Add ESLint 9 support and migrate to flat config

[sjinks]

This PR:

• migrates the code base to ESLint 9 (ESLint 8's EoL was on October 5; it is no longer supported);
• migrates our configuration to the flat format. The traditional configuration files are deprecated; ESLint expects flat configs.

Unfortunately, these changes are breaking; consumers' configuration files will have to be migrated manually.

The plan is to release 1.0.0-rc@next and test it on some of our code bases; once we ensure everything is good, publish 1.0.

[Copilot]

Copilot reviewed 15 out of 30 changed files in this pull request and generated 2 comments.

Files not reviewed (15)

• .eslintignore: Language not supported
• .nvmrc: Language not supported
• tests/snapshots/check-fixtures.js.snap: Language not supported
• .eslintrc.js: Evaluated as low risk
• configs/weak-testing.js: Evaluated as low risk
• tests/check-fixtures.js: Evaluated as low risk
• tests/rules/nestjs-route-prefix.ts: Evaluated as low risk
• tests/rules/no-async-foreach.ts: Evaluated as low risk
• configs/cli.js: Evaluated as low risk
• configs/formatting.js: Evaluated as low risk
• configs/index.js: Evaluated as low risk
• configs/weak-typescript.js: Evaluated as low risk
• configs/jsdoc.js: Evaluated as low risk
• configs/weak-javascript.js: Evaluated as low risk
• configs/react.js: Evaluated as low risk

Comments suppressed due to low confidence (3)

configs/javascript.js:23

• The 'ecmaVersion' property is set twice in the 'languageOptions' object. The second instance on line 32 should be removed to avoid redundancy and potential conflicts.

ecmaVersion: 2022,

configs/testing.js:28

• The rule 'jest/unbound-method' is not a valid rule in the Jest plugin. The correct rule name is '@typescript-eslint/unbound-method'.

'jest/unbound-method': 'error',

configs/recommended.js:22

• The debugLog function call has been removed. This function was used to log the configurations being used, which could be useful for debugging purposes. Consider re-adding this logging functionality.

module.exports = configs;

[GaryJones]

@copilot Please review again, particularly the feedback to your comments.

[Copilot]

@GaryJones I've opened a new pull request, #224, to work on those changes. Once the pull request is ready, I'll request review from you.

[Copilot]

Pull request overview

Copilot reviewed 28 out of 30 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

rules/no-unguarded-get-range-at.js:11

• The meta.messages object should be added with a message definition, and the context.report call on line 19 should use messageId instead of message. This ensures consistency with ESLint 9's rule API and matches the pattern used in nestjs-route-prefix.js.

Example fix:

meta: {
  type: 'problem',
  docs: {
    description: 'Avoid unguarded getRangeAt',
  },
  messages: {
    unguarded: 'Avoid unguarded getRangeAt',
  },
  schema: [],
},

Then update the report call to use messageId: 'unguarded'.

	meta: {
		type: 'problem',
		schema: [],
	},

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[Copilot]

Pull request overview

Copilot reviewed 28 out of 30 changed files in this pull request and generated 8 comments.

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[Copilot]

Pull request overview

Copilot reviewed 28 out of 30 changed files in this pull request and generated 1 comment.

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[sjinks]

@GaryJones I have created a PR in the VIP CLI repo to test this PR: Automattic/vip-cli#2650

It even seems to work 😆