eslint

name

eslint

description

Professional-grade ESLint development for JavaScript and TypeScript projects. Use when working with ESLint for (1) configuring ESLint in projects, (2) understanding or fixing ESLint errors and warnings, (3) creating or modifying ESLint rules, (4) integrating ESLint into build systems or editors, (5) migrating ESLint configurations, (6) setting up custom parsers or plugins, (7) troubleshooting ESLint issues, (8) implementing code quality standards, or (9) any task involving ESLint setup, configuration, or usage. Covers all ESLint versions with focus on v9+ flat config format.

ESLint Development

Professional ESLint integration for JavaScript and TypeScript codebases. This skill provides comprehensive guidance for configuring, using, and extending ESLint to enforce code quality standards.

Latest ESLint version: 9.32.2 (December 2025)

Quick Start

Install ESLint

npm install --save-dev eslint
npx eslint --init

Basic Configuration (Flat Config - ESLint 9+)

// eslint.config.js
export default [
  {
    files: ["**/*.js"],
    rules: {
      "no-unused-vars": "error",
      "no-console": "warn"
    }
  }
];

Run ESLint

npx eslint .                 # Lint all files
npx eslint --fix .           # Auto-fix issues
npx eslint src/**/*.js       # Lint specific files

Core Workflows

1. Project Setup

New projects:

1. Install ESLint: npm install --save-dev eslint
2. Initialize config: npx eslint --init (interactive)
3. Review generated eslint.config.js
4. Run first lint: npx eslint .

Existing projects:

1. Review current configuration in eslint.config.js or .eslintrc.*
2. Understand applied rules and plugins
3. Migrate to flat config if using legacy format (see references/use/configure/migration-guide.md)

2. Configuration

ESLint uses flat config format (eslint.config.js) in v9+. Legacy formats (.eslintrc.*) are deprecated.

Key configuration areas:

• Files: Specify which files to lint
• Rules: Enable/disable specific rules and set severity
• Language options: Parser, source type, ECMAScript version
• Plugins: Extend with custom rules
• Ignore patterns: Exclude files from linting

See references/use/configure/configuration-files.md for complete guide.

3. Rule Management

Rule severity levels:

• "off" or 0 - Disable rule
• "warn" or 1 - Warning (doesn't affect exit code)
• "error" or 2 - Error (exit code 1)

Configure rules:

export default [
  {
    rules: {
      "no-unused-vars": "error",
      "quotes": ["error", "double"],
      "semi": ["error", "always"],
      "no-console": "off"
    }
  }
];

Find specific rule documentation:

• All 300+ core rules documented in references/rules/
• Rule names are kebab-case (e.g., no-unused-vars.md, prefer-const.md)

4. Fixing Issues

Auto-fix:

npx eslint --fix .           # Fix all auto-fixable issues
npx eslint --fix src/        # Fix specific directory
npx eslint --fix-dry-run .   # Preview fixes without applying

Manual fixes:

1. Read error message and rule name
2. Look up rule in references/rules/[rule-name].md
3. Understand the issue and correct code examples
4. Apply fix or disable rule if not applicable

Disable rules:

// Disable for one line
// eslint-disable-next-line no-console
console.log("debug");

// Disable for entire file
/* eslint-disable no-console */

// Disable specific rule in block
/* eslint-disable no-unused-vars */
const temp = getData();
/* eslint-enable no-unused-vars */

5. Integration

Editor integration:

• Install ESLint extension for your editor (VS Code, Sublime, etc.)
• Enables real-time linting and auto-fix on save

Build system integration:

• Add to npm scripts: "lint": "eslint ."
• CI/CD: Run npm run lint in build pipeline
• Pre-commit hooks: Use with husky or lint-staged

See references/use/integrations.md for editor and tool integrations.

Advanced Usage

Custom Rules

Create project-specific rules to enforce custom patterns:

// eslint.config.js
import myCustomRule from './rules/my-custom-rule.js';

export default [
  {
    plugins: {
      local: { rules: { 'my-custom-rule': myCustomRule } }
    },
    rules: {
      'local/my-custom-rule': 'error'
    }
  }
];

See references/extend/custom-rules.md for complete guide.

Plugins

Extend ESLint with community plugins for frameworks and libraries:

import react from 'eslint-plugin-react';
import typescript from '@typescript-eslint/eslint-plugin';

export default [
  {
    plugins: { react, typescript },
    rules: {
      'react/jsx-uses-react': 'error',
      '@typescript-eslint/no-unused-vars': 'error'
    }
  }
];

See references/extend/plugins.md for plugin development and usage.

TypeScript

For TypeScript projects, use @typescript-eslint:

npm install --save-dev @typescript-eslint/parser @typescript-eslint/eslint-plugin

import tseslint from '@typescript-eslint/eslint-plugin';
import parser from '@typescript-eslint/parser';

export default [
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      parser: parser,
      parserOptions: {
        project: './tsconfig.json'
      }
    },
    plugins: { '@typescript-eslint': tseslint },
    rules: {
      '@typescript-eslint/no-explicit-any': 'warn',
      '@typescript-eslint/explicit-function-return-type': 'error'
    }
  }
];

See references/extend/custom-parsers.md for parser configuration.

Documentation Organization

Complete ESLint documentation is organized in references/:

Core Usage

• references/use/getting-started.md - Initial setup and installation
• references/use/command-line-interface.md - CLI options and flags
• references/use/core-concepts/ - Core ESLint concepts and terminology

Configuration

• references/use/configure/configuration-files.md - Flat config format (v9+)
• references/use/configure/rules.md - Rule configuration patterns
• references/use/configure/language-options.md - Parser and language settings
• references/use/configure/plugins.md - Plugin configuration
• references/use/configure/ignore.md - Ignoring files and directories
• references/use/configure/migration-guide.md - Migrating from legacy config

Rules Reference

• references/rules/ - All 300+ core ESLint rules
  • Each rule has its own file (e.g., no-console.md, prefer-const.md)
  • Includes description, examples, options, and use cases
  • Organized alphabetically by rule name

Extension & Customization

• references/extend/custom-rules.md - Creating custom ESLint rules
• references/extend/custom-parsers.md - Building custom parsers
• references/extend/plugins.md - Plugin development and publishing
• references/extend/shareable-configs.md - Creating shareable configurations
• references/extend/selectors.md - AST selectors for advanced rules

Integration

• references/integrate/nodejs-api.md - Programmatic ESLint API usage
• references/use/integrations.md - Editor and tool integrations

Troubleshooting

• references/use/troubleshooting/ - Common error messages and solutions
  • Covers plugin loading errors, config resolution issues, and more

Migration Guides

• references/use/migrate-to-9.0.0.md - Migrating to ESLint 9.x
• references/use/migrate-to-8.0.0.md - Migrating to ESLint 8.x
• Additional migration guides for older versions

Common Patterns

Monorepo Configuration

export default [
  {
    files: ["packages/*/src/**/*.js"],
    rules: { "no-console": "error" }
  },
  {
    files: ["packages/cli/src/**/*.js"],
    rules: { "no-console": "off" }  // Allow console in CLI package
  }
];

Environment-Specific Rules

export default [
  {
    files: ["src/**/*.js"],
    rules: { "no-console": "error" }
  },
  {
    files: ["**/*.test.js", "**/*.spec.js"],
    rules: { "no-console": "off" }  // Allow console in tests
  }
];

Shared Configuration

// config/base.js
export default {
  rules: {
    "no-unused-vars": "error",
    "semi": ["error", "always"]
  }
};

// eslint.config.js
import baseConfig from './config/base.js';

export default [
  baseConfig,
  {
    files: ["src/**/*.js"],
    rules: {
      "no-console": "warn"
    }
  }
];

Best Practices

1. Start with recommended config - Use eslint:recommended as baseline
2. Enable auto-fix - Configure editor to fix on save for productivity
3. Use strict mode gradually - Start with warnings, upgrade to errors iteratively
4. Document exceptions - Add comments when disabling rules
5. Keep config organized - Split large configs into multiple files
6. Test rule changes - Run linter on entire codebase before committing config changes
7. Update regularly - Keep ESLint and plugins up to date for latest rules and fixes
8. Use flat config - Migrate to eslint.config.js format (v9+ standard)

Workflow for Fixing Errors

When ESLint reports errors:

1. Identify the rule - Look for rule name in error message (e.g., no-unused-vars)
2. Read rule docs - Check references/rules/[rule-name].md
3. Review examples - Examine correct/incorrect examples in the docs
4. Apply fix - Either fix code or configure rule if not applicable
5. Verify - Re-run ESLint to confirm error is resolved

Updating Documentation

To update this skill's documentation when new ESLint versions are released:

See scripts/docs-updater/USAGE.md for complete instructions on extracting updated documentation from the ESLint repository.

Key Differences Between Versions

ESLint 9.x (Flat Config)

• New flat config format (eslint.config.js)
• Simplified configuration structure
• Better TypeScript support
• Improved performance

ESLint 8.x (Legacy)

• Uses .eslintrc.* files (deprecated in v9+)
• Different plugin loading mechanism
• Legacy config format

Migration: See references/use/configure/migration-guide.md for migrating from v8 to v9.

Working with This Skill

This skill provides:

1. Comprehensive rule reference - All 300+ ESLint rules with examples
2. Configuration patterns - Flat config examples and best practices
3. Integration guides - Editor, build system, and CI/CD integration
4. Troubleshooting - Common errors and solutions
5. Migration guides - Version upgrade assistance
6. Extension patterns - Custom rules, plugins, and parsers

For specific rule details, configuration options, or integration patterns, consult the organized reference documentation in references/.