Bug: [flat config] `.mjs` extension for config file

Environment

Environment Info:

Node version: v19.1.0 npm version: v8.19.3 Local ESLint version: v8.28.0 (Currently used) Global ESLint version: Not found Operating System: darwin 22.1.0

What parser are you using?

Default (Espree)

What did you do?

My npm package is commonjs, with .js extensions, alongside ESM with .mjs extensions for running tests. Running npx eslint . results in Node failing to parse the configuration file, as it assumes it to be commonjs.

What did you expect to happen?

The config file should only support a .mjs extension (eslint.config.mjs) to avoid this problem since Node will always execute it as ESM regardless of the type field defined in package.json.

What actually happened?

(Shortened)

Warning: To load an ES module, set "type": "module" in the package.json or use the .mjs extension.
...
eslint.config.js:1
export default [
^^^^^^

SyntaxError: Unexpected token 'export'

Participation

• I am willing to submit a pull request for this issue.

Additional comments

No response

[jaydenseric]

If your project does not have a package type field of "module", and your ESM is in .mjs files because .js is in CJS mode, and if you need to use a pure ESM dev dependency within the project ESLint config file, you are screwed. The eslint.config.js file is in CJS mode, meaning you can’t require the ESM dev dependency. ESLint must add support for eslint.config.mjs files.

If in the future ESLint decides to support only one config file name and format that any project could use, it would be eslint.config.mjs because that will always work as ESM no matter what the package type field is configured to, and the ESM format is capable of importing from both ESM as well as CJS dev dependencies.

[dburles]

@dburles you’ll need to use the environment variable to force the file to be interpreted as flat config.

Whoops! that does it, thanks. That’s a decent workaround.