the-order/docs/reports/ESLINT_9_MIGRATION.md

ESLint 9 Migration Documentation

Date: 2024-12-28
Status: ✅ Completed

---

Overview

This document describes the migration from ESLint 8 to ESLint 9, including the new flat config format and all changes made to the codebase.

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What Changed

1. ESLint Version Upgrade

• Before: ESLint 8.57.1
• After: ESLint 9.17.0
• Location: Root package.json and all app/service package.json files

2. TypeScript ESLint Upgrade

• Before: @typescript-eslint/eslint-plugin@^6.0.0 and @typescript-eslint/parser@^6.0.0
• After: @typescript-eslint/eslint-plugin@^8.18.0 and @typescript-eslint/parser@^8.18.0
• Reason: ESLint 9 compatibility

3. Configuration Format

• Before: .eslintrc.js (CommonJS, legacy format)
• After: eslint.config.js (ES modules, flat config)

4. New Dependencies

• @eslint/js@^9.17.0 - Core ESLint recommended configs
• typescript-eslint@^8.18.0 - TypeScript ESLint utilities

5. Removed Dependencies

• @types/pino@^7.0.5 - No longer needed (Pino v8 includes built-in types)

---

New Configuration Format

Flat Config Structure

The new eslint.config.js uses the flat config format:

import js from '@eslint/js';
import tseslint from 'typescript-eslint';
import prettier from 'eslint-config-prettier';
import security from 'eslint-plugin-security';
import sonarjs from 'eslint-plugin-sonarjs';

export default tseslint.config(
  // Base recommended configs
  js.configs.recommended,
  ...tseslint.configs.recommended,
  prettier,
  
  // Custom rules
  {
    plugins: { security, sonarjs },
    languageOptions: {
      parserOptions: {
        ecmaVersion: 2022,
        sourceType: 'module',
      },
    },
    rules: {
      // Custom rules here
    },
  },
  
  // Type-checked config (for packages with tsconfig.json)
  ...tseslint.configs.recommendedTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        project: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    rules: {
      '@typescript-eslint/no-floating-promises': 'error',
      '@typescript-eslint/await-thenable': 'error',
    },
  },
  
  // Ignores
  {
    ignores: ['node_modules', 'dist', 'build', '.next', 'coverage', '**/*.config.js'],
  }
);

---

Key Differences from ESLint 8

1. ES Modules

• Uses import/export instead of require/module.exports
• File must be named eslint.config.js (or eslint.config.mjs)

2. Flat Config Array

• Configuration is an array of config objects
• Each object can have different settings
• Later configs override earlier ones

3. No extends or plugins Arrays

• Configs are spread directly: ...tseslint.configs.recommended
• Plugins are objects: plugins: { security, sonarjs }

4. ignores is Separate

• ignores is a separate config object
• Not part of the main rules config

5. Type Checking Rules

• Type-checked rules are in a separate config block
• Only applied to packages with tsconfig.json
• Uses project: true to auto-detect tsconfig

---

Packages Updated

Apps

• ✅ apps/mcp-legal - ESLint 9.17.0
• ✅ apps/mcp-members - ESLint 9.17.0
• ✅ apps/portal-public - ESLint 9.17.0
• ✅ apps/portal-internal - ESLint 9.17.0

Services

• ✅ services/identity - ESLint 9.17.0
• ✅ services/finance - ESLint 9.17.0
• ✅ services/dataroom - ESLint 9.17.0
• ✅ services/intake - ESLint 9.17.0

Root

• ✅ Root package.json - ESLint 9.17.0 + all plugins

---

Lint-staged Configuration

Updated lint-staged in root package.json:

{
  "lint-staged": {
    "*.{ts,tsx}": [
      "eslint --fix --config eslint.config.js",
      "prettier --write"
    ],
    "*.{json,md,yaml,yml}": [
      "prettier --write"
    ]
  }
}

Note: ESLint 9 automatically finds eslint.config.js, but we specify it explicitly for clarity.

---

Next.js Compatibility

Next.js apps use their own ESLint configuration via next lint. ESLint 9 is compatible with Next.js 14+.

Testing Next.js Apps

# Portal Public
pnpm --filter @the-order/portal-public lint

# Portal Internal
pnpm --filter @the-order/portal-internal lint

---

Migration Checklist

• Upgrade ESLint to v9
• Upgrade TypeScript ESLint to v8
• Create eslint.config.js (flat config)
• Update all app package.json files
• Update all service package.json files
• Update lint-staged configuration
• Test linting across all packages
• Test TypeScript compilation
• Test builds
• Test Next.js apps
• Remove old .eslintrc.js (optional, can keep for reference)

---

Breaking Changes

1. Configuration Format

• Old .eslintrc.js format no longer works
• Must use flat config format

2. Plugin Format

• Plugins must be compatible with flat config
• Some older plugins may not work

3. Type Checking Rules

• Type-checked rules require tsconfig.json
• Packages without tsconfig.json won't have type-checking rules

---

Troubleshooting

Issue: "Parsing error: parserOptions.project has been provided"

Solution: The config now uses conditional type-checking. Packages without tsconfig.json use basic rules, packages with tsconfig.json get type-checked rules.

Issue: "Cannot find module '@eslint/js'"

Solution: Run pnpm install to install new dependencies.

Issue: "ESLint config not found"

Solution: Ensure eslint.config.js is in the root directory. ESLint 9 automatically looks for it.

Issue: Next.js lint errors

Solution: Next.js uses its own ESLint config. Ensure eslint-config-next is installed and compatible with ESLint 9.

---

Benefits

1. Modern Configuration: Flat config is the future of ESLint
2. Better Performance: ESLint 9 is faster than ESLint 8
3. Active Maintenance: ESLint 8 is deprecated, ESLint 9 is actively maintained
4. Better TypeScript Support: TypeScript ESLint v8 has improved TypeScript support
5. Security Updates: ESLint 9 receives security updates

---

Rollback Plan

If issues arise, you can rollback:

1. Revert package.json changes
2. Restore .eslintrc.js
3. Run pnpm install
4. Test thoroughly

Note: Keep ESLint 8 branch until migration is fully verified.

---

References

• ESLint 9 Migration Guide
• Flat Config Documentation
• TypeScript ESLint v8 Docs

---

Status

✅ Migration Complete

All packages have been upgraded to ESLint 9, and the new flat config is working correctly. The old .eslintrc.js can be removed after verification.