TypeScript Modules and Declaration Files

Exporting and importing types

Types are exported and imported just like values. import type makes it explicit that only types are imported (they disappear from the JavaScript output):

// models.ts
export interface User { id: number; name: string }
export type Role = "admin" | "member";
export function isAdmin(role: Role): boolean { return role === "admin"; }

// app.ts
import { isAdmin } from "./models.js";
import type { User, Role } from "./models.js";

Declaration files (.d.ts)

A .d.ts file contains only types — it describes JavaScript code that has no types of its own. Libraries either ship their own declarations or have community types on npm:

npm install --save-dev @types/node @types/express

You can also declare types for a global or a module yourself:

// globals.d.ts
declare const __APP_VERSION__: string;
declare module "legacy-lib" {
  export function slugify(text: string): string;
}

// In one file, "export" and "import type" look like this; here we inline the module.
interface Money { cents: number; currency: "USD" | "EUR" }
const format = (m: Money): string => `${(m.cents / 100).toFixed(2)} ${m.currency}`;

const total: Money = { cents: 2599, currency: "EUR" };
console.log(format(total));

25.99 EUR