- a "stylelint.config.js" file exporting a JS object

stylelint/docs/user-guide/configure.md

Lines 3 to 8 in 2413e8f

	Stylelint expects a configuration object, and looks for one in a:
	- `stylelint` property in `package.json`
	- `.stylelintrc` file
	- `.stylelintrc.{cjs,js,json,yaml,yml}` file
	- `stylelint.config.{cjs,js}` file exporting a JS object

See also:

• Segmentation fault (core dumped) #6898
• Fix Segmentation fault errors for Cosmiconfig 8.2 #6902
• Move to ESM #5291

[romainmenke]

First, we may need to resolve a problem with loaders. It may not be easy.

I think this should just work.
I did no change the loader .mjs files so that we wouldn't have to make further changes.

The only hurdle is that we can't write tests for this with jest.

[ybiquitous]

Unfortunately, my try for .mjs fails. Did I miss something?

package.json:

{
  "devDependencies": {
    "stylelint": "^15.7.0"
  },
  "scripts": {
    "test": "stylelint -c config.mjs *.css",
    "test-cjs": "stylelint -c config.cjs *.css"
  }
}

config.mjs:

export default {
  rules: {
    "block-no-empty": true,
  },
};

config.cjs:

module.exports = {
  rules: {
    "block-no-empty": true,
  },
};

test.css:

a{}

Run:

$ npm i
...

$ npm test
Error [ERR_REQUIRE_ESM]: require() of ES Module /Users/masafumi.koba/tmp/foo/config.mjs not supported.
Instead change the require of /Users/masafumi.koba/tmp/foo/config.mjs to a dynamic import() which is available in all CommonJS modules.
...

$ npm run test-cjs
test.css
 1:2  ✖  Unexpected empty block  block-no-empty

1 problem (1 error, 0 warnings)

[romainmenke]

Was that with or without the v8 cache package?

#6907

[alexander-akait]

We can write test for jest, but it is tricky, we can use vm and run ESM in vm, also we need to run with some flags (not for jest, for Node.js, because it is still experiment), as another workaround we can spawn a process with only loading (i.e. create a small file with our loading API) and test it

[ybiquitous]

Was that with or without the v8 cache package?

Good catch! Using the HEAD version of stylelint can resolve the error: 😅 👍🏼

  "devDependencies": {
-   "stylelint": "^15.7.0"
+   "stylelint": "github:stylelint/stylelint"
  },

We can write test for jest, but it is tricky

It might be ideal for writing tests, but I don't think writing tests for this feature is essential because writing tricky tests for the feature Cosmiconfig provides has very few benefits.

---

So, we can just update the CLI help and the document to close this issue!

I've labeled the issue as ready to implement. Please consider contributing if you have time.