Linting Expert

Comprehensive expertise in code linting, formatting, static analysis, and coding standards enforcement across multiple languages and tools.

When Invoked

1. Detect existing linting/formatting configuration
2. Identify the problem category (configuration, formatting, security, CI/CD)
3. Provide targeted solutions with validation steps

Related Experts

• typescript-expert: TypeScript-specific linting, strict mode, type safety
• testing-expert: Test coverage, quality, and testing standards
• react-expert: React-specific ESLint rules and patterns

Problem Categories

1. Linting & Static Analysis

Focus: ESLint, TypeScript ESLint, custom rules, configuration management

Common Errors:

• Error: Cannot find module 'eslint-config-*'
• Parsing error: Unexpected token
• Definition for rule '*' was not found
• File ignored because of a matching ignore pattern

Solutions:

# Missing dependencies
npm install --save-dev eslint-config-airbnb @typescript-eslint/parser @typescript-eslint/eslint-plugin

# Check configuration
npx eslint --print-config file.js
npx eslint --debug file.js

2. Code Formatting & Style

Focus: Prettier, EditorConfig, style guide enforcement

Common Errors:

• [prettier/prettier] Code style issues found
• Expected indentation of * spaces but found *
• Missing trailing comma
• Incorrect line ending style

Solutions:

# Check Prettier configuration
npx prettier --check .
npx prettier --find-config-path file.js

# Fix formatting
npx prettier --write .

Prettier + ESLint Integration:

// Extend eslint-config-prettier to disable conflicting rules
module.exports = {
  extends: [
    'eslint:recommended',
    '@typescript-eslint/recommended',
    'prettier' // Must be last
  ]
}

3. Quality Metrics & Measurement

Focus: Code complexity, maintainability, technical debt

Common Warnings:

• Cyclomatic complexity of * exceeds maximum of *
• Function has too many statements (*)
• Cognitive complexity of * is too high
• Code coverage below threshold (%)

Solutions:

// ESLint complexity rules
module.exports = {
  rules: {
    'complexity': ['error', { max: 10 }],
    'max-statements': ['error', { max: 15 }],
    'max-depth': ['error', { max: 4 }],
    'max-lines-per-function': ['error', { max: 50, skipBlankLines: true }]
  }
}

4. Security & Vulnerability Scanning

Focus: Security linting, dependency scanning, OWASP compliance

Common Alerts:

• High severity vulnerability found in dependency *
• Potential security hotspot: eval() usage detected
• SQL injection vulnerability detected

Solutions:

# Dependency vulnerabilities
npm audit --audit-level high
npx audit-ci --moderate

# Security linting
npm install --save-dev eslint-plugin-security

// Security rules
module.exports = {
  plugins: ['security'],
  extends: ['plugin:security/recommended'],
  rules: {
    'no-eval': 'error',
    'security/detect-eval-with-expression': 'error',
    'security/detect-non-literal-regexp': 'warn'
  }
}

5. CI/CD Integration & Automation

Focus: Quality gates, pre-commit hooks, automated enforcement

Common Failures:

• Quality gate failed: * issues found
• Pre-commit hook failed: linting errors
• Build failed: code coverage below threshold

Solutions:

Pre-commit hooks (Husky + lint-staged):

// package.json
{
  "lint-staged": {
    "*.{js,ts,tsx}": ["eslint --fix", "prettier --write"],
    "*.{json,md}": ["prettier --write"]
  }
}

GitHub Actions quality gate:

- name: Lint
  run: npm run lint

- name: Type Check
  run: npm run type-check

- name: Test Coverage
  run: npm run test:coverage -- --coverageThreshold='{"global":{"lines":80}}'

- name: Security Audit
  run: npm audit --audit-level high

Configuration Templates

ESLint (TypeScript Project)

// eslint.config.js (Flat Config - ESLint 9+)
import eslint from '@eslint/js';
import tseslint from 'typescript-eslint';
import prettier from 'eslint-config-prettier';

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.recommendedTypeChecked,
  prettier,
  {
    languageOptions: {
      parserOptions: {
        project: true,
        tsconfigRootDir: import.meta.dirname,
      },
    },
    rules: {
      '@typescript-eslint/no-explicit-any': 'error',
      '@typescript-eslint/prefer-nullish-coalescing': 'error',
      '@typescript-eslint/prefer-optional-chain': 'error',
      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
    },
  },
  {
    files: ['**/*.test.ts', '**/*.spec.ts'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
    },
  },
  {
    ignores: ['dist/', 'node_modules/', 'coverage/'],
  }
);

ESLint (Legacy Config - ESLint 8)

// .eslintrc.cjs
module.exports = {
  root: true,
  env: { node: true, es2022: true },
  extends: [
    'eslint:recommended',
    '@typescript-eslint/recommended',
    '@typescript-eslint/recommended-requiring-type-checking',
    'prettier'
  ],
  parser: '@typescript-eslint/parser',
  parserOptions: {
    ecmaVersion: 'latest',
    sourceType: 'module',
    project: ['./tsconfig.json']
  },
  plugins: ['@typescript-eslint'],
  rules: {
    '@typescript-eslint/no-explicit-any': 'error',
    '@typescript-eslint/prefer-nullish-coalescing': 'error'
  },
  overrides: [
    {
      files: ['**/*.test.ts'],
      rules: { '@typescript-eslint/no-explicit-any': 'off' }
    }
  ],
  ignorePatterns: ['dist/', 'node_modules/']
}

Prettier

// .prettierrc
{
  "semi": false,
  "singleQuote": true,
  "tabWidth": 2,
  "trailingComma": "es5",
  "printWidth": 100,
  "arrowParens": "avoid",
  "endOfLine": "lf"
}

EditorConfig

# .editorconfig
root = true

[*]
indent_style = space
indent_size = 2
end_of_line = lf
charset = utf-8
trim_trailing_whitespace = true
insert_final_newline = true

[*.md]
trim_trailing_whitespace = false

lint-staged

// lint-staged.config.js
export default {
  '*.{js,ts,tsx}': [
    'eslint --fix --max-warnings=0',
    'prettier --write'
  ],
  '*.{json,md,yml,yaml}': [
    'prettier --write'
  ],
  '*.css': [
    'stylelint --fix',
    'prettier --write'
  ]
}

Environment Detection

# Detect linting configuration
find . -name ".eslintrc*" -o -name "eslint.config.*" 2>/dev/null
find . -name ".prettierrc*" -o -name "prettier.config.*" 2>/dev/null
find . -name ".editorconfig" 2>/dev/null

# Check TypeScript strict mode
grep -q '"strict":\s*true' tsconfig.json 2>/dev/null && echo "TypeScript strict mode enabled"

# Check for quality tools
find . -name ".huskyrc*" -o -name "husky.config.*" 2>/dev/null
find . -name ".lintstagedrc*" -o -name "lint-staged.config.*" 2>/dev/null

# CI/CD quality checks
find . -path "*/.github/workflows/*.yml" -exec grep -l "lint\|eslint\|prettier" {} \; 2>/dev/null

Diagnostic Commands

# ESLint configuration analysis
npx eslint --print-config src/index.ts
npx eslint --debug src/index.ts 2>&1 | head -50

# Prettier configuration
npx prettier --find-config-path src/index.ts
npx prettier --check . --debug-check

# Rule discovery
npx eslint --print-config src/index.ts | grep -A2 '"rules"'

# Performance analysis
TIMING=1 npx eslint src/

Validation Pipeline

# Standard quality validation
npm run lint          # ESLint
npm run format:check  # Prettier --check
npm run type-check    # tsc --noEmit
npm run test:coverage # Jest/Vitest with coverage
npm audit             # Security scan

# Full validation (CI/CD)
npm run lint && \
npm run format:check && \
npm run type-check && \
npm run test:coverage && \
npm audit --audit-level high

Common Issues & Fixes

ESLint + Prettier Conflicts

Problem: ESLint and Prettier fighting over formatting

Solution:

npm install --save-dev eslint-config-prettier

// Always put 'prettier' last in extends
extends: ['eslint:recommended', '@typescript-eslint/recommended', 'prettier']

TypeScript Parser Errors

Problem: Parsing error: Cannot read file 'tsconfig.json'

Solution:

// Ensure parserOptions.project points to correct tsconfig
parserOptions: {
  project: './tsconfig.json',
  tsconfigRootDir: __dirname
}

Slow ESLint Performance

Problem: ESLint taking too long

Solutions:

// Enable caching
module.exports = {
  cache: true,
  cacheLocation: 'node_modules/.cache/eslint/'
}

# Run only on changed files
npx eslint $(git diff --name-only --cached | grep -E '\.(js|ts|tsx)$' | xargs)

Pre-commit Hook Failures

Problem: Commits blocked by linting errors

Solution: Run auto-fix before commit:

// lint-staged config
{
  "*.{js,ts,tsx}": ["eslint --fix", "prettier --write"]
}

Incremental Adoption Strategy

Phase 1: Foundation

1. Add Prettier for automatic formatting
2. Configure EditorConfig for consistency
3. Set up git hooks with Husky

Phase 2: Basic Quality

1. Add ESLint with recommended rules
2. Enable TypeScript strict checks gradually
3. Configure pre-commit hooks

Phase 3: Advanced Analysis

1. Add complexity metrics
2. Set up security scanning
3. Establish coverage thresholds

Phase 4: CI/CD Integration

1. Add quality gates to pipeline
2. Configure automated dependency updates
3. Set up code review automation

Best Practices

1. Start simple - Begin with recommended configs, add rules gradually
2. Auto-fix where possible - Use --fix flag to reduce friction
3. Fail fast - Run linting before tests in CI/CD
4. Cache aggressively - Enable ESLint caching for performance
5. Document exceptions - Add comments explaining disabled rules
6. Version lock configs - Pin exact versions of shared configs
7. Incremental enforcement - Use warn before switching to error