@eslint/eslintrc 3.3.6: Translate legacy ESLintRC configuration to Flat Config array using FlatCompat

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: Translate legacy ESLintRC configuration to Flat Config array using FlatCompat
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/%40eslint/eslintrc@3.3.6
Demonstrate these symbols/APIs:
  - FlatCompat
Constraints:
  - executionContext: node
Required runtime conditions:
  - ecosystem: npm
  - language: javascript
  - moduleSystem: cjs
  - runtime: node@22.23.2

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:d8083cb66ce57ffa64fdd646c578a1ed164da35e6ce3196a96ad7b4bf687f988","constraints":{"executionContext":"node"},"contract":["FlatCompat initializes with baseDirectory configuration","env method translates legacy environment flags into flat config objects with languageOptions globals","config method translates legacy eslintrc rules and globals into flat config objects","extends method translates built-in or custom configuration presets into flat config arrays"],"goal":"Translate legacy ESLintRC configuration to Flat Config array using FlatCompat","kind":"HOW","packages":["pkg:npm/%40eslint/eslintrc@3.3.6"],"schemaVersion":1,"symbols":["FlatCompat"]},"contractCommand":["node","test/contract.cjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","executionContext":"node","language":"javascript","libc":"glibc","libcVersion":"2.39","moduleSystem":"cjs","os":"linux","osVersionBucket":"24","packageManager":"npm","runtime":"node","runtimeVersion":"22.23.2","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/%40eslint/eslintrc@3.3.6"],"schemaVersion":1,"subject":"pkg:npm/%40eslint/eslintrc@3.3.6","symbols":["FlatCompat"],"verifierAdapter":"node-typescript@1"}

'use strict';

const { FlatCompat } = require('@eslint/eslintrc');

function createCompat(options) {
  return new FlatCompat(options);
}

function translateLegacyConfig(legacyConfig, options) {
  const compat = new FlatCompat(options || { baseDirectory: process.cwd() });
  return compat.config(legacyConfig);
}

module.exports = {
  FlatCompat,
  createCompat,
  translateLegacyConfig
};

{
  "name": "sample-eslint-eslintrc",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample-eslint-eslintrc",
      "version": "1.0.0",
      "license": "MIT-0",
      "dependencies": {
        "@eslint/eslintrc": "3.3.6"
      }
    },
    "node_modules/@eslint/eslintrc": {
      "version": "3.3.6",
      "resolved": "https://registry.npmjs.org/@eslint/eslintrc/-/eslintrc-3.3.6.tgz",
      "integrity": "sha512-l2Ul9PrHsPCKcEY/ac7VgFj9D80C7S68sOKc618SyHDPK36s1XcFebXY0iTzUVn4Yq+YbwvSnDmCz9yxjX+QrA==",
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        "ignore": "^5.2.0",
        "import-fresh": "^3.2.1",
        "js-yaml": "^4.3.0",
        "minimatch": "^3.1.5",
        "strip-json-comments": "^3.1.1"
      },
      "engines": {
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      "funding": {
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}

{
  "name": "sample-eslint-eslintrc",
  "version": "1.0.0",
  "description": "Translate legacy ESLintRC configuration to Flat Config array using FlatCompat",
  "main": "index.js",
  "scripts": {
    "test": "node test/contract.cjs"
  },
  "license": "MIT-0",
  "dependencies": {
    "@eslint/eslintrc": "3.3.6"
  }
}

{
  "schemaVersion": 1,
  "goal": "Translate legacy ESLintRC configuration to Flat Config array using FlatCompat",
  "kind": "HOW",
  "packages": [
    "pkg:npm/%40eslint/eslintrc@3.3.6"
  ],
  "symbols": [
    "FlatCompat"
  ],
  "constraints": {
    "executionContext": "node"
  },
  "runtimeConditions": {
    "ecosystem": "npm",
    "language": "javascript",
    "moduleSystem": "cjs",
    "runtime": "node@22.23.2"
  }
}

'use strict';

const assert = require('node:assert/strict');
const { FlatCompat, createCompat, translateLegacyConfig } = require('../index.js');

// 1. FlatCompat initializes with baseDirectory configuration
const compat = new FlatCompat({
  baseDirectory: process.cwd(),
  recommendedConfig: {
    rules: {
      'no-debugger': 'error'
    }
  },
  allConfig: {
    rules: {
      'no-debugger': 'error',
      'no-alert': 'error'
    }
  }
});
assert.ok(compat instanceof FlatCompat, 'compat must be an instance of FlatCompat');

const helperCompat = createCompat({ baseDirectory: process.cwd() });
assert.ok(helperCompat instanceof FlatCompat, 'createCompat must return FlatCompat instance');

// 2. env method translates legacy environment flags into flat config objects with languageOptions globals
const envConfigs = compat.env({ browser: true, es2021: true });
assert.ok(Array.isArray(envConfigs), 'compat.env must return an array');
assert.ok(envConfigs.length > 0, 'envConfigs array must not be empty');

const browserConfig = envConfigs.find(c => c.languageOptions && c.languageOptions.globals && 'window' in c.languageOptions.globals);
assert.ok(browserConfig, 'browser environment must populate languageOptions.globals');
assert.strictEqual(browserConfig.languageOptions.globals.window, false, 'window global must be defined as readonly (false)');

const es2021Config = envConfigs.find(c => c.languageOptions && c.languageOptions.ecmaVersion === 12);
assert.ok(es2021Config, 'es2021 environment must set languageOptions.ecmaVersion to 12');

// 3. config method translates legacy eslintrc rules and globals into flat config objects
const legacyConfig = {
  env: { node: true },
  rules: {
    'no-unused-vars': 'warn',
    semi: ['error', 'always']
  },
  globals: {
    customGlobalVar: 'readonly'
  }
};

const translatedConfigs = compat.config(legacyConfig);
assert.ok(Array.isArray(translatedConfigs), 'compat.config must return an array');

const rulesConfig = translatedConfigs.find(c => c.rules && c.rules['no-unused-vars']);
assert.ok(rulesConfig, 'translated configs must contain rules definition');
assert.strictEqual(rulesConfig.rules['no-unused-vars'], 'warn', 'no-unused-vars rule must be set to warn');
assert.deepStrictEqual(rulesConfig.rules.semi, ['error', 'always'], 'semi rule configuration must match');
assert.strictEqual(rulesConfig.languageOptions.globals.customGlobalVar, 'readonly', 'custom global must be preserved in languageOptions.globals');

const helperTranslated = translateLegacyConfig(legacyConfig, { baseDirectory: process.cwd() });
assert.ok(Array.isArray(helperTranslated), 'translateLegacyConfig helper must return translated config array');

// 4. extends method translates built-in or custom configuration presets into flat config arrays
const extendedRecommended = compat.extends('eslint:recommended');
assert.ok(Array.isArray(extendedRecommended), 'compat.extends must return an array');
assert.ok(extendedRecommended.length > 0, 'extendedRecommended must contain config objects');
assert.strictEqual(extendedRecommended[0].rules['no-debugger'], 'error', 'extended recommendedConfig rules must be translated');

const extendedAll = compat.extends('eslint:all');
assert.ok(Array.isArray(extendedAll), 'compat.extends all must return an array');
assert.strictEqual(extendedAll[0].rules['no-alert'], 'error', 'extended allConfig rules must be translated');

console.log('Contract verification succeeded.');