---

name: "TypeScript" description: "TypeScript language rules with strict mode, testing, linting, and CI/CD best practices" version: "1.0.0" category: "languages" author: "Rulebook" tags: ["typescript", "javascript", "node", "strict", "testing", "vitest", "eslint"] dependencies: [] conflicts: []

TypeScript Project Rules

Agent Automation Commands

CRITICAL: Execute these commands after EVERY implementation (see AGENT_AUTOMATION module for full workflow).

# Complete quality check sequence:
npm run type-check        # Type checking
npm run lint              # Linting (0 warnings required)
npm run format            # Code formatting
npm test                  # All tests (100% pass required)
npm run test:coverage     # Coverage check (95%+ required)
npm run build             # Build verification

# Security audit:
npm audit --production    # Vulnerability scan
npm outdated              # Check outdated deps (informational)

TypeScript Configuration

CRITICAL: Use TypeScript 5.3+ with strict mode enabled.

• Version: TypeScript 5.3+
• Mode: Strict mode enabled
• Target: ES2022 or later
• Module: ESNext with Node16 module resolution

tsconfig.json Requirements

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "node",
    "strict": true,
    "esModuleInterop": true,
    "skipLibCheck": true,
    "forceConsistentCasingInFileNames": true,
    "resolveJsonModule": true,
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

Code Quality Standards

Mandatory Quality Checks

CRITICAL: After implementing ANY feature, you MUST run these commands in order.

IMPORTANT: These commands MUST match your GitHub Actions workflows to prevent CI/CD failures!

# Pre-Commit Checklist (MUST match .github/workflows/*.yml)

# 1. Type check (matches workflow)
npm run type-check  # or: tsc --noEmit

# 2. Lint (MUST pass with no warnings - matches workflow)
npm run lint

# 3. Format check (matches workflow - use same command as CI)
npx prettier --check 'src/**/*.ts' 'tests/**/*.ts'

# 4. Run all tests (MUST pass 100% - matches workflow)
npm test

# 5. Build (MUST succeed - matches workflow)
npm run build

# 6. Check coverage (MUST meet threshold)
npm run test:coverage

If ANY of these fail, you MUST fix the issues before committing.

Linting

• Use ESLint with TypeScript plugin
• Configuration in eslint.config.js or .eslintrc.json
• Must pass with no warnings: eslint src/**/*.ts
• Fix automatically when possible: eslint src/**/*.ts --fix

Example ESLint config:

{
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended"
  ],
  "parser": "@typescript-eslint/parser",
  "plugins": ["@typescript-eslint"],
  "rules": {
    "@typescript-eslint/no-unused-vars": ["error", { "argsIgnorePattern": "^_" }],
    "@typescript-eslint/explicit-function-return-type": "warn",
    "@typescript-eslint/no-explicit-any": "warn"
  }
}

Formatting

• Use Prettier for code formatting
• Configuration in .prettierrc.json
• Integrate with ESLint for consistency
• Format before committing: prettier --write "src/**/*.ts"

Example Prettier config:

{
  "semi": true,
  "trailingComma": "es5",
  "singleQuote": true,
  "printWidth": 100,
  "tabWidth": 2
}

Testing

• Framework: Vitest (recommended) or Jest
• Location: /tests directory or co-located *.test.ts files
• Coverage: Must meet project threshold (default 95%)
• Watch Mode: Use vitest or vitest --watch for development
• CI Mode: CRITICAL - Default npm test command MUST include --run flag
  • This prevents Vitest from entering watch mode, which never terminates
  • In package.json: "test": "vitest --run"
  • In CI workflows: use npm test (do NOT add --run argument)
  • For manual development, use npm run test:watch

Example test structure:

import { describe, it, expect, beforeEach, afterEach } from 'vitest';
import { myFunction } from './my-module';

describe('myFunction', () => {
  beforeEach(() => {
    // Setup
  });

  afterEach(() => {
    // Cleanup
  });

  it('should handle valid input', () => {
    const result = myFunction('input');
    expect(result).toBe('expected');
  });

  it('should throw on invalid input', () => {
    expect(() => myFunction('')).toThrow('Invalid input');
  });
});

Package Management

CRITICAL: Use consistent package manager across team.

• Default: npm (most compatible, built-in)
• Alternative: pnpm (fast, disk-efficient) or yarn
• Lockfile: Always commit lockfile (package-lock.json, pnpm-lock.yaml, or yarn.lock)
• Workspaces: Use for monorepos
• CI/CD: Update GitHub Actions workflows to match your package manager

Dependencies

1. Check for latest versions:

   • Use Context7 MCP tool if available
   • Check npm registry: npm view <package> versions
   • Review changelog for breaking changes

2. Dependency Guidelines:

   • Use exact versions for applications ("1.2.3")
   • Use semver for libraries ("^1.2.3")
   • Keep dependencies updated regularly
   • Use npm audit or pnpm audit for security
   • Don't use deprecated packages
   • Don't add unnecessary dependencies

Type Safety

• No any: Avoid any type - use unknown and type guards
• Strict null checks: Handle null/undefined explicitly
• Type assertions: Minimize use of as - prefer type guards
• Generics: Use for reusable type-safe code

Example type-safe code:

// Good: Type guard
function isString(value: unknown): value is string {
  return typeof value === 'string';
}

function process(input: unknown): string {
  if (isString(input)) {
    return input.toUpperCase();
  }
  throw new Error('Invalid input');
}

// Bad: Type assertion
function processUnsafe(input: unknown): string {
  return (input as string).toUpperCase(); // Runtime error if not string
}

Error Handling

• Create custom error classes
• Use type guards for error checking
• Document errors in JSDoc/TSDoc
• Never swallow errors silently

Example:

export class ValidationError extends Error {
  constructor(
    message: string,
    public readonly field: string
  ) {
    super(message);
    this.name = 'ValidationError';
  }
}

export function validate(data: unknown): Data {
  if (!isValidData(data)) {
    throw new ValidationError('Invalid data structure', 'data');
  }
  return data;
}

Project Structure

project/
├── package.json        # Package manifest
├── tsconfig.json       # TypeScript config
├── vitest.config.ts    # Test config
├── README.md           # Project overview
├── CHANGELOG.md        # Version history
├── AGENTS.md           # AI assistant rules
├── src/
│   ├── index.ts        # Main entry point
│   ├── types.ts        # Type definitions
│   └── ...
├── tests/              # Test files
├── dist/               # Compiled output (gitignored)
└── docs/               # Project documentation

Module System

• Use ES modules (import/export)
• Set "type": "module" in package.json
• Use .js extensions in imports for Node.js compatibility
• Configure moduleResolution: "node" in tsconfig.json

Example:

// Good: ES modules with .js extension
import { myFunction } from './my-module.js';

export { myFunction };
export default class MyClass {}

CI/CD Requirements

CRITICAL: GitHub Actions cache: 'npm' requires package-lock.json to be committed.

• Lockfile Requirement: Remove package-lock.json from .gitignore
• Cache Strategy: Use cache: 'npm' in setup-node action
• Install Command: Use npm ci (not npm install) for reproducible builds

Must include GitHub Actions workflows for:

1. Testing (typescript-test.yml)
2. Linting (typescript-lint.yml)
3. Build (typescript-build.yml)