ESLint & Prettier Configuration

Setup consistent code quality and formatting with ESLint and Prettier.

Core Workflow

1. Install dependencies: ESLint, Prettier, plugins
2. Configure ESLint: Rules and extends
3. Configure Prettier: Formatting options
4. Integrate tools: ESLint + Prettier
5. Setup scripts: Lint and format commands
6. Add hooks: Pre-commit validation

ESLint Flat Config (v9+)

// eslint.config.mjs
import js from '@eslint/js';
import typescript from '@typescript-eslint/eslint-plugin';
import tsParser from '@typescript-eslint/parser';
import react from 'eslint-plugin-react';
import reactHooks from 'eslint-plugin-react-hooks';
import reactRefresh from 'eslint-plugin-react-refresh';
import importPlugin from 'eslint-plugin-import';
import prettier from 'eslint-config-prettier';

export default [
  // Ignore patterns
  {
    ignores: [
      '**/node_modules/**',
      '**/dist/**',
      '**/build/**',
      '**/.next/**',
      '**/coverage/**',
      '**/*.config.js',
      '**/*.config.mjs',
    ],
  },

  // Base JavaScript config
  js.configs.recommended,

  // TypeScript files
  {
    files: ['**/*.ts', '**/*.tsx'],
    languageOptions: {
      parser: tsParser,
      parserOptions: {
        ecmaVersion: 'latest',
        sourceType: 'module',
        ecmaFeatures: {
          jsx: true,
        },
        project: './tsconfig.json',
      },
    },
    plugins: {
      '@typescript-eslint': typescript,
      'import': importPlugin,
    },
    rules: {
      // TypeScript rules
      ...typescript.configs.recommended.rules,
      '@typescript-eslint/no-unused-vars': ['error', {
        argsIgnorePattern: '^_',
        varsIgnorePattern: '^_',
      }],
      '@typescript-eslint/no-explicit-any': 'warn',
      '@typescript-eslint/explicit-function-return-type': 'off',
      '@typescript-eslint/explicit-module-boundary-types': 'off',
      '@typescript-eslint/no-non-null-assertion': 'warn',
      '@typescript-eslint/consistent-type-imports': ['error', {
        prefer: 'type-imports',
        fixStyle: 'inline-type-imports',
      }],

      // Import rules
      'import/order': ['error', {
        groups: [
          'builtin',
          'external',
          'internal',
          ['parent', 'sibling'],
          'index',
          'type',
        ],
        'newlines-between': 'always',
        alphabetize: { order: 'asc', caseInsensitive: true },
      }],
      'import/no-duplicates': 'error',
    },
  },

  // React files
  {
    files: ['**/*.tsx', '**/*.jsx'],
    plugins: {
      'react': react,
      'react-hooks': reactHooks,
      'react-refresh': reactRefresh,
    },
    settings: {
      react: {
        version: 'detect',
      },
    },
    rules: {
      ...react.configs.recommended.rules,
      ...reactHooks.configs.recommended.rules,
      'react/react-in-jsx-scope': 'off',
      'react/prop-types': 'off',
      'react/jsx-uses-react': 'off',
      'react/jsx-no-target-blank': 'error',
      'react/self-closing-comp': 'error',
      'react/jsx-curly-brace-presence': ['error', {
        props: 'never',
        children: 'never',
      }],
      'react-hooks/rules-of-hooks': 'error',
      'react-hooks/exhaustive-deps': 'warn',
      'react-refresh/only-export-components': ['warn', {
        allowConstantExport: true,
      }],
    },
  },

  // Test files
  {
    files: ['**/*.test.ts', '**/*.test.tsx', '**/*.spec.ts', '**/*.spec.tsx'],
    rules: {
      '@typescript-eslint/no-explicit-any': 'off',
      '@typescript-eslint/no-non-null-assertion': 'off',
    },
  },

  // Disable rules that conflict with Prettier
  prettier,
];

ESLint Legacy Config (.eslintrc)

// .eslintrc.json
{
  "root": true,
  "env": {
    "browser": true,
    "es2022": true,
    "node": true
  },
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended",
    "plugin:@typescript-eslint/recommended-type-checked",
    "plugin:react/recommended",
    "plugin:react-hooks/recommended",
    "plugin:import/recommended",
    "plugin:import/typescript",
    "plugin:jsx-a11y/recommended",
    "prettier"
  ],
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "ecmaVersion": "latest",
    "sourceType": "module",
    "ecmaFeatures": {
      "jsx": true
    },
    "project": ["./tsconfig.json"]
  },
  "plugins": [
    "@typescript-eslint",
    "react",
    "react-hooks",
    "import",
    "jsx-a11y"
  ],
  "settings": {
    "react": {
      "version": "detect"
    },
    "import/resolver": {
      "typescript": {
        "alwaysTryTypes": true
      }
    }
  },
  "rules": {
    // TypeScript
    "@typescript-eslint/no-unused-vars": ["error", {
      "argsIgnorePattern": "^_",
      "varsIgnorePattern": "^_"
    }],
    "@typescript-eslint/consistent-type-imports": ["error", {
      "prefer": "type-imports",
      "fixStyle": "inline-type-imports"
    }],
    "@typescript-eslint/no-explicit-any": "warn",
    "@typescript-eslint/no-floating-promises": "error",
    "@typescript-eslint/await-thenable": "error",

    // React
    "react/react-in-jsx-scope": "off",
    "react/prop-types": "off",
    "react/self-closing-comp": "error",

    // Imports
    "import/order": ["error", {
      "groups": ["builtin", "external", "internal", "parent", "sibling", "index", "type"],
      "newlines-between": "always",
      "alphabetize": { "order": "asc" }
    }],
    "import/no-duplicates": "error",
    "import/no-unresolved": "error",

    // General
    "no-console": ["warn", { "allow": ["warn", "error"] }],
    "prefer-const": "error",
    "no-var": "error"
  },
  "overrides": [
    {
      "files": ["*.test.ts", "*.test.tsx", "*.spec.ts", "*.spec.tsx"],
      "rules": {
        "@typescript-eslint/no-explicit-any": "off",
        "@typescript-eslint/no-floating-promises": "off"
      }
    }
  ],
  "ignorePatterns": [
    "node_modules",
    "dist",
    "build",
    ".next",
    "coverage"
  ]
}

Prettier Configuration

// .prettierrc
{
  "semi": true,
  "singleQuote": true,
  "trailingComma": "es5",
  "tabWidth": 2,
  "useTabs": false,
  "printWidth": 100,
  "bracketSpacing": true,
  "bracketSameLine": false,
  "arrowParens": "always",
  "endOfLine": "lf",
  "quoteProps": "as-needed",
  "jsxSingleQuote": false,
  "proseWrap": "preserve",
  "htmlWhitespaceSensitivity": "css",
  "embeddedLanguageFormatting": "auto"
}

// .prettierignore
node_modules
dist
build
.next
coverage
*.min.js
*.min.css
pnpm-lock.yaml
package-lock.json
yarn.lock

Package Dependencies

// package.json (partial)
{
  "scripts": {
    "lint": "eslint . --ext .ts,.tsx --max-warnings 0",
    "lint:fix": "eslint . --ext .ts,.tsx --fix",
    "format": "prettier --write \"**/*.{ts,tsx,json,md,css}\"",
    "format:check": "prettier --check \"**/*.{ts,tsx,json,md,css}\"",
    "typecheck": "tsc --noEmit",
    "check": "npm run typecheck && npm run lint && npm run format:check"
  },
  "devDependencies": {
    "@eslint/js": "^9.0.0",
    "@typescript-eslint/eslint-plugin": "^7.0.0",
    "@typescript-eslint/parser": "^7.0.0",
    "eslint": "^9.0.0",
    "eslint-config-prettier": "^9.1.0",
    "eslint-import-resolver-typescript": "^3.6.0",
    "eslint-plugin-import": "^2.29.0",
    "eslint-plugin-jsx-a11y": "^6.8.0",
    "eslint-plugin-react": "^7.33.0",
    "eslint-plugin-react-hooks": "^4.6.0",
    "eslint-plugin-react-refresh": "^0.4.0",
    "prettier": "^3.2.0",
    "typescript": "^5.3.0"
  }
}

Git Hooks with Husky

# Install Husky
npm install -D husky lint-staged
npx husky init

// package.json
{
  "lint-staged": {
    "*.{ts,tsx}": [
      "eslint --fix --max-warnings 0",
      "prettier --write"
    ],
    "*.{json,md,css,scss}": [
      "prettier --write"
    ]
  }
}

# .husky/pre-commit
npx lint-staged

# .husky/commit-msg
npx commitlint --edit $1

Commitlint Configuration

// commitlint.config.js
module.exports = {
  extends: ['@commitlint/config-conventional'],
  rules: {
    'type-enum': [
      2,
      'always',
      [
        'feat',     // New feature
        'fix',      // Bug fix
        'docs',     // Documentation
        'style',    // Formatting
        'refactor', // Code restructuring
        'perf',     // Performance
        'test',     // Tests
        'build',    // Build system
        'ci',       // CI configuration
        'chore',    // Maintenance
        'revert',   // Revert changes
      ],
    ],
    'subject-case': [2, 'always', 'lower-case'],
    'subject-max-length': [2, 'always', 72],
    'body-max-line-length': [2, 'always', 100],
  },
};

VS Code Integration

// .vscode/settings.json
{
  "editor.formatOnSave": true,
  "editor.defaultFormatter": "esbenp.prettier-vscode",
  "editor.codeActionsOnSave": {
    "source.fixAll.eslint": "explicit",
    "source.organizeImports": "never"
  },
  "[typescript]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  },
  "[typescriptreact]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  },
  "[json]": {
    "editor.defaultFormatter": "esbenp.prettier-vscode"
  },
  "eslint.validate": [
    "javascript",
    "javascriptreact",
    "typescript",
    "typescriptreact"
  ],
  "typescript.preferences.importModuleSpecifier": "relative",
  "typescript.suggest.autoImports": true
}

// .vscode/extensions.json
{
  "recommendations": [
    "dbaeumer.vscode-eslint",
    "esbenp.prettier-vscode",
    "bradlc.vscode-tailwindcss",
    "streetsidesoftware.code-spell-checker"
  ]
}

Next.js Specific

// .eslintrc.js (Next.js)
module.exports = {
  extends: [
    'next/core-web-vitals',
    'plugin:@typescript-eslint/recommended',
    'prettier',
  ],
  parser: '@typescript-eslint/parser',
  plugins: ['@typescript-eslint'],
  rules: {
    '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
    '@typescript-eslint/consistent-type-imports': 'error',
    'import/order': ['error', {
      groups: ['builtin', 'external', 'internal', 'parent', 'sibling', 'index'],
      'newlines-between': 'always',
    }],
  },
};

Node.js/API Specific

// eslint.config.mjs (Node.js API)
import js from '@eslint/js';
import typescript from '@typescript-eslint/eslint-plugin';
import tsParser from '@typescript-eslint/parser';
import nodePlugin from 'eslint-plugin-n';
import prettier from 'eslint-config-prettier';

export default [
  js.configs.recommended,
  {
    files: ['**/*.ts'],
    languageOptions: {
      parser: tsParser,
      parserOptions: {
        project: './tsconfig.json',
      },
    },
    plugins: {
      '@typescript-eslint': typescript,
      'n': nodePlugin,
    },
    rules: {
      ...typescript.configs.recommended.rules,
      'n/no-process-exit': 'error',
      'n/no-unsupported-features/es-syntax': 'off',
      '@typescript-eslint/no-floating-promises': 'error',
      '@typescript-eslint/require-await': 'error',
    },
  },
  prettier,
];

Custom Rules Examples

// Custom ESLint rule example (eslint-plugin-custom/no-direct-env.js)
module.exports = {
  meta: {
    type: 'problem',
    docs: {
      description: 'Disallow direct process.env access',
    },
    messages: {
      noDirectEnv: 'Use config module instead of direct process.env access',
    },
  },
  create(context) {
    return {
      MemberExpression(node) {
        if (
          node.object.type === 'MemberExpression' &&
          node.object.object.name === 'process' &&
          node.object.property.name === 'env'
        ) {
          context.report({
            node,
            messageId: 'noDirectEnv',
          });
        }
      },
    };
  },
};

Best Practices

1. Extend carefully: Order matters in extends
2. Use type-checking: Enable type-aware rules
3. Consistent imports: Enforce import ordering
4. Prettier last: Always extend prettier last
5. Auto-fix on save: Configure VS Code
6. Pre-commit hooks: Validate before commit
7. CI validation: Run lint in CI pipeline
8. Progressive adoption: Start with warnings

Output Checklist

Every ESLint/Prettier setup should include:

• ESLint configuration (flat or legacy)
• TypeScript-ESLint integration
• React/React Hooks rules
• Import ordering rules
• Prettier configuration
• ESLint-Prettier integration
• VS Code settings
• Husky pre-commit hooks
• lint-staged configuration
• CI lint validation
• Ignore patterns
• npm scripts for lint/format