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name: typescript-pro description: Advanced TypeScript development with full type safety. Use when: (1) designing complex generic types or utility types, (2) achieving end-to-end type safety (tRPC, GraphQL codegen), (3) optimizing TypeScript build performance, (4) migrating JavaScript to TypeScript, (5) authoring type-safe libraries, (6) debugging complex type errors.

TypeScript Pro

Workflow

1. Audit - Review tsconfig strictness, type coverage, build times
2. Design - Define domain types first, derive others with mapped/conditional types
3. Implement - Use type inference, avoid explicit types where inferred
4. Validate - Test types with expectType, check edge cases
5. Optimize - Profile tsc, use project references for large codebases

Strict Mode Essentials

Copy// tsconfig.json - non-negotiable settings
{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noImplicitReturns": true,
    "noFallthroughCasesInSwitch": true
  }
}

Type Patterns

Branded Types (nominal typing)

Copytype UserId = string & { readonly __brand: 'UserId' };
type OrderId = string & { readonly __brand: 'OrderId' };

// Prevents mixing up IDs even though both are strings
function getUser(id: UserId) { /* ... */ }

Discriminated Unions (state machines)

Copytype State =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: Data }
  | { status: 'error'; error: Error };

// Exhaustive checking with never
function handle(state: State) {
  switch (state.status) {
    case 'idle': return /* ... */;
    case 'loading': return /* ... */;
    case 'success': return state.data;
    case 'error': return state.error;
    default: return state satisfies never;
  }
}

Const Assertions

Copy// Without: routes is string[]
const routes = ['home', 'about', 'contact'];

// With: routes is readonly ['home', 'about', 'contact']
const routes = ['home', 'about', 'contact'] as const;
type Route = typeof routes[number]; // 'home' | 'about' | 'contact'

Build Performance

End-to-End Type Safety

CopyDatabase → ORM types → API types → Client types
         Prisma       tRPC/Zod    Inferred

• tRPC: Types flow from backend to frontend automatically
• Zod: Runtime validation + TypeScript types from one source
• Prisma: Database schema generates TypeScript types

Common Type Errors Decoded

Library Authoring Checklist

• declaration: true in tsconfig
• Export types explicitly in package.json exports
• Test types with @ts-expect-error comments
• Support both ESM and CJS if needed
• Document generic constraints in JSDoc