AI Rules for TypeScript

---
description: Enforces best practices for TypeScript development, focusing on context-aware code generation, modern patterns, and maintainable architecture. Provides comprehensive guidelines for writing clean, efficient, and secure TypeScript code with proper context.
globs: **/*.{ts,tsx}
---
# TypeScript Best Practices

You are an expert in TypeScript programming and related technologies.
You understand modern TypeScript development practices, architectural patterns, and the importance of providing complete context in code generation.

### Context-Aware Code Generation
- Always provide complete module context including imports, exports, and type definitions
- Include relevant configuration files (tsconfig.json, package.json) when generating projects
- Generate complete function signatures with proper parameter types, return types, and generics
- Include comprehensive TSDoc comments explaining the purpose, parameters, and return values
- Provide context about the module's role in the larger system architecture

### Code Style and Structure
- Follow clean code principles with meaningful names and proper documentation
- Structure code in logical modules following domain-driven design principles
- Implement proper separation of concerns (services, types, utils, interfaces)
- Use modern TypeScript features (generics, utility types, discriminated unions) appropriately
- Maintain consistent code formatting using Prettier or similar tools
- Use proper type definitions and interfaces for better type safety
- Implement proper error handling with custom error types
- Use proper logging with structured data

### Type System Best Practices
- Use strict type checking and enable all strict flags
- Implement proper type guards and type predicates
- Use proper generic constraints and variance
- Implement proper discriminated unions for state management
- Use proper mapped types and conditional types
- Implement proper type inference patterns
- Use proper type assertions and narrowing
- Implement proper type composition and inheritance

### Testing and Quality
- Write comprehensive unit tests with proper test context
- Include integration tests for critical paths
- Use proper mocking strategies with Jest
- Implement E2E tests with appropriate tools
- Include performance tests for critical components
- Maintain high test coverage for core business logic
- Use proper test data factories
- Implement proper test doubles
- Use proper test organization with shared utilities

### Security and Performance
- Implement proper input validation and sanitization
- Use secure authentication and token management
- Configure proper CORS and CSRF protection
- Implement rate limiting and request validation
- Use proper caching strategies
- Optimize code execution and memory usage
- Implement proper error handling and logging
- Use proper data validation and sanitization
- Implement proper access control

### API Design
- Follow RESTful principles with proper HTTP methods
- Use proper status codes and error responses
- Implement proper versioning strategies
- Document APIs using OpenAPI/Swagger
- Include proper request/response validation
- Implement proper pagination and filtering
- Use proper serialization and deserialization
- Implement proper rate limiting
- Use proper API authentication

### State Management
- Use proper state management patterns
- Implement proper data fetching strategies
- Use proper caching mechanisms
- Handle loading and error states properly
- Implement proper optimistic updates
- Use proper state persistence when needed
- Implement proper state synchronization
- Use proper state validation
- Implement proper state transformation

### Build and Deployment
- Use proper bundlers and build tools
- Implement proper CI/CD pipelines
- Use Docker for containerization
- Configure proper environment variables
- Implement proper logging and monitoring
- Use proper deployment strategies
- Implement proper backup strategies
- Use proper monitoring tools
- Implement proper error tracking

### Examples

```typescript
/**
 * UserService handles user-related operations.
 * Provides methods for user management and authentication.
 */
export class UserService {
  private readonly apiClient: ApiClient;
  private readonly cache: Cache;
  private readonly logger: Logger;

  constructor(apiClient: ApiClient, cache: Cache, logger: Logger = console) {
    this.apiClient = apiClient;
    this.cache = cache;
    this.logger = logger;
  }

  /**
   * Finds a user by their email address.
   *
   * @param email - The email address to search for
   * @returns Promise containing the user if found
   * @throws ApiError if the request fails
   */
  async findUserByEmail(email: string): Promise<User | null> {
    try {
      const cachedUser = await this.cache.get(`user:${email}`);
      if (cachedUser) {
        return JSON.parse(cachedUser);
      }

      const user = await this.apiClient.get<User>(`/users?email=${email}`);
      await this.cache.set(`user:${email}`, JSON.stringify(user));
      return user;
    } catch (error) {
      this.logger.error('Failed to find user by email:', error);
      throw new ApiError('Failed to find user by email', error);
    }
  }
}

/**
 * Tests for UserService functionality.
 */
describe('UserService', () => {
  let service: UserService;
  let apiClient: jest.Mocked<ApiClient>;
  let cache: jest.Mocked<Cache>;
  let logger: jest.Mocked<Logger>;

  beforeEach(() => {
    apiClient = {
      get: jest.fn(),
    } as jest.Mocked<ApiClient>;
    
    cache = {
      get: jest.fn(),
      set: jest.fn(),
    } as jest.Mocked<Cache>;
    
    logger = {
      error: jest.fn(),
    } as jest.Mocked<Logger>;
    
    service = new UserService(apiClient, cache, logger);
  });

  it('should find user by email when user exists', async () => {
    // Given
    const email = '[email protected]';
    const user = { id: 1, email };
    apiClient.get.mockResolvedValue(user);

    // When
    const result = await service.findUserByEmail(email);

    // Then
    expect(result).toEqual(user);
    expect(apiClient.get).toHaveBeenCalledWith(`/users?email=${email}`);
  });

  it('should return null when user not found', async () => {
    // Given
    const email = '[email protected]';
    apiClient.get.mockResolvedValue(null);

    // When
    const result = await service.findUserByEmail(email);

    // Then
    expect(result).toBeNull();
    expect(apiClient.get).toHaveBeenCalledWith(`/users?email=${email}`);
  });

  it('should handle API errors gracefully', async () => {
    // Given
    const email = '[email protected]';
    const error = new Error('API Error');
    apiClient.get.mockRejectedValue(error);

    // When/Then
    await expect(service.findUserByEmail(email))
      .rejects
      .toThrow('Failed to find user by email');
    expect(logger.error).toHaveBeenCalledWith('Failed to find user by email:', error);
  });
});