ESLint v9 Migration Discussion

Hi!

I'd like to get your thoughts on migrating to ESLint v9.

Currently, react.dev uses ESLint 7.x, which reached end-of-life in 2024. It's not causing immediate problems, but it's becoming a limitation for adopting new linting tools going forward.

For example, ko.react.dev introduced eslint-markdown to help maintain translation quality in markdown docs. However, these newer plugins only support ESLint 9's flat config. So right now, ko.react.dev is in an awkward situation where it uses ESLint 7 as the base, but has to run npx eslint@9 separately just for markdown linting:

"textlint-lint": "yarn textlint ... && npx --yes eslint@9 -c eslint.config.mjs"

When translation projects try to add more lint rules to improve documentation quality, ESLint 7 becomes a bottleneck. Official ESLint plugins like @eslint/markdown and @eslint/json are all flat config only.

This came up recently in a ko.react.dev discussion (reactjs/ko.react.dev#1406), where a maintainer mentioned:

"If you want CI/CD integration and improvements in this area, please submit an ESLint v9 migration PR to the react.dev repository first, so we can pull it in later."

They also noted that maintaining ESLint 7, ESLint 9, and Textlint together is becoming difficult, and an upstream migration would help simplify things for translation projects.

Also, some dependencies like babel-eslint and eslint-config-react-app are already deprecated and not even being used, so they'll need to be cleaned up at some point anyway.

Of course, this change would also affect other translation repositories, so I understand it needs careful consideration.

What do you think about upgrading to ESLint v9? If you're open to the idea, we might be able to help with the migration work.

The issue mentions react.dev's ESLint 7.x setup, the ko.react.dev eslint.config.mjs entry point, and the textlint-lint script, but no react.dev files or tests. Start by locating the repository's ESLint dependencies and lint scripts, then define the migration scope and validation needed across translation repositories before implementation.