# TypeScript Modules and Namespaces

Organizing code is crucial for maintainability. TypeScript provides two main ways to organize code: **Modules** (based on the ES6 standard) and **Namespaces** (a TypeScript-specific way to group code).

## 1. Modules (Recommended)

Modules are the standard way to organize code in modern JavaScript and TypeScript. A file is considered a module if it contains at least one top-level `import` or `export`. Variables, functions, classes, etc., are private to the module unless explicitly exported.

### Exporting

```typescript
// mathUtils.ts
export function add(x: number, y: number): number {
  return x + y;
}

export const PI = 3.14;

// Default export
export default class Calculator {
  // ...
}
```

### Importing

```typescript
// app.ts
import Calculator, { add, PI } from "./mathUtils";

console.log(add(10, 5));
const calc = new Calculator();
```

### Renaming Imports

```typescript
import { add as sum } from "./mathUtils";
```

## 2. Namespaces (Legacy/Specific Use Cases)

Namespaces (formerly "Internal Modules") are a TypeScript-specific way to organize code using the `namespace` keyword. They compile into JavaScript objects (IIFEs) to prevent global scope pollution.

They are useful for grouping utilities logically without using a module loader, or for structuring type definitions (`.d.ts`) for third-party libraries.

### Syntax

```typescript
// Validation.ts
namespace Validation {
  export interface StringValidator {
    isAcceptable(s: string): boolean;
  }

  const lettersRegexp = /^[A-Za-z]+$/;

  export class LettersOnlyValidator implements StringValidator {
    isAcceptable(s: string) {
      return lettersRegexp.test(s);
    }
  }
}

// Usage
let validators: { [s: string]: Validation.StringValidator; } = {};
validators["Letters only"] = new Validation.LettersOnlyValidator();
```

### Multi-file Namespaces

You can split namespaces across files using reference tags (though this is rarely used in modern projects).

```typescript
/// <reference path="Validation.ts" />
```

## 3. Modules vs. Namespaces

| Feature | Modules | Namespaces |
| :--- | :--- | :--- |
| **Standard** | ECMAScript Standard (ES6) | TypeScript Specific |
| **Loading** | Handled by Node.js or Bundlers (Webpack) | Just global objects (script tags) |
| **Recommendation** | **Use for almost everything** | Use for legacy apps or `.d.ts` files |

[[programming/javascript/typescript/typescript]]