💻

• Would you like to work on a fix?

How are you using Babel?

babel-loader (webpack)

Input code

You can create any dummy.cjs file and run npx eslint . on it and you'll get the following error:

  0:0  error  Parsing error: .sourceType must be "module", "script", "unambiguous", or undefined

This happens when you do not specify a sourceType in your eslint flat config file under languageOptions>parserOptions.

Configuration file name

No response

Configuration

My eslint.config.mjs file looks like:

import babelParser from "@babel/eslint-parser";

export default [
    {
        rules: {
            'no-unassigned-vars': 'error'
        },
        languageOptions: {
            parser: babelParser,
            parserOptions: {
                requireConfigFile: false,
            }
        }
    }
]

Current and expected behavior

Simple repro steps:

  mkdir repro
  cd repro
  npm init
  npm install eslint
  npm install @babel/eslint-parser
  touch dummy.cjs
  echo "let a;\nif(a === 'ready') console.log('ok');" > dummy.cjs
  npx eslint dummy.cjs

and then create a eslint.config.mjs with the above contents shown above.

Then run

npx eslint dummy.cjs

Environment

~/temp/repro: npm --version  
10.9.2
~/temp/repro: node --version  
v23.6.1
~/temp/repro: sw_vers         
ProductName:		macOS
ProductVersion:		15.5
BuildVersion:		24F74
~/temp/repro: cat package.json
{
  "name": "repro",
  "dependencies": {
    "@babel/eslint-parser": "^7.27.1",
    "eslint": "^9.27.0"
  }
}

Possible solution

If I alter my eslint.config.mjs config file to have sourceType: undefined, in the parserOptions like so:

import babelParser from "@babel/eslint-parser";

export default [
    {
        rules: {
            'no-unassigned-vars': 'error'
        },
        languageOptions: {
            parser: babelParser,
            parserOptions: {
                sourceType: undefined,  // Magically fixes the issue
                requireConfigFile: false,
            }
        }
    }
]

then I then get the correct eslint evaluation output without the parser error.

Doing some debugging, I noticed that if I modify the code over at https://github.com/babel/babel/blob/main/packages/babel-core/src/config/validation/option-assertions.ts#L132
to instead spit out the value:

      `${msg(loc)} must be "module", "script", "unambiguous", or undefined. Value was "${value}"`,

then when I go to my original repro steps (without sourceType specified) then I get:

  0:0  error  Parsing error: .sourceType must be "module", "script", "unambiguous", or undefined. Value was "commonjs"

and doing some reading I believe the ESLint 9 now automatically fills in the sourceType depending on the file extension if it isn't specified.

I believe the solution is that babel needs to map "commonjs" or whatever else ESLint auto fills in with to its corresponding "script", "module", or "unambiguous" values.

Additional context

No response