Eslint

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/.