New: Configuring Additional Lint Targets with .eslintrc

designs/2019-additional-lint-targets/README.md

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+- Start Date: 2019-05-12
+- RFC PR:
+- Authors: Toru Nagashima ([@mysticatea](https://github.com/mysticatea))
+
+# Configuring Additional Lint Targets with `.eslintrc`
+
+## Summary
+
+This proposal adds the ability to specify additional target files into configuration files. This enhancement will solve the pain that people have to use the `--ext` option with wanted file extensions even if they use plugins which support additional file types.
+
+## Motivation
+
+People have to use the `--ext` option or glob patterns to check wanted files even if they use plugins which support additional file types.
+
+```yml
+plugins:
+  - markdown
+  - html
+  - "@typescript-eslint"
+  - react
+  - vue
+```
+
+```bash
+# ESLint checks only `*.js`.
+eslint src docs
+
+# Needs `--ext` option
+eslint src docs --ext .js,.md,.html,.ts,.jsx,.tsx,.vue
+```
+
+However, using plugins which support additional file types is the intention that wants to check those files. The requirement of redundant CLI options is not reasonable.
+
+Per "[Related Discussions](#related-discussions)" section, this is very regularly requested; I want to configure additional target files with `.eslintrc`.
+
+## Detailed Design
+
+This proposal enhances `overrides` property of the config file.
+
+- If a config file in a directory has `overrides` property, ESLint checks the files which are matched by any of override entries (i.e., `files`/`excludedFiles` criteria) additionally in the directory.
+    - If any of `files` value of a override entry ended with `*`, ESLint ignores the override entry to avoid checking all files.
+    - This enhancement affects only the directory path. If people gave glob patterns, ESLint behaves the same as currently.
+    - The ignoring configuration (`.eslintignore`) is prior to this enhancement. If `.eslintignore` contains the additional target files, ESLint just ignores those as same as currently.
+
+The `overrides` property means that people intend to check those files. So this behavior is intuitive.
+
+<table><td>
+💡 <b>Example</b>:
+<pre lang="yml">
+overrides:
+  - files: "*.ts"
+    parser: "plugin:@typescript-eslint/parser"
+  - files: "tests/**/*"
+    env: { mocha: true }
+</pre>
+
+With the above config, `eslint .` command will check `*.ts` files additionally. But the command doesn't check all files inside `tests` directory.
+</td></table>
+
+### Implementation
+
+We can add the check to [lib/cli-engine/file-enumerator.js#L410](https://github.com/eslint/eslint/blob/553795712892c8350b1780e947f65d3c019293a7/lib/cli-engine/file-enumerator.js#L410). It's very easy.
+
+### With file extension processors
+
+If a plugin has file extension processors, [the config array factory yields elements which have `files` property](https://github.com/eslint/eslint/blob/553795712892c8350b1780e947f65d3c019293a7/lib/cli-engine/config-array-factory.js#L870-L902). Therefore this enhancement affects file extension processors.
+
+<table><td>
+💡 <b>Example</b>:
+<pre lang="yml">
+plugins:
+  - vue # has `.vue` processor
+</pre>
+
+With the above config, `eslint .` command will check `*.vue` files additionally.
+</td></table>
+
+Similarly, if a shareable config (including the recommended config of plugins) has `overrides` property, it affects to the config file which extends the shareable config.
+
+### For code blocks
+
+If a code block that processors extracted has the virtual filename, ESLint filters the file extension of the virtual filename with `--ext` option. This enhancement affects to that check. ESLint lints the code block if the `overrides` matches the virtual filename.
+
+## Documentation
+
+This enhancement needs migration guide because of a breaking change.
+
+- If your config contains `overrides` property or plugins which have file extension processors, `eslint` command with directory paths lints the files which are matched automatically. This may increase errors of your command.<br>
+  If you don't want to add file types to check, please use glob patterns instead of directory paths.
+
+This enhancement, so it needs to update some documents.
+
+- In the description of `--ext` CLI option, it should say that your config file may add file types automatically.
+- In the description of `overrides` property, it should say that the `overrides[].files` property adds target files automatically.
+- In the description of `plugins` property, it should say that plugins which have file extension processors add target files automatically.
+- In the "working with plugins" page, it should say that file extension processors add target files automatically.
+
+## Drawbacks
+
+- This is a breaking change.
+- Implicit behavior may be confusing people.
+
+## Backwards Compatibility Analysis
+
+In the following situation, `eslint` command increases errors.
+
+- Their configuration has `overrides` property (with `files` property which doesn't end with `*`) or plugins which have file extension processors.
+- Their `eslint` command is using directory paths without `--ext` option.
+
+I guess that the impact is limited because people use `--ext` option or glob patterns if they use configuration which affects files other than `*.js`.
+
+## Alternatives
+
+- To add `extensions` property that behaves as same as `--ext` option. Explicit reduces confusion. However, plugins and shareable configs cannot add the `extensions` property without the breaking change that drops old ESLint support.
+
+## Open Questions
+
+-
+
+## Frequently Asked Questions
+
+-
+
+## Related Discussions
+
+- [eslint/eslint#801](https://github.com/eslint/eslint/issues/801) - Configurable Extension Filter
+- [eslint/eslint#1674](https://github.com/eslint/eslint/issues/1674) - Allow setting file extensions in .eslintrc
+- [eslint/eslint#2274](https://github.com/eslint/eslint/issues/2274) - Configure file extensions in .eslintrc
+- [eslint/eslint#2419](https://github.com/eslint/eslint/issues/2419) - Allow configuration of default JS file types
+- [eslint/eslint#7324](https://github.com/eslint/eslint/issues/7324) - File extensions in .eslintrc
+- [eslint/eslint#8399](https://github.com/eslint/eslint/issues/8399) - Add .jsx to the default --ext extensions list
+- [eslint/eslint#10828](https://github.com/eslint/eslint/issues/10828) - Support specifying extensions in the config
+- [eslint/eslint#11223](https://github.com/eslint/eslint/issues/11223) - Add --ext to `eslintrc` or other config files