What are modules in TypeScript?

TypeScript uses JavaScript's module system with enhanced type support. A file is a module if it has at least one top-level import or export statement; otherwise it is a script (global scope). Exports: export const PI = 3.14;, export function greet() {}, export interface User {}, export default class App {}. Imports: import { PI, greet } from "./utils";, import App from "./App";, import * as Utils from "./utils";. TypeScript also supports type-only imports/exports (TS 3.8+): import type { User } from "./types"; — these are completely erased at compile time, which helps bundlers optimize tree-shaking. The module option in tsconfig controls the module format (commonjs, es2020, node16, etc.). TypeScript understands declaration files (.d.ts) that provide type information for JavaScript packages without rewriting them.