name: writing-typescript
description: Idiomatic TypeScript development. Use when writing TypeScript code, Node.js services, React apps, or discussing TS patterns. Emphasizes strict typing, composition, and modern tooling (bun/vite).
allowed-tools: Read, Bash, Grep, Glob

TypeScript Development (2025)

Core Principles

• Strict typing: Enable all strict checks
• Parse, don't validate: Transform untrusted data at boundaries
• Composition over inheritance: Small, focused functions
• Explicit over implicit: No any, prefer unknown

Toolchain

bun          # Runtime + package manager (fast)
vite         # Frontend bundling
vitest       # Testing
eslint       # Linting
prettier     # Formatting

Quick Patterns

Type Guards

function isUser(value: unknown): value is User {
  return typeof value === "object" && value !== null && "id" in value;
}

Discriminated Unions

type Result<T, E = Error> = { ok: true; value: T } | { ok: false; error: E };

function processResult<T>(result: Result<T>): T {
  if (result.ok) return result.value;
  throw result.error;
}

Utility Types

type UserUpdate = Partial<User>;
type UserSummary = Pick<User, "id" | "name">;
type UserWithoutPassword = Omit<User, "password">;
type ReadonlyUser = Readonly<User>;

tsconfig.json Essentials

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "moduleResolution": "bundler",
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noImplicitReturns": true,
    "isolatedModules": true
  }
}

References

• PATTERNS.md - Code patterns and style
• REACT.md - React component patterns
• TESTING.md - Testing with vitest

Commands

bun install              # Install deps
bun run build            # Build
bun test                 # Test
bun run lint             # Lint
bun run format           # Format