ES Modules in TypeScript

// Named exports
export interface User {
  id: number;
  name: string;
  email: string;
}

export type UserRole = "admin" | "editor" | "viewer";

export function createUser(name: string, email: string): User {
  return { id: Date.now(), name, email };
}

export const MAX_USERS = 1000;

// Default export (one per file)
export default class UserService {
  private users: User[] = [];

  add(user: User): void {
    this.users.push(user);
  }

  findById(id: number): User | undefined {
    return this.users.find(u => u.id === id);
  }
}

// Importing
import UserService, { User, UserRole, createUser } from "./userService";
import type { User } from "./userService"; // Type-only import (erased at runtime)

// Rename imports
import { User as UserType } from "./userService";

// Import everything
import * as UserModule from "./userService";
const user: UserModule.User = UserModule.createUser("Alice", "a@test.com");

// Re-export (barrel file — index.ts)
export { User, UserRole, createUser } from "./userService";
export { Product, ProductCategory } from "./productService";
export { OrderService } from "./orderService";
// Consumers import from one place: import { User, Product } from "./models";

// Dynamic imports (code splitting)
async function loadEditor() {
  const { Editor } = await import("./editor");
  return new Editor();
}

// Type-only exports
export type { User, UserRole }; // Only exports the types, not values