ES Modules in Node.js

ES Modules (ESM) is the modern JavaScript module system. Node.js supports it fully.

Enabling ES Modules

**Option 1:** Use .mjs extension ```javascript // math.mjs export function add(a, b) { return a + b; } ```

**Option 2:** Add "type": "module" to package.json ```json { "name": "my-app", "type": "module" } ```

Now .js files use ESM: ```javascript // math.js (with "type": "module") export function add(a, b) { return a + b; } ```

Named Exports

```javascript // utils.js export function formatDate(date) { return date.toISOString(); }

export function capitalize(str) { return str.charAt(0).toUpperCase() + str.slice(1); }

export const VERSION = '1.0.0'; ```

```javascript // app.js import { formatDate, capitalize, VERSION } from './utils.js';

console.log(capitalize('hello')); // Hello console.log(VERSION); // 1.0.0 ```

Default Export

```javascript // User.js export default class User { constructor(name) { this.name = name; } } ```

```javascript // app.js import User from './User.js'; // No curly braces

const user = new User('Alice'); ```

Mixed Exports

```javascript // api.js export default function fetchData() { /* ... */ } export const BASE_URL = 'https://api.example.com'; export function formatResponse(data) { /* ... */ } ```

```javascript // app.js import fetchData, { BASE_URL, formatResponse } from './api.js'; ```

Import Everything

```javascript import * as utils from './utils.js';

utils.formatDate(new Date()); utils.capitalize('hello'); console.log(utils.VERSION); ```

CommonJS vs ES Modules

| Feature | CommonJS | ES Modules | |---------|----------|------------| | Syntax | require/module.exports | import/export | | Loading | Synchronous | Asynchronous | | File extension | .js | .mjs or "type": "module" | | Top-level await | ❌ | ✅ |

Importing CommonJS in ESM

```javascript // ESM file importing CommonJS import express from 'express'; // Works!

// Named imports might not work import { Router } from 'express'; // May fail const { Router } = express; // Use this instead ```

File Extensions Required

ESM requires full file paths: ```javascript // ❌ Won't work in ESM import { add } from './math';

// ✅ Works import { add } from './math.js'; ```

Key Takeaway

ES Modules is the modern standard. Use `export` to share code, `import` to use it. Enable with `"type": "module"` in package.json or .mjs files. Remember to include .js extensions in imports!