ESLint 6.2.0 + babel-eslint + no-unused-vars false positive with for-in loop

[feross]

There's a new issue in ESLint 6.2.0 just caught by the standard test suite. Issue did not exist in ESLint 6.1.0.

Tell us about your environment

• ESLint Version: 6.2.0
• Node Version: 10.16.3
• npm Version: 6.9.0

What parser (default, Babel-ESLint, etc.) are you using? babel-eslint

Please show your full configuration:

Configuration

{
  "parserOptions": {
    "ecmaVersion": 2019,
    "ecmaFeatures": {
      "jsx": true
    },
    "sourceType": "module"
  },
  "parser": "babel-eslint",
  "rules": {
    "no-unused-vars": ["error", { "vars": "all", "args": "none", "ignoreRestSiblings": true }]
  }
}

What did you do? Please include the actual source code causing the issue, as well as the command that you used to run ESLint.

function scoreAudioCoverFile (imgFile) {
  const fileName = path.basename(imgFile.name, path.extname(imgFile.name)).toLowerCase()
  const relevanceScore = {
    cover: 80,
    folder: 80,
    album: 80,
    front: 80,
    back: 20,
    spectrogram: -80
  }

  for (const keyword in relevanceScore) {
    if (fileName === keyword) {
      return relevanceScore[keyword]
    }
    if (fileName.indexOf(keyword) !== -1) {
      return relevanceScore[keyword]
    }
  }
  return 0
}

npx eslint --config eslintrc.json t.js

What did you expect to happen?

Just one error:

  1:10  error  'scoreAudioCoverFile' is defined but never used  no-unused-vars

What actually happened? Please include the actual, raw output from ESLint.

Two errors:

   1:10  error  'scoreAudioCoverFile' is defined but never used  no-unused-vars
  12:14  error  'keyword' is defined but never used              no-unused-vars

The variable keyword from the for-in loop is seen as not used.

This may be a bug in babel-eslint since the issue does not happen when the default parser is used. However, this issue did not exist with babel-eslint + 6.1.0 and does exist with babel-eslint + 6.2.0.

Issue opened on babel-eslint: babel/babel-eslint#791

Are you willing to submit a pull request to fix this bug?
Yes

[h4emp3]

I just noticed this as well, maybe a second example might help in finding the problem.

test.js

export const foo = bar => {
    const baz = {};
    for (const key of bar) {
        baz[key] = [];
    }
    return baz;
};

eslint --parser babel-eslint --rule '{"no-unused-vars": ["error"]}' --no-eslintrc test.js

[feross]

This may have been caused by the change to the no-unused-vars rule introduced in this PR: #12055 cc @mdjermanovic

[h4emp3]

Well, at least the rule doesn't result in a false positive for my example when I just add the additional condition again in a otherwise unchanged v6.2.0.

[tmorehouse]

We are getting man false positives with 6.2.0 as well:

6:14 error 'arg' is defined but never used no-unused-vars

  for (const arg of args) {
    if (arg !== undefined) {
      return arg
    }
  }

111:16 error 'type' is defined but never used no-unused-vars

    for (const type of ['a', 'b']) {
      const x = Array.isArray(options[type]) ? options[type] : []

[mdjermanovic]

babel-eslint is using "eslint-scope": "3.7.1"

[mdjermanovic]

This may have been caused by the change to the no-unused-vars rule introduced in this PR: #12055 cc @mdjermanovic

Most likely it is that commit, with babel-eslint in the keyword example there is a TDZ scope between function and for, probably because it's using the old version of eslint-scope

[kaicataldo]

@feross Any chance you can test the same code out with the default parser to determine if this issue should be fixed in babel-eslint or here? @mdjermanovic's assessment is most likely what's going on here.

[hellomrbigshot]

The same question! I use 6.1.0 instead and it works.

[feross]

@kaicataldo The issue doesn't exist with the default parser.

[xsf0105]

It's a bug in eslint, just upgrade eslint to v6.7.1.

[MarkPare]

I upgraded to eslint v6.7.2 and still seeing this.

[platinumazure]

It's a bug in eslint, just upgrade eslint to v6.7.1.

@allan2coder This is not accurate, or at least not fully accurate. babel-eslint did have to write a fix on their side as well. Please ensure you have latest babel-eslint.

I upgraded to eslint v6.7.2 and still seeing this.

@MarkPare See above: You do need to upgrade babel-eslint to latest as well. If you upgrade babel-eslint and still run into this issue, please open a new issue and fill out our issue template so we can take a look at your configuration and help you more effectively. Thanks!

[jgreen210]

...i.e. I also had to remove the .eslintcache directory.

[lffg]

I am using ESLint v6.7.2 and this error is happening when I use a for..of loop.

[RobTheFiveNine]

If anyone is still running into this issue with React Native due to @react-native-community/eslint-config having a hard reference to version 10.0.1 of babel-eslint and you're using yarn, add this to your package.json, remove node_modules and re-run yarn:

  "resolutions": {
    "@react-native-community/eslint-config/babel-eslint": "^10.0.3"
  }