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# Configuration
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Comprehensive configuration system supporting all ESLint CLI options with automatic detection of ESLint configuration type (legacy vs flat config).
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## Capabilities
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### Configuration Loading
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Configuration loading system using cosmiconfig to find and parse jest-runner-eslint configuration.
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```javascript { .api }
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/**
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 * Loads ESLint options from configuration files
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 * Supports package.json, .jest-runner-eslintrc, and jest-runner-eslint.config.js
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 * @param configType - ESLint configuration type ('flat' or 'legacy')
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 * @param config - Jest configuration object containing rootDir
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 * @returns Normalized ESLint options object
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 */
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function getESLintOptions(
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  configType: 'flat' | 'legacy',
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  config: { rootDir: string }
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): { cliOptions: ESLintCliOptions };
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```
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**Usage Examples:**
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```javascript
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// package.json configuration
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{
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  "jest-runner-eslint": {
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    "cliOptions": {
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      "fix": true,
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      "cache": true,
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      "format": "codeframe"
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    }
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  }
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}
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// jest-runner-eslint.config.js
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module.exports = {
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  cliOptions: {
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    fix: true,
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    cache: true,
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    cacheLocation: './eslint-cache',
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    maxWarnings: 0,
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    format: 'stylish'
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  }
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};
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// .jest-runner-eslintrc (JSON)
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{
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  "cliOptions": {
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    "quiet": true,
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    "fix": false,
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    "ext": [".js", ".jsx", ".ts", ".tsx"]
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  }
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}
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```
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### Configuration Normalization
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System for normalizing configuration options based on ESLint configuration type.
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```javascript { .api }
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/**
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 * Normalizes configuration object with proper CLI options
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 * @param configType - ESLint configuration type
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 * @param config - Raw configuration object
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 * @returns Normalized configuration with cliOptions
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 */
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function normalizeConfig(
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  configType: 'flat' | 'legacy',
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  config: Partial<JestRunnerESLintConfig>
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): JestRunnerESLintConfig;
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/**
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 * Normalizes CLI options based on configuration type
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 * @param configType - ESLint configuration type
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 * @param rawConfig - Raw CLI options object
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 * @returns Normalized CLI options object
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 */
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function normalizeCliOptions(
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  configType: 'flat' | 'legacy',
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  rawConfig: Partial<ESLintCliOptions>
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): ESLintCliOptions;
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```
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### Configuration Schemas
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Configuration schemas for different ESLint configuration types.
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```javascript { .api }
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/** Base configuration schema shared by both config types */
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const BASE_CONFIG: ConfigSchema;
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/** Legacy ESLint configuration schema (ESLint <8.57.0) */
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const LEGACY_CONFIG: ConfigSchema;
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/** Flat ESLint configuration schema (ESLint >=8.57.0) */
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const FLAT_CONFIG: ConfigSchema;
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interface ConfigSchema {
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  [key: string]: {
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    name?: string;
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    default: any;
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    transform?: (value: any) => any;
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  };
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}
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```
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### ESLint CLI Options
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Complete interface for ESLint CLI options supported by jest-runner-eslint.
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```javascript { .api }
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interface ESLintCliOptions {
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  // Basic options (supported in both legacy and flat config)
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  /** Enable ESLint caching */
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  cache?: boolean;
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  /** Location of cache file */
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  cacheLocation?: string;
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  /** Path to ESLint configuration file */
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  config?: string;
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  /** Automatically fix problems */
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  fix?: boolean;
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  /** Fix problems without saving files */
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  fixDryRun?: boolean;
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  /** Output format for ESLint results */
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  format?: string;
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  /** Maximum number of warnings allowed */
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  maxWarnings?: number;
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  /** Disable warning output, only show errors */
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  quiet?: boolean;
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  /** Disable ignore files and patterns */
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  noIgnore?: boolean;
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  /** Disable inline configuration comments */
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  noInlineConfig?: boolean;
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  // Legacy config options (ESLint <8.57.0)
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  /** File extensions to lint */
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  ext?: string | string[];
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  /** Environment presets to enable */
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  env?: string | string[];
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  /** Global variables to define */
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  global?: string | string[];
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  /** Path to ignore file */
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  ignorePath?: string;
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  /** Ignore patterns to apply */
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  ignorePattern?: string[];
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  /** Disable .eslintrc files */
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  noEslintrc?: boolean;
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  /** Parser to use for code analysis */
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  parser?: string;
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  /** Parser options object */
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  parserOptions?: Record<string, any>;
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  /** ESLint plugins to load */
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  plugin?: string | string[];
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  /** Report unused disable directives */
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  reportUnusedDisableDirectives?: boolean;
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  /** Base path for resolving plugins */
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  resolvePluginsRelativeTo?: string;
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  /** Rules configuration object */
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  rules?: Record<string, any>;
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  /** Additional rules directories */
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  rulesdir?: string | string[];
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}
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interface JestRunnerESLintConfig {
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  /** CLI options for ESLint */
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  cliOptions?: ESLintCliOptions;
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}
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```
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**Usage Examples:**
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```javascript
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// Comprehensive configuration example
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module.exports = {
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  cliOptions: {
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    // Basic options
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    cache: true,
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    cacheLocation: './node_modules/.cache/eslint',
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    format: 'codeframe',
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    fix: false,
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    maxWarnings: 10,
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    quiet: false,
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    // Legacy config options (if using ESLint <8.57.0)
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    ext: ['.js', '.jsx', '.ts', '.tsx'],
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    env: ['browser', 'node'],
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    global: ['window', 'document'],
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    plugin: ['react', 'import'],
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    rules: {
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      'no-console': 'warn',
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      'prefer-const': 'error'
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    },
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    // File patterns
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    ignorePattern: ['dist/*', 'build/*'],
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    noIgnore: false,
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    noInlineConfig: false
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  }
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};
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```
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### Transformation Functions
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Utility functions for transforming configuration values.
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```javascript { .api }
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/** Identity function - returns value unchanged */
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const identity: <T>(value: T) => T;
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/** Negation function - returns boolean opposite */
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const negate: (value: boolean) => boolean;
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/** Array conversion function - converts string to single-item array */
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const asArray: (value: string | string[]) => string[];
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/** Integer conversion function - converts string to number */
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const asInt: (value: string | number) => number;
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```
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**Usage Examples:**
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```javascript
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// Configuration with type transformations
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const config = {
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  cliOptions: {
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    ext: '.js', // Transformed to ['.js'] by asArray
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    maxWarnings: '5', // Transformed to 5 by asInt
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    noEslintrc: true, // Transformed to useEslintrc: false by negate
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  }
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};
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// Dot-prop path transformation for nested properties
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const legacyConfig = {
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  env: ['browser'], // Becomes overrideConfig.env
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  global: ['window'], // Becomes overrideConfig.globals
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  rules: { 'no-console': 'warn' } // Becomes overrideConfig.rules
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};
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```
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### Configuration Discovery
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Configuration files are discovered using cosmiconfig in the following order:
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1. `jest-runner-eslint` property in `package.json`
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2. `.jest-runner-eslintrc` file (JSON)
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3. `.jest-runner-eslintrc.json` file
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4. `.jest-runner-eslintrc.js` file
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5. `jest-runner-eslint.config.js` file
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The search starts from the Jest `rootDir` and traverses up the directory tree until a configuration is found or the file system root is reached.