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:

• Install ESLint: npm install --save-dev eslint

• Initialize config: npx eslint --init (interactive)

• Review generated eslint.config.js

• Run first lint: npx eslint .

Existing projects:

• Review current configuration in eslint.config.js or .eslintrc.*

• Understand applied rules and plugins

• 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:

• Read error message and rule name

• Look up rule in references/rules/[rule-name].md

• Understand the issue and correct code examples

• 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

• Start with recommended config - Use eslint:recommended as baseline

• Enable auto-fix - Configure editor to fix on save for productivity

• Use strict mode gradually - Start with warnings, upgrade to errors iteratively

• Document exceptions - Add comments when disabling rules

• Keep config organized - Split large configs into multiple files

• Test rule changes - Run linter on entire codebase before committing config changes

• Update regularly - Keep ESLint and plugins up to date for latest rules and fixes

• Use flat config - Migrate to eslint.config.js format (v9+ standard)

Workflow for Fixing Errors

When ESLint reports errors:

• Identify the rule - Look for rule name in error message (e.g., no-unused-vars )

• Read rule docs - Check references/rules/[rule-name].md

• Review examples - Examine correct/incorrect examples in the docs

• Apply fix - Either fix code or configure rule if not applicable

• 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:

• Comprehensive rule reference - All 300+ ESLint rules with examples

• Configuration patterns - Flat config examples and best practices

• Integration guides - Editor, build system, and CI/CD integration

• Troubleshooting - Common errors and solutions

• Migration guides - Version upgrade assistance

• Extension patterns - Custom rules, plugins, and parsers

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