nodaguti / babel-eslint-scoping-bug

Babel-eslint's Scope Mishandlling with eslint@3 and eslint-scope Installed

This might be a really rare case, but if your project contains eslint@3 and eslint-scope and uses flow with babel-eslint, eslint starts to complain about almost all of your flow typings saying there are no-undef, no-unused-vasr, no-shadow, no-dupe-args, no-redeclare violations.

This is probably because while eslint@3 uses escope to determine scopes, babel-eslint want to use eslint-scope if it is available and this mismatch causes the trouble.

Note that some packages, such as webpack@4, install eslint-scope as their dependencies even though you don't use it directly.

Steps to reproduce

With eslint@3 and eslint-scope

Run

$ git checkout master  # make sure you are in master
$ yarn install
$ yarn run eslint src

and you will get the following errors:

babel-eslint-scoping-bug/src/index.js
   3:8  error  'ImportedFooParam' is defined but never used               no-unused-vars
  15:6  error  'ImportedFooParam' is already declared in the upper scope  no-shadow
  20:1  error  Duplicate param 'FooParam'                                 no-dupe-args
  25:6  error  'FooParam' is already defined                              no-redeclare

babel-eslint-scoping-bug/src/type.js
  2:13  error  'ImportedFooParam' is not defined  no-undef

✖ 5 problems (5 errors, 0 warnings)

With eslint@3 and without eslint-scope

Run

$ git checkout without-eslint-scope
$ yarn install
$ yarn run eslint src

and you will get no errors.

With eslint@latest and eslint-scope

Run

$ git checkout latest-eslint
$ yarn install
$ yarn run eslint src

and you will get no errors.