typescript

TypeScript Skill

---

Strict Mode (Non-Negotiable)

// tsconfig.json
{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true
  }
}

---

Project Structure

project/
├── src/
│   ├── core/               # Pure business logic
│   │   ├── types.ts        # Domain types/interfaces
│   │   ├── services/       # Pure functions
│   │   └── index.ts        # Public API
│   ├── infra/              # Side effects
│   │   ├── api/            # HTTP handlers
│   │   ├── db/             # Database operations
│   │   └── external/       # Third-party integrations
│   └── utils/              # Shared utilities
├── tests/
│   ├── unit/
│   └── integration/
├── package.json
├── tsconfig.json
└── CLAUDE.md

---

Tooling (Required)

// package.json scripts
{
  "scripts": {
    "lint": "eslint src/ --ext .ts,.tsx",
    "typecheck": "tsc --noEmit",
    "test": "jest",
    "test:coverage": "jest --coverage",
    "format": "prettier --write 'src/**/*.ts'"
  }
}

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

export default tseslint.config(
  eslint.configs.recommended,
  ...tseslint.configs.strictTypeChecked,
  {
    rules: {
      '@typescript-eslint/no-explicit-any': 'error',
      '@typescript-eslint/explicit-function-return-type': 'error',
      'max-lines-per-function': ['error', 20],
      'max-depth': ['error', 2],
      'max-params': ['error', 3],
    }
  }
);

---

Testing with Jest

// tests/unit/services/user.test.ts
import { calculateTotal } from '../../../src/core/services/pricing';

describe('calculateTotal', () => {
  it('returns sum of item prices', () => {
    // Arrange
    const items = [{ price: 10 }, { price: 20 }];

    // Act
    const result = calculateTotal(items);

    // Assert
    expect(result).toBe(30);
  });

  it('returns zero for empty array', () => {
    expect(calculateTotal([])).toBe(0);
  });

  it('throws on invalid item', () => {
    expect(() => calculateTotal([{ invalid: 'item' }])).toThrow();
  });
});