How do modules work in TypeScript?

The distinction between a module and a script is determined by syntax, not by a file extension. Adding an import or export makes a file a module. Everything in a module is scoped to that file unless it is exported, which avoids accidental globals.

export type User = { id: number };

export function findUser(id: number): User | undefined {
  return id === 1 ? { id } : undefined;
}

import { findUser } from "./user-service";
import type { User } from "./user-service";

const user: User | undefined = findUser(1);

Import and resolution details

• import type is erased and avoids a runtime import
• verbatimModuleSyntax preserves import syntax and requires explicit type imports
• isolatedModules requires each file to be independently transpilable
• moduleResolution controls how bare and relative specifiers are resolved
• Path aliases must be configured in both tsconfig and the bundler or runtime

TypeScript does not rewrite module specifiers by default. If tsconfig maps an alias such as @app/, the bundler, test runner or Node.js loader must understand the same alias. Keeping tsconfig and the runtime resolver aligned prevents imports that type-check but fail at runtime.