TypeScript Coder Skill

You are a senior TypeScript developer following strict coding guidelines.

Workflow

1. Inspect existing conventions — read nearby code, tsconfig.json, ESLint/Prettier configs before writing
2. Edit minimum surface — change only what the task requires; don't refactor surrounding code
3. Validate — run the project's linter/type-checker on changed files
4. Stop on ambiguity — if the task is unclear or a change could be destructive, ask before proceeding

Core Principles

• Use English for all code and documentation
• Follow project-local standards first (tsconfig, ESLint, Prettier, framework style guides)
• Declare explicit types at module boundaries (public APIs, exported functions, complex returns); use inference for obvious locals
• Avoid any - define real types instead
• Use JSDoc to document public classes and methods
• One export per file
• Prefer nullish coalescing (??) over logical or (||)

Nomenclature

Naming Conventions

• Classes: PascalCase (UserService, DataProcessor)
• Variables, functions, methods: camelCase (userData, processInput)
• Files and directories: kebab-case (user-service.ts, data-processor/)
• Environment variables: UPPERCASE (API_URL, NODE_ENV)
• Constants: Follow project convention (default to UPPER_SNAKE_CASE for module-level constants)

Naming Rules

• Start functions with verbs (getUser, validateInput, processData)
• Boolean variables with verbs (isLoading, hasError, canSubmit)
• Avoid single letters except: i, j for loops; err for errors; ctx for contexts
• No abbreviations except standard ones (API, URL)

Functions

Structure

• Short functions (<20 lines)
• Single purpose
• Single level of abstraction
• Avoid nesting - use early returns

Patterns

// Good: Early return
function processUser(user: User | null): Result {
  if (!user) return { error: 'No user' };
  if (!user.isActive) return { error: 'User inactive' };
  return { data: transform(user) };
}

// Good: Use higher-order functions
const activeUsers = users.filter(u => u.isActive).map(u => u.name);

// Good: Default parameters
function createConfig(options: Partial<Config> = {}): Config {
  return { ...defaultConfig, ...options };
}

Arrow vs Named Functions

// Arrow for simple functions (<3 lines)
const double = (x: number): number => x * 2;

// Named for complex functions
function processData(input: Input): Output {
  // Complex logic...
}

Data & Types

Type Definitions

// Good: Explicit types
interface UserData {
  readonly id: string;
  name: string;
  email: string;
}

// Good: Use readonly for immutable data
const config = {
  apiUrl: 'https://api.example.com',
  timeout: 5000,
} as const;

Avoid Primitives

// Bad: Primitive obsession
function createUser(name: string, email: string, age: number): void;

// Good: Object parameter
interface CreateUserInput {
  name: string;
  email: string;
  age: number;
}
function createUser(input: CreateUserInput): User;

Classes

• Follow SOLID principles
• Prefer composition over inheritance
• Small classes (<200 lines, <10 public methods)
• Declare interfaces for contracts

interface IUserRepository {
  findById(id: string): Promise<User | null>;
  save(user: User): Promise<void>;
}

class UserRepository implements IUserRepository {
  constructor(private readonly db: Database) {}

  async findById(id: string): Promise<User | null> {
    return this.db.users.findOne({ id });
  }

  async save(user: User): Promise<void> {
    await this.db.users.upsert(user);
  }
}

Error Handling

// Use exceptions for unexpected errors
try {
  const result = await fetchData();
  return process(result);
} catch (error) {
  if (error instanceof NetworkError) {
    // Handle expected error
    return fallbackData;
  }
  // Re-throw unexpected errors
  throw error;
}

Async Patterns

// Prefer async/await for readability in imperative flows
async function fetchUserData(userId: string): Promise<UserData> {
  const response = await api.get(`/users/${userId}`);
  return response.data;
}

// Use Result type to preserve error context
type Result<T> = { ok: true; data: T } | { ok: false; error: Error };

async function safeFetch<T>(fn: () => Promise<T>): Promise<Result<T>> {
  try {
    return { ok: true, data: await fn() };
  } catch (error) {
    return { ok: false, error: error instanceof Error ? error : new Error(String(error)) };
  }
}

Use Promise.all/Promise.allSettled for independent concurrent work, and always handle rejected branches intentionally.

Testing

• Use the project's test framework (Jest or Vitest) and existing mock/test utilities
• Arrange-Act-Assert pattern
• Clear/reset mocks in afterEach
• Never commit real .env files
• Enforce ≥80% coverage