ESLint v9 Support

[bradzacher]

ESLint has released v9.

• https://eslint.org/blog/2024/04/eslint-v9.0.0-released/

Note that it contains breaking changes and as such we do not currently support it. It may work or it may not.

We are not accepting issues against v9 until we have official support - so please do not file issues with crashes or bugs for now and please do not comment on this issue telling us about crashes.

If you believe you have a bug when using ESLint v8 - please file a new issue.
If you need some help setting up / configuring your project - please visit our discord.

If you'd like to help contribute towards support feel free to comment!

---

Ongoing Work

• ESLint Flat Config Support #7694 -- feat: add support for flat configs #7935
• feat(utils): add types for both flat and legacy eslint classes #8861
• feat(eslint-plugin): replace no-new-symbol with no-new-native-nonconstructor #8895
• feat: support ESLint v9 #9002

Breaking Changes

• correct camelcase rule schema for allow option
• move AST traversal into SourceCode
• disallow multiple configuration comments for same rule
• default for enforceForClassMembers in no-useless-computed-key
• no-unused-vars default caughtErrors to ‘all’: Enhancement: [no-unused-vars] Update for ESLint 9 defaults #8967
• no-unused-vars varsIgnorePattern behavior with catch arguments: Enhancement: [no-unused-vars] Update for ESLint 9 defaults #8967
• no-restricted-imports allow multiple config entries for same path
• no-sequences rule schema correction
• no-invalid-regexp make allowConstructorFlags case-sensitive
• no-inner-declaration new default behaviour and option
• add two more cases to no-implicit-coercion
• Remove valid-jsdoc and require-jsdoc
• handle --output-file for empty output when saving to disk
• upgrade eslint-scope@8.0.0
• configuration comments with just severity should retain options
• Remove CodePath#currentSegments
• remove no-inner-declarations from eslint:recommended
• Rename FlatESLint to ESLint
• skip running warnings in --quiet mode
• Set default schema: [], drop support for function-style rules
• Switch Linter to flat config by default: Enhancement(utils): Update types ESLint v9 Linter class (flat config) #8968
• Parsing ‘exported’ comment using parseListConfig
• Behavior of CLI when no arguments are passed
• Update shouldUseFlatConfig and CLI so flat config is default
• deprecate no-new-symbol, recommend no-new-native-nonconstructor
• Update eslint:recommended configuration
• drop support for string configurations in flat config array
• Remove SourceCode#getComments()
• Remove deprecated context methods
• Swap FlatESLint-ESLint, FlatRuleTester-RuleTester in API Enhancement(utils): Swap FlatESLint-ESLint, FlatRuleTester-RuleTester in API #8970
• remove formatters except html, json(-with-metadata), and stylish
• Require Node.js ^18.18.0 || ^20.9.0 || >=21.1.0

Changes we're treating as non-breaking

Per discussion in #8211, these augmentations fix previous gaps in the RuleTester. We'll treat them as non-breaking bug fixes so they can land in the v7 major version ahead of v8.

• [RuleTester] Rule Tester checks for missing placeholder data in the message: Enhancement(rule-tester): check for missing placeholder data in the message #9020
• [RuleTester] Stricter rule test validations: Enhancement(rule-tester): Stricter rule test validations #9016
• [RuleTester] FlatRuleTester -> RuleTester: Enhancement(rule-tester): FlatRuleTester -> RuleTester #9017
• [RuleTester] detect duplicate test cases: Enhancement(rule-tester): detect duplicate test cases #9018
• [RuleTester] check for parsing errors in suggestion fixes: Enhancement(rule-tester): check for parsing errors in suggestion fixes #9019
• [RuleTester] assert suggestion messages are unique in rule testers: Enhancement(rule-tester): assert suggestion messages are unique #8969

[bradzacher]

If you glance at the implementation you'll see that the helper is the identity function -- if you don't use the extends feature it adds.

That means that writing export default tseslint.config(...) is semantically the same as export default [...];. The difference is that the latter is untyped, but the former has strict types defined. This means your config can be checked -- mistakes will be flagged and you'll get auto-complete with each property fully documented via jsdoc.

We introduced this util because it provides a simple way for people to typecheck their configs. Otherwise people need to annotate the array with a jsdoc type which is very cumbersome and verbose:

/** @type {import('@typescript-eslint/utils').TSESLint.FlatConfigArray} */
export default [...];

// vs

export default tseslint.config(...);

We can definitely clarify this in the docs if it wasn't clear that the util isn't required.

[bradzacher]

v9 has officially released
https://eslint.org/blog/2024/04/eslint-v9.0.0-released/

Note that it contains breaking changes and as such we do not currently support it. It may work or it may not.

This issue tracks our official support.

Note that we are not accepting issues against v9 until we have official support - so please do not file issues with crashes or bugs for now and please do not comment on this issue telling us about crashes.

If you'd like to help contribute towards support feel free to comment!
See the comment below for more info

[jamesgpearce]

Don't want to add to the noise, but did want to say thank you for being so clear and organized about what is supported and what the status is. Saved me a morning of wrestling with the immediate v9.0 fallout; I know to wait! 🙏