Modules and Namespaces

As TypeScript applications grow, organising code into modules becomes essential. TypeScript fully supports ECMAScript modules (ESM) and adds features such as type-only imports and ambient module declarations. This lesson covers the module system, namespaces (and why they are largely historical), and declaration files.

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ES Modules

TypeScript uses the standard ES module syntax. Any file containing a top-level import or export is treated as a module:

Exporting

// utils.ts
export function formatDate(date: Date): string {
  return date.toISOString().split('T')[0];
}

export const MAX_RETRIES = 3;

export interface Logger {
  log(message: string): void;
  error(message: string): void;
}

Importing

// app.ts
import { formatDate, MAX_RETRIES, Logger } from './utils';
import type { Logger } from './utils'; // type-only import (see below)

console.log(formatDate(new Date()));

Default Exports

A module can have one default export:

// database.ts
export default class Database {
  constructor(private connectionString: string) {}

  connect(): void {
    console.log(`Connecting to ${this.connectionString}`);
  }
}

// app.ts
import Database from './database';
const db = new Database('postgres://localhost:5432/mydb');

Re-exports

You can re-export from other modules to create barrel files: