ESLint

Pluggable linting for JavaScript and TypeScript.

When to Use

• Enforcing code style
• Catching bugs early
• TypeScript type checking
• CI/CD quality gates

Quick Start

# Install
npm init @eslint/config@latest

# Run
npx eslint .
npx eslint --fix .

Core Concepts

Flat Config (ESLint 9+)

// eslint.config.js
import js from "@eslint/js";
import typescript from "@typescript-eslint/eslint-plugin";
import tsParser from "@typescript-eslint/parser";
import react from "eslint-plugin-react";

export default [
  js.configs.recommended,
  {
    files: ["**/*.{ts,tsx}"],
    languageOptions: {
      parser: tsParser,
      parserOptions: {
        project: "./tsconfig.json",
      },
    },
    plugins: {
      "@typescript-eslint": typescript,
      react,
    },
    rules: {
      "@typescript-eslint/no-unused-vars": "error",
      "@typescript-eslint/no-explicit-any": "warn",
      "react/prop-types": "off",
    },
  },
  {
    ignores: ["dist/**", "node_modules/**"],
  },
];

Custom Rules

// Common rule configurations
{
  rules: {
    // Errors
    'no-console': ['error', { allow: ['warn', 'error'] }],
    'no-unused-vars': 'off',
    '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],

    // Warnings
    'max-lines': ['warn', { max: 300 }],
    'complexity': ['warn', { max: 10 }],

    // Style
    'quotes': ['error', 'single'],
    'semi': ['error', 'always'],
  }
}

Common Patterns

VS Code Integration

// .vscode/settings.json
{
  "eslint.validate": [
    "javascript",
    "javascriptreact",
    "typescript",
    "typescriptreact"
  ],
  "editor.codeActionsOnSave": {
    "source.fixAll.eslint": "explicit"
  }
}

Pre-commit Hook

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

Best Practices

Do:

• Use TypeScript-aware rules
• Configure per-project
• Integrate with CI/CD
• Use format on save

Don't:

• Disable rules globally
• Ignore all errors
• Mix legacy and flat config
• Skip TypeScript plugin for TS

Troubleshooting

Issue	Cause	Solution
Parser error	Wrong parser	Use @typescript-eslint/parser
Rule not found	Missing plugin	Install plugin package
Slow linting	Type-aware rules	Limit project scope

References

• ESLint Documentation
• TypeScript ESLint