no-unused-vars rule don't work while extending the airbnb's config

[artptr]

This repo reproduces the bug in eslint-config-airbnb.
There is an error while checking the test.js:

  1:5  error  'Foo' is defined but never used  no-unused-vars

The error appears despite that the parent config contains appropriate no-unused-vars setting.
It disappears only if we remove the airbnb's config (default or legacy) from the extends property.

The error appears on the ESLint 1.10.3 and 2.2.0

[ljharb]

According to the eslint docs, setting the "vars" option should still warn about unused local vars - it seems that error is correct, and that var is unused, meaning that line could be deleted and your program would work the same.

[ljharb]

( I'm referring both to https://github.com/artptr/eslint-airbnb-no-unused-vars-bug/blob/master/.eslint-project-rules.json#L3 and to the redundant https://github.com/artptr/eslint-airbnb-no-unused-vars-bug/blob/master/.eslintrc.json#L8 )

btw thanks for the reproduction repo, that makes it super easy to understand what's going on!

[vkrol]

@ljharb Foo is the global variable, not local.

[ljharb]

Every file in node is in a module context, which means it's not at all a global variable.

[ljharb]

@vkrol you can verify that by checking global.Foo against Foo inside that file.

[artptr]

So the airbnb/legacy is not for a browser environment, but for an old Node.js environment, right?

[ljharb]

@artptr it's for both. but eslint runs with node, and assumes your files are run with node. You should be using browserify or the like to wrap your files anyway such that nothing in your browser runs directly in the global scope.

[artptr]

@ljharb but if we remove the "airbnb/legacy" string from extends property then all will be ok and local "no-unused-vars": [2, { vars: "local" } ] rule will be work because it will decide that Foo is a global variable in a script in a browser.

[artptr]

Also it may be returned to assumed behavior by adding

"env": {
  "node": false
}

[ljharb]

Right - this isn't a bug, because in 2016 everything should be run in a node context, and only after running through browserify/webpack should it be shipped to a browser.

You're certainly free to override the env settings in your eslint config, however!