Scripts: Add missing root flag to the default Eslint config

[gziolo]

Description

Extracted from #13394 based on a suggestion from @aduth:
https://github.com/WordPress/gutenberg/pull/13394/files#r249797548

This PR adds missing root flag in the default ESlint config which works as explained in the docs (https://eslint.org/docs/user-guide/configuring):

By default, ESLint will look for configuration files in all parent folders up to the root directory. This can be useful if you want all of your projects to follow a certain convention, but can sometimes lead to unexpected results. To limit ESLint to a specific project, place "root": true inside the eslintConfig field of the package.json file or in the .eslintrc.* file at your project’s root level. ESLint will stop looking in parent folders once it finds a configuration with "root": true.

Testing

Not sure how to test with the recent configs added which depend on the .eslintrc.js in the root folder. Otherwise I would remove it and check whether npm run lint-js works.

[aduth]

Trying to test this, I still see that ESLint will inherit rules from parent directories.

I think we need to use--either instead of or in addition-- the --no-eslintrc-js parameter when falling back to the default-provided configuration.

https://eslint.org/docs/user-guide/command-line-interface#--no-eslintrc

Here:

gutenberg/packages/scripts/scripts/lint-js.js

Line 30 in c153cc4

[ '--config', fromConfigRoot( '.eslintrc.js' ) ] :

To:

[ '--no-eslintrc', '--config', fromConfigRoot( '.eslintrc.js' ) ] :

[aduth]

Added the proposed change. Feel free to merge if it makes sense to you.

[gziolo]

I see this line in ESlint docs and it suddenly makes everything clear:

If .eslintrc.* and/or package.json files are also used for configuration (i.e., -no-eslintrc was not specified), the configurations will be merged. Options from this configuration file have precedence over the options from .eslintrc.* and package.json files.

Thanks for catching it. I didn't think about configs located outside of the project at all. Never stop learning :)

[gziolo]

Kudos @aduth for changes applied 🥇

I will merge as soon as Travis will turn green.