chore(deps): FE-454 Update to ESLint 9

[PascalZajac]

What & Why?

Migrate from ESLint 8 to ESLint 9 with the new flat config format.

BREAKING CHANGE: This package now requires ESLint 9.x

Why ESLint 9?

• ESLint 8 reaches end-of-life
• Flat config is simpler and more explicit
• Better TypeScript integration
• Improved performance

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📖 Review Guide

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Files	Why Skip
*.snap files	Auto-generated, format change only
yarn.lock	Dependency lockfile

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Files	What Changed
configs/base.js	Core rules → flat config format
configs/typescript.js	TS rules → flat config format
configs/react.js	React rules → flat config format
index.js	Entry point, conditional config loading
utils/index.js	hasPackage() helper

🟡 Glance At (~300 lines)

Files	What Changed
test/*.spec.js	Updated for new ESLint API
README.md	Usage instructions updated
package.json	"type": "module", peer deps

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Changes Summary

Config Structure

- module.exports = { extends: ['...'], rules: {...} }
+ export default [{ rules: {...} }, ...]

ES Modules

- const foo = require('foo');
- module.exports = { ... };
+ import foo from 'foo';
+ export default [...];

Entry Point

- module.exports = { extends: [...] }
+ export default (async () => {
+   const configs = [...baseConfig];
+   if (await hasPackage('react')) configs.push(...reactConfig);
+   return configs;
+ })();

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Examples

Before (ESLint 8)

// .eslintrc.js
require('@bigcommerce/eslint-config/patch');

module.exports = {
  extends: ['@bigcommerce/eslint-config'],
};

After (ESLint 9)

// eslint.config.js
import eslintConfig from '@bigcommerce/eslint-config';

export default [...(await eslintConfig)];

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Testing

• ✅ All 28 tests passing
• ✅ Lint clean
• ✅ Verified in store-control-panel#3915

[Copilot]

Pull Request Overview

This PR updates the repository to support ESLint 9 with its new flat config format, replacing the legacy ESLint 8 configuration system. The migration includes converting all modules to ES modules ("type": "module"), replacing CommonJS with ES module syntax, and restructuring configuration files to use the flat config format.

Key Changes:

• Updated ESLint peer dependency from ^8.0.0 to ^9.0.0 across all packages
• Converted all configuration files from CommonJS to ES modules
• Restructured ESLint configs to use flat config format instead of the legacy .eslintrc format

Reviewed Changes

Copilot reviewed 33 out of 41 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
package.json	Updated ESLint to 9.x, added module type, modified test script for ES modules
packages/eslint-plugin/package.json	Updated ESLint peer dependency to 9.x and added module type
packages/eslint-config/package.json	Updated ESLint peer dependency to 9.x, added module type, replaced patch dependency with @eslint/js and globals
packages/eslint-plugin/index.js	Converted to ES modules with flat config format for plugin configuration
packages/eslint-plugin/rules/jsx-short-circuit-conditionals.js	Converted to ES modules and removed deprecated config properties
packages/eslint-plugin/tests/jsx-short-circuit-conditionals.spec.js	Converted to ES modules with compatibility handling for default exports
packages/eslint-config/index.js	Converted to async ES module returning flat config array
packages/eslint-config/configs/*.js	Converted all config files to ES modules with flat config format
packages/eslint-config/utils/index.js	Converted to async ES module using dynamic imports
packages/eslint-config/test/*.spec.js	Updated test files to ES modules with proper async handling
packages/eslint-config/prettier.config.js	Converted to ES module export
packages/eslint-config/jest.config.js	Added transform config for ES modules
eslint.config.js	New flat config file replacing .eslintrc.js
README.md	Updated documentation for ESLint 9 migration

Comments suppressed due to low confidence (1)

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[oliverdowie]

Any estimate on when this upgrade will be available? I'm currently converting our Catalyst project to Nextjs 16 and ESLint 9 but getting a little stuck on the BigCommerce configs.
Thanks :)

[PascalZajac]

@oliverdowie thanks for the question! It actually helps to know there are other people out there interested / waiting on this. We'll aim to get it finalised next week.

[icatalina]

hey @danielochoa-epam , can you please rebase instead of merging master into here?