Cannot load CJS custom rules with rulesdir when project is "type": "module"

[hsablonniere]

Hello ESLint team 😉

Env

• Node version: v14.15.0
• npm version: v6.14.8
• Local ESLint version: v7.15.0 (Currently used)
• Global ESLint version: Not found

The Context

• I want to add "type": "module" to the package.json of my project
• I renamed my config files to .eslintrc.cjs and it's almost working

The "problem"

• I have a custom rule inside a directory
• This custom rule is a CommonJS script with a .js extention eslint-rules/my-rule.js
• I run this npm script: eslint --rulesdir eslint-rules src

I get this error:

Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: /home/hsablonniere/dev/clever-components/eslint-rules/sort-lit-element-css-declarations.js
require() of ES modules is not supported.
require() of /home/hsablonniere/dev/clever-components/eslint-rules/sort-lit-element-css-declarations.js from /home/hsablonniere/dev/clever-components/node_modules/eslint/lib/cli-engine/load-rules.js is an ES module file as it is a .js file whose nearest parent package.json contains "type": "module" which defines all .js files in that package scope as ES modules.
Instead rename sort-lit-element-css-declarations.js to end in .cjs, change the requiring code to use import(), or remove "type": "module" from /home/hsablonniere/dev/clever-components/package.json.

The diagnostic

• My project has "type": "module"
• ESLint CLI uses require() for scripts in rulesdir (see here)

From what I understand from Node.js' docs and how ESM vs CJS works, with this setup:

• the required file must be a CJS file
• the required file must have a .cjs extension

It's already a CJS file but I cannot change the extension to .cjs because ESLint ignores files that aren't .js (see here).

The proposition

Could we maybe add .cjs as allowed extensions in this check?

It tried this locally and it fixes my problem:

fs.readdirSync(rulesDir).forEach(file => {
  if (path.extname(file) !== ".js" && path.extname(file) !== ".cjs") {
    return;
  }
  rules[path.parse(file).name] = require(path.join(rulesDir, file));
});

Questions

1. What do you think about this problem?
2. What do you think about my proposition?
3. Do you want me to write a PR?

Cheers...

[mdjermanovic]

Hi @hsablonniere, thanks for the issue!

It looks like this use case wasn't covered by eslint/rfcs#43.

I think we could support this, but the --rulesdir feature will be anyway removed in some of the future ESLint versions (per eslint/rfcs#9), so I'm not sure should we be adding some enhancements to this feature now if there's at least some workaround for "type": "module" projects.

Can you add an empty package.json file to the directory where the custom rules are, and would that work for you? (extensions should be still .js).

[hsablonniere]

Hello @mdjermanovic,

• I just had a quick look at the RFC and the replacement for ˋ—rulesdirˋ seems really nice.
• I totally understand how my proposition is not relevant if this is going away
• I created a « local » plugin with a ˋfile:ˋ dependency but your solution feels simpler and smarter thanks very much 👍

Thanks for your quick feedback and answer and best regards to the team.