TypeScript at Scale: Type-Safe Architecture

// Monorepo type structure
// packages/
//   types/
//     shared/          # Cross-cutting types
//     api/            # API contract types
//     domain/         # Domain-specific types
//   apps/
//     web/           # Web app (uses types package)
//     mobile/        # Mobile app (uses types package)

// packages/types/shared/index.ts
export interface BaseEntity {
  id: string;
  createdAt: Date;
  updatedAt: Date;
}

export type ID<T extends BaseEntity> = T["id"];

// packages/types/api/contracts.ts
export interface ApiResponse<T> {
  data: T;
  meta?: {
    pagination?: {
      page: number;
      limit: number;
      total: number;
    };
  };
  errors?: ApiError[];
}

export interface ApiError {
  code: string;
  message: string;
  field?: string;
}

// packages/types/domain/user.ts
import { BaseEntity } from "../shared";

export interface User extends BaseEntity {
  email: string;
  name: string;
  role: "admin" | "user" | "guest";
  preferences: UserPreferences;
}

export interface UserPreferences {
  theme: "light" | "dark";
  notifications: boolean;
}

// Type boundaries
// Each domain owns its types
// Cross-domain types go in shared
// API types are separate from domain types

Organize types in a monorepo with clear boundaries. Separate shared types, domain types, and API contract types. Establish ownership and versioning strategies for types at scale.

// API contract types
// contracts/api/users.ts
export interface GetUserRequest {
  userId: string;
}

export interface GetUserResponse {
  user: User;
}

export interface CreateUserRequest {
  email: string;
  name: string;
  role?: UserRole;
}

export interface CreateUserResponse {
  user: User;
  token: string;
}

// Contract validation
type RequiredFields<T, K extends keyof T> = T & Required<Pick<T, K>>;

function validateRequest<T>(
  request: unknown,
  schema: ZodSchema<T>
): T {
  return schema.parse(request);
}

// Type-safe API client
type ApiEndpoints = {
  "GET /users/:id": {
    params: { id: string };
    response: GetUserResponse;
  };
  "POST /users": {
    body: CreateUserRequest;
    response: CreateUserResponse;
  };
};

class TypeSafeApiClient {
  async request<
    E extends keyof ApiEndpoints,
    R = ApiEndpoints[E]["response"]
  >(
    endpoint: E,
    options: E extends keyof ApiEndpoints
      ? ApiEndpoints[E] extends { params: infer P }
        ? { params: P }
        : ApiEndpoints[E] extends { body: infer B }
        ? { body: B }
        : {}
      : never
  ): Promise<R> {
    // Implementation
    return {} as R;
  }
}

// Usage - fully type-safe
const client = new TypeSafeApiClient();
const user = await client.request("GET /users/:id", {
  params: { id: "123" },
});
// user is GetUserResponse

// GraphQL type generation
// schema.graphql -> generated/types.ts (via codegen)
// Automatic type generation from GraphQL schema
// Ensures types stay in sync with schema

API contract types ensure type safety across services. Define contracts explicitly, generate types from schemas (OpenAPI, GraphQL), and use type-safe clients. This catches breaking changes at compile time and keeps services in sync.

// packages/@company/types/package.json
{
  "name": "@company/types",
  "version": "2.0.0",
  "types": "./dist/index.d.ts",
  "exports": {
    ".": "./dist/index.js",
    "./api": "./dist/api/index.d.ts",
    "./domain": "./dist/domain/index.d.ts"
  },
  "dependencies": {},
  "peerDependencies": {}
}

// packages/@company/types/index.ts
// Public API - carefully curated exports
export * from "./shared";
export * from "./api";
export * from "./domain";

// Versioned API types
export * as v1 from "./api/v1";
export * as v2 from "./api/v2";

// Usage in apps
import { User, ApiResponse } from "@company/types";
import { GetUserResponse } from "@company/types/api/v2";

// Type-only imports (tree-shakeable)
import type { User } from "@company/types";

// Module augmentation for extending types
declare module "@company/types" {
  interface User {
    customField?: string;
  }
}

// Type guards in shared library
export function isUser(value: unknown): value is User {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value &&
    "email" in value
  );
}

Shared type libraries provide organization-wide type safety. Design clear public APIs, version types carefully, support multiple versions during migrations, and enable module augmentation for extensibility. Type-only packages have no runtime cost.

// Type governance checklist

// 1. Type Review Process
// - All exported types reviewed
// - Breaking changes documented
// - Migration paths provided

// 2. Naming Conventions
// - Interfaces: PascalCase (User, ApiResponse)
// - Types: PascalCase (UserRole, ApiEndpoint)
// - Type parameters: Single uppercase letter (T, K, V)

// 3. Documentation Standards
/**
 * Represents a user in the system.
 * 
 * @example
 * const user: User = {
 *   id: "123",
 *   email: "user@example.com",
 *   name: "John Doe"
 * };
 */
export interface User {
  /** Unique identifier */
  id: string;
  /** User's email address */
  email: string;
  /** User's full name */
  name: string;
}

// 4. Complexity Guidelines
// - Prefer composition over complex types
// - Limit conditional type nesting
// - Use utility types instead of inline complex types
// - Extract complex types to named types

// 5. Breaking Change Detection
// tools/check-breaking-changes.ts
import { compareTypes } from "./type-comparator";

const oldTypes = loadTypes("v1.0.0");
const newTypes = loadTypes("v2.0.0");

const breakingChanges = compareTypes(oldTypes, newTypes);

if (breakingChanges.length > 0) {
  console.error("Breaking changes detected:", breakingChanges);
  process.exit(1);
}

// 6. Type Coverage
// Track type coverage across codebase
// Enforce "no any" policy
// Monitor type errors in CI

Type governance ensures quality and consistency at scale. Establish review processes, naming conventions, documentation standards, complexity guidelines, and automated checking. Monitor breaking changes and type coverage across the organization.

// Multi-version support
// Support multiple API versions simultaneously
type ApiVersion = "v1" | "v2" | "v3";

type VersionedApi<T extends ApiVersion> = {
  v1: V1ApiTypes;
  v2: V2ApiTypes;
  v3: V3ApiTypes;
}[T];

function createApiClient<T extends ApiVersion>(version: T): ApiClient<VersionedApi<T>> {
  // Implementation
}

const v2Client = createApiClient("v2"); // Type-safe v2 client

// Backward compatibility types
type BackwardCompatible<T, U> = T & Partial<U>;

// Migration utilities
type Migrate<T, U> = {
  from: T;
  to: U;
  migrate: (value: T) => U;
};

// Cross-service type sharing
// services/
//   user-service/types.ts
//   order-service/types.ts
//   shared-types/        # Shared across services

// Type registry for discovery
export const TypeRegistry = {
  User: import("@company/types/domain/user"),
  Order: import("@company/types/domain/order"),
  // ... more types
} as const;

// Type-safe service communication
type ServiceContract = {
  "user-service": {
    "getUser": (id: string) => Promise<User>;
    "createUser": (data: CreateUserRequest) => Promise<User>;
  };
  "order-service": {
    "createOrder": (data: CreateOrderRequest) => Promise<Order>;
  };
};

function callService<
  S extends keyof ServiceContract,
  M extends keyof ServiceContract[S]
>(
  service: S,
  method: M,
  ...args: Parameters<ServiceContract[S][M]>
): ReturnType<ServiceContract[S][M]> {
  // Implementation
}

Enterprise patterns handle complex scenarios: multi-version support, backward compatibility, gradual migrations, and cross-service type sharing. Design type systems that evolve safely while maintaining compatibility and type safety.