ESLint - Quick Reference

When to Use This Skill

Configure linting for JavaScript/TypeScript projects
Create custom rules
Integrate with Prettier and other tools

When NOT to Use This Skill

ESLint 9 flat config + Biome - Use eslint-biome skill for modern setup
TypeScript-specific rules - Use typescript-eslint skill
Code quality principles - Use quality-common for SOLID/Clean Code
SonarQube integration - Use sonarqube skill

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: eslint for comprehensive documentation.

Basic Setup

npm install -D eslint @eslint/js
npx eslint --init

Flat Config (ESLint 9+)

eslint.config.js

import js from '@eslint/js';
import tseslint from 'typescript-eslint';

export default [
  js.configs.recommended,
  ...tseslint.configs.recommended,
  {
    files: ['**/*.ts', '**/*.tsx'],
    rules: {
      '@typescript-eslint/no-unused-vars': 'error',
      '@typescript-eslint/explicit-function-return-type': 'warn',
    },
  },
  {
    ignores: ['dist/', 'node_modules/', '*.config.js'],
  },
];

With Prettier

import eslintConfigPrettier from 'eslint-config-prettier';

export default [
  js.configs.recommended,
  ...tseslint.configs.recommended,
  eslintConfigPrettier, // Must be last
];

Legacy Config (.eslintrc)

{
  "root": true,
  "env": { "browser": true, "es2021": true, "node": true },
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended"
  ],
  "parser": "@typescript-eslint/parser",
  "plugins": ["@typescript-eslint"],
  "rules": {
    "no-console": "warn",
    "@typescript-eslint/no-explicit-any": "error"
  }
}

CLI Usage

# Lint files
npx eslint src/

# Fix auto-fixable issues
npx eslint src/ --fix

# Check specific files
npx eslint "src/**/*.{ts,tsx}"

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
Using legacy .eslintrc in new projects	Deprecated in ESLint 9	Use flat config (eslint.config.mjs)
No TypeScript type checking	Misses type-aware issues	Use `recommendedTypeChecked` preset
Disabling rules with comments everywhere	Code smell, defeats purpose	Fix the issue or adjust rule config
Not caching in CI	Slow linting	Use `--cache` flag
Conflicting Prettier rules	Formatting wars	Use `eslint-config-prettier`
Ignoring warnings	Accumulate tech debt	Treat warnings as errors in CI

Quick Troubleshooting

Issue	Likely Cause	Solution
"Failed to load config" error	Wrong config format	Check eslint.config.mjs syntax
Type-aware rules not working	Missing parserOptions	Add `projectService: true` to config
Linting very slow	No caching, type checking all files	Enable cache, limit type checking to TS files
Rules from plugin not found	Plugin not in flat config format	Check plugin compatibility with ESLint 9
Prettier conflicts	Both formatting same code	Add `eslint-config-prettier` last
File not being linted	In ignores array	Check `ignores` in config