ESLint 9 & Biome Linting

When NOT to Use This Skill

• Legacy ESLint (.eslintrc) - Use eslint skill for old config format
• TypeScript-only rules - Use typescript-eslint skill for deep TypeScript linting
• Java/Kotlin linting - Use sonarqube skill
• Code quality principles - Use quality-common for SOLID/Clean Code

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

Official References

ESLint 9 Flat Config

Basic Setup

npm install --save-dev eslint @eslint/js typescript typescript-eslint

// eslint.config.mjs
import eslint from '@eslint/js';
import tseslint from 'typescript-eslint';

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommended,
);

TypeScript with Type Checking

// eslint.config.mjs
import eslint from '@eslint/js';
import tseslint from 'typescript-eslint';

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommendedTypeChecked,
  {
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
  },
);

Full Configuration Example

// eslint.config.mjs
import eslint from '@eslint/js';
import tseslint from 'typescript-eslint';
import reactPlugin from 'eslint-plugin-react';
import reactHooksPlugin from 'eslint-plugin-react-hooks';

export default tseslint.config(
  // Ignores
  {
    ignores: ['dist/**', 'node_modules/**', '*.config.js'],
  },

  // Base configs
  eslint.configs.recommended,
  ...tseslint.configs.strictTypeChecked,
  ...tseslint.configs.stylisticTypeChecked,

  // TypeScript files
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      parserOptions: {
        projectService: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    rules: {
      // Prevent bugs
      '@typescript-eslint/no-floating-promises': 'error',
      '@typescript-eslint/no-misused-promises': 'error',
      '@typescript-eslint/await-thenable': 'error',

      // Code quality
      '@typescript-eslint/no-explicit-any': 'error',
      '@typescript-eslint/explicit-function-return-type': 'warn',
      '@typescript-eslint/consistent-type-imports': 'error',

      // Complexity
      'complexity': ['warn', 10],
      'max-depth': ['warn', 4],
      'max-lines-per-function': ['warn', 50],
    },
  },

  // React files
  {
    files: ['**/*.tsx'],
    plugins: {
      react: reactPlugin,
      'react-hooks': reactHooksPlugin,
    },
    rules: {
      ...reactPlugin.configs.recommended.rules,
      ...reactHooksPlugin.configs.recommended.rules,
      'react/react-in-jsx-scope': 'off',
    },
    settings: {
      react: { version: 'detect' },
    },
  },

  // Test files
  {
    files: ['**/*.test.ts', '**/*.test.tsx', '**/*.spec.ts'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
      'max-lines-per-function': 'off',
    },
  },
);

typescript-eslint Presets

Key Type-Checked Rules

rules: {
  // Async/Promise safety
  '@typescript-eslint/no-floating-promises': 'error',
  '@typescript-eslint/no-misused-promises': 'error',
  '@typescript-eslint/await-thenable': 'error',
  '@typescript-eslint/require-await': 'error',

  // Type safety
  '@typescript-eslint/no-unsafe-argument': 'error',
  '@typescript-eslint/no-unsafe-assignment': 'error',
  '@typescript-eslint/no-unsafe-call': 'error',
  '@typescript-eslint/no-unsafe-member-access': 'error',
  '@typescript-eslint/no-unsafe-return': 'error',

  // Best practices
  '@typescript-eslint/no-unnecessary-condition': 'error',
  '@typescript-eslint/prefer-nullish-coalescing': 'error',
  '@typescript-eslint/prefer-optional-chain': 'error',
}

Biome

Installation

npm install --save-dev @biomejs/biome
npx @biomejs/biome init

Configuration (biome.json)

{
  "$schema": "https://biomejs.dev/schemas/1.9.0/schema.json",
  "organizeImports": {
    "enabled": true
  },
  "linter": {
    "enabled": true,
    "rules": {
      "recommended": true,
      "complexity": {
        "noExcessiveCognitiveComplexity": {
          "level": "warn",
          "options": { "maxAllowedComplexity": 15 }
        }
      },
      "correctness": {
        "noUnusedImports": "error",
        "noUnusedVariables": "error",
        "useExhaustiveDependencies": "warn"
      },
      "suspicious": {
        "noExplicitAny": "error",
        "noConsoleLog": "warn"
      },
      "style": {
        "useConst": "error",
        "noNonNullAssertion": "warn"
      },
      "security": {
        "noDangerouslySetInnerHtml": "error"
      }
    }
  },
  "formatter": {
    "enabled": true,
    "indentStyle": "space",
    "indentWidth": 2
  },
  "javascript": {
    "formatter": {
      "quoteStyle": "single",
      "semicolons": "always"
    }
  }
}

Rule Categories

Commands

# Check all
npx biome check .

# Fix auto-fixable issues
npx biome check --write .

# Lint only
npx biome lint .

# Format only
npx biome format --write .

# CI mode (no writes)
npx biome ci .

Migration: ESLint to Biome

# Automatic migration
npx @biomejs/biome migrate eslint --write

# With inspired rules
npx @biomejs/biome migrate eslint --include-inspired --write

# Migrate prettier config too
npx @biomejs/biome migrate prettier --write

Supported ESLint Plugins

Rule Name Mapping

ESLint vs Biome

When to Use ESLint

• Need specific plugins (import sorting, testing rules)
• Require full type-aware linting
• Have complex dynamic configuration

When to Use Biome

• Speed is critical (large codebase, CI)
• Want unified linting + formatting
• Don't need extensive plugin ecosystem

CI Integration

ESLint (GitHub Actions)

- name: Lint
  run: npx eslint . --max-warnings=0

# With caching
- name: Lint with cache
  run: npx eslint . --cache --cache-location node_modules/.cache/eslint

Biome (GitHub Actions)

- name: Setup Biome
  uses: biomejs/setup-biome@v2
  with:
    version: latest

- name: Run Biome
  run: biome ci .

Checklist

ESLint Setup

• Using flat config (eslint.config.mjs)
• Type-checked rules enabled
• No deprecated eslintrc format
• Strict TypeScript rules
• CI caching configured

Biome Setup

• biome.json configured
• Recommended rules enabled
• Formatter settings match team style
• CI mode in pipeline

Metrics

Anti-Patterns

Quick Troubleshooting

Related Skills

• TypeScript
• Quality Principles
• GitHub Actions