Migrate to eslint.config.js

[JLHwung]

Q	A
License	MIT

In this PR we migrate to the flat ESLint config eslint.config.js. Most transitions are done according to https://eslint.org/blog/2022/08/new-config-system-part-2/

• The .eslintignore is now replaced by top level .ignores pattern as flat ESLint config does not support .eslintignore now. I removed unused patterns such as packages/*/dist
• Migrating extends config with a files filter is a bit tricky. Currently I have to manually assign our config to the eslint-plugin-jest's recommended config, otherwise the eslint-plugin-jest:recommended will be enabled on source files if it is cascaded with top level config items.
  /cc @nzakas if I miss the proper approach

[JLHwung]

The Flow globals are also removed as we are not using Flow anymore.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54836/

[JLHwung]

excludedFiles is replaced by ignores because the former is not supported in flat config.
If I understand correctly, excludedFiles will exclude the matched file from this override config item but ESLint will continue matching it with other config items, while ignores will stop ESLint matching further config items to this file. @nzakas Is that correct?

Here it can be replaced because there are no more rules matching packages/babel-register/**/*.{js,ts}.

[nzakas]

Almost. When both files and ignores are present in the same object, ignores acts like excludedFiles in that it simply filters out results that match files.

When ignores is present with any other key (i.e. rules) then it also acts like excludedFiles in that it matches every file except the ones matching patterns in ignores.

When ignores is present without any other keys in the same object then those files will always be ignored and never match any config objects.

[JLHwung]

Thank you! That is very helpful. I can't find much docs on ignores on the website, should I open a PR to update https://eslint.org/docs/latest/use/configure/ignore?

[nzakas]

It's on this page:
https://eslint.org/docs/latest/use/configure/configuration-files-new#specifying-files-and-ignores

Once we officially make flat config the default, we'll update the URL so this one is the default and the eslintrc one is marked as deprecated.

[JLHwung]

Oh thank you! Now I might understand why searching "ignores" on the website does not return the options docs: most of them are rules options.

Are they supposed to show up once the flat config is made default?

[nzakas]

Yes. When we release v9.0.0, we will rearrange the documentation so that we favor everything related to flat config and the eslintrc docs will be made less prominent. So when you look up "ignores" it will show you docs related to flat config.

We're just in this weird intermediate state right now.

[JLHwung]

Previous config item:

{
      files: [
        ...testFiles,
        "packages/babel-helper-transform-fixture-test-runner/src/helpers.{ts,js}",
        "test/**/*.js",
      ],
      env: {
        jest: true,
      },
      extends: "plugin:jest/recommended",
      rules: {
        "jest/expect-expect": "off",
        "jest/no-identical-title": "off",
        "jest/no-standalone-expect": "off",
        "jest/no-test-callback": "off",
        "jest/valid-describe": "off",
        "import/extensions": ["error", "always"],
        "import/no-extraneous-dependencies": "off",
        "no-restricted-imports": ["error", { patterns: ["**/src/**"] }],
      },
    },

[nzakas]

This looks good! It can still be a bit tricky to use eslintrc-style configs (like the one in eslint-plugin-jest) and modify it to work differently, but fortunately, you have the full power of JavaScript available to help. Eventually, once plugins start exporting flat configs instead, this will get much easier.

[JLHwung]

Heads up for contributors:
If you are using VSCode, please add this line to your project config after this PR gets merged, otherwise the linter extension will not work.

[liuxingbaoyu]

Amazing!

[nicolo-ribaudo]

Thank you!