Extending ESLint is not supporting overrides

Extending ESLint got introduced originally in https://github.com/facebook/create-react-app/pull/7036 by @mrmckeb. It’s stated that this is limited currently to users configuring their ESLint in package.json:

• this ain’t quite true - because used eslintCli.getConfigForFile is not limited to package.json configs
• there is no such mention in the docs - not a biggie but brought a little bit of confusion for me.

What is actually stated in the docs and what doesn’t actually hold true is that overrides can be used to support TS rules etc (there is example of that).

This is not quite true, because whatever config gets loaded & resolved through that eslintCli.getConfigForFile call (with paths.appIndexJs as argument) gets set as “global” config for the whole eslint-loader.

So depending on what paths.appIndexJs is we either can get:

• all TS-related rules being applied to ALL files (including JS) - when paths.appIndexJs is TS
• no TS rules are applied at all - when paths.appIndexJs is JS

For this to work correctly it would be the easiest to allow ESLint to load configs on its own - without passing explicit single config to the eslint-loader.

[mrmckeb]

@Andarist, sorry I think we were confused about what you were reporting.

I’ve managed to reproduce now and understand. Sorry for the inconvenience.

So, in short:

• The config itself is fine.
• Instructions need to be updated.
• Return undefined to baseConfig when process.env.EXTEND_ESLINT === 'true'.
• Change useEslintrc: false to useEslintrc: process.env.EXTEND_ESLINT === 'true'.

How do these changes sound to everyone? I’ve tested them by modifying the Webpack config (in node_modules directly) - if others can do the same and report back, I can make the change this weekend.

[ianschmitz]

Hi @Andarist. Sorry this fell through the cracks. Let me try to address your comments:

Extending ESLint got introduced originally in #7036 by @mrmckeb. It’s stated that this is limited currently to users configuring their ESLint in package.json:

Agreed. This should be clarified in the docs.

What is actually stated in the docs and what doesn’t actually hold true is that overrides can be used to support TS rules etc (there is example of that).

Yeah this should be explicitly stated in the language above that example.

So depending on what paths.appIndexJs is we either can get:

• all TS-related rules being applied to ALL files (including JS) - when paths.appIndexJs is TS
• no TS rules are applied at all - when paths.appIndexJs is JS

Can you confirm this? I seem to remember when testing this that getConfigForFile() resolves the entire configuration (both TS, and JS) irrespective of the file extension. That would be the intent anyways so that we support mixed JS/TS code bases.

Thanks for bringing this up. We’d gladly accept PRs to improve the documentation for points you raised.