refactor(project): update to eslint v9

[joshblack]

Closes https://github.com/github/primer/issues/4155

Update the project from eslint v7 to eslint v9, including a transformation to the new flat config.

Changelog

New

Changed

• Update eslint configuration to use the new flat config
• Update plugins in order to work with new flat config
• Update files with changes rules or remove rules that no longer apply

Removed

Rollout strategy

• None; if selected, include a brief description as to why

This is a change to our internal eslint config

[changeset-bot]

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Latest commit: ea4d28f

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[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[Copilot]

Pull Request Overview

This PR updates the project to ESLint v9 by migrating to the new flat config while ensuring code consistency with updated linting rules. Key changes include:

• Replacing legacy ESLint configuration (.eslintrc.js) with a comprehensive flat config (eslint.config.mjs).
• Updating event handlers in various ActionList and ActionMenu components from onClick to onSelect per new guidelines.
• Transforming React import statements and ESLint ignore comments (e.g. switching from @ts-ignore to @ts-expect-error) to adhere to updated best practices.

Reviewed Changes

Copilot reviewed 122 out of 124 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/react/src/AnchoredOverlay/AnchoredOverlay.tsx	Updated React import to "import type React" for improved type usage.
packages/react/src/ActionMenu/ActionMenu.examples.stories.tsx	Replaced onClick with onSelect for action items.
packages/react/src/ActionList/Item.tsx	Revised the function type suppression comment to reflect the updated rule and event handling changes.
packages/react/src/ActionBar/ActionBar.tsx	Changed onClick to onSelect for overflow menu actions in the ActionBar.
packages/react/.storybook/src/accessibility-tool.tsx	Removed optional chaining for window.localStorage for consistency with browser environments.
packages/postcss-preset-primer/src/index.js	Modified ESLint suppression comments to use @ts-expect-error with explanatory messages.
eslint.config.mjs	Introduced the new flat ESLint configuration aligning with ESLint v9 requirements.
.eslintrc.js	Removed the legacy ESLint configuration in favor of the new setup.
examples/theming/src/app/registry.tsx & examples/nextjs/src/app/registry.tsx	Updated React imports to separate type and value imports.

Files not reviewed (2)

• .eslintignore: Language not supported
• package.json: Language not supported

Comments suppressed due to low confidence (4)

packages/postcss-preset-primer/src/index.js:9

• The switch from '@ts-ignore' to '@ts-expect-error' is good for clarity; verify that similar updates are applied consistently across other parts of the code.

import customPropertiesFallback from 'postcss-custom-properties-fallback'

.eslintrc.js:1

• The removal of the legacy .eslintrc.js is expected with the migration to ESLint v9; ensure that any custom overrides are fully ported into the new flat config.

module.exports = { ... }

packages/react/.storybook/src/accessibility-tool.tsx:14

• Since window.localStorage is guaranteed in browser environments, this explicit removal of optional chaining is appropriate; ensure the environment consistently supports localStorage.

const currentValue = showSurroundingElements ?? window.localStorage.getItem('showSurroundingElements') === 'true'

eslint.config.mjs:29

• Review that the new flat config accurately reflects all necessary overrides and custom rules from the previous configuration to prevent unexpected linting behavior.

const config = defineConfig([

[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	99.68 KB (-0.09% 🔽)
packages/react/dist/browser.umd.js	100.07 KB (+0.15% 🔺)