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name: typescript-pro description: Implements advanced TypeScript type systems, creates custom type guards, utility types, and branded types, and configures tRPC for end-to-end type safety. Use when building TypeScript applications requiring advanced generics, conditional or mapped types, discriminated unions, monorepo setup, or full-stack type safety with tRPC. license: MIT metadata: author: https://github.com/Jeffallan version: "1.1.0" domain: language triggers: TypeScript, generics, type safety, conditional types, mapped types, tRPC, tsconfig, type guards, discriminated unions role: specialist scope: implementation output-format: code related-skills: fullstack-guardian, api-designer

TypeScript Pro

Core Workflow

1. Analyze type architecture - Review tsconfig, type coverage, build performance
2. Design type-first APIs - Create branded types, generics, utility types
3. Implement with type safety - Write type guards, discriminated unions, conditional types; run tsc --noEmit to catch type errors before proceeding
4. Optimize build - Configure project references, incremental compilation, tree shaking; re-run tsc --noEmit to confirm zero errors after changes
5. Test types - Confirm type coverage with a tool like type-coverage; validate that all public APIs have explicit return types; iterate on steps 3–4 until all checks pass

Reference Guide

Load detailed guidance based on context:

| Topic | Reference | Load When | |-------|-----------|-----------| | Advanced Types | references/advanced-types.md | Generics, conditional types, mapped types, template literals | | Type Guards | references/type-guards.md | Type narrowing, discriminated unions, assertion functions | | Utility Types | references/utility-types.md | Partial, Pick, Omit, Record, custom utilities | | Configuration | references/configuration.md | tsconfig options, strict mode, project references | | Patterns | references/patterns.md | Builder pattern, factory pattern, type-safe APIs |

Code Examples

Branded Types

// Branded type for domain modeling
type Brand<T, B extends string> = T & { readonly __brand: B };
type UserId  = Brand<string, "UserId">;
type OrderId = Brand<number, "OrderId">;

const toUserId  = (id: string): UserId  => id as UserId;
const toOrderId = (id: number): OrderId => id as OrderId;

// Usage — prevents accidental id mix-ups at compile time
function getOrder(userId: UserId, orderId: OrderId) { /* ... */ }

Discriminated Unions & Type Guards

type LoadingState = { status: "loading" };
type SuccessState = { status: "success"; data: string[] };
type ErrorState   = { status: "error";   error: Error };
type RequestState = LoadingState | SuccessState | ErrorState;

// Type predicate guard
function isSuccess(state: RequestState): state is SuccessState {
  return state.status === "success";
}

// Exhaustive switch with discriminated union
function renderState(state: RequestState): string {
  switch (state.status) {
    case "loading": return "Loading…";
    case "success": return state.data.join(", ");
    case "error":   return state.error.message;
    default: {
      const _exhaustive: never = state;
      throw new Error(`Unhandled state: ${_exhaustive}`);
    }
  }
}

Custom Utility Types

// Deep readonly — immutable nested objects
type DeepReadonly<T> = {
  readonly [K in keyof T]: T[K] extends object ? DeepReadonly<T[K]> : T[K];
};

// Require exactly one of a set of keys
type RequireExactlyOne<T, Keys extends keyof T = keyof T> =
  Pick<T, Exclude<keyof T, Keys>> &
  { [K in Keys]-?: Required<Pick<T, K>> & Partial<Record<Exclude<Keys, K>, never>> }[Keys];

Recommended tsconfig.json

{
  "compilerOptions": {
    "target": "ES2022",
    "module": "NodeNext",
    "moduleResolution": "NodeNext",
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "exactOptionalPropertyTypes": true,
    "isolatedModules": true,
    "declaration": true,
    "declarationMap": true,
    "incremental": true,
    "skipLibCheck": false
  }
}

Constraints

MUST DO

• Enable strict mode with all compiler flags
• Use type-first API design
• Implement branded types for domain modeling
• Use satisfies operator for type validation
• Create discriminated unions for state machines
• Use Annotated pattern with type predicates
• Generate declaration files for libraries
• Optimize for type inference

MUST NOT DO

• Use explicit any without justification
• Skip type coverage for public APIs
• Mix type-only and value imports
• Disable strict null checks
• Use as assertions without necessity
• Ignore compiler performance warnings
• Skip declaration file generation
• Use enums (prefer const objects with as const)

Output Templates

When implementing TypeScript features, provide:

1. Type definitions (interfaces, types, generics)
2. Implementation with type guards
3. tsconfig configuration if needed
4. Brief explanation of type design decisions

Knowledge Reference

TypeScript 5.0+, generics, conditional types, mapped types, template literal types, discriminated unions, type guards, branded types, tRPC, project references, incremental compilation, declaration files, const assertions, satisfies operator