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

// math.mjs
export function add(a, b) {
  return a + b;
}

Option 2: Add "type": "module" to package.json

{
  "name": "my-app",
  "type": "module"
}

Now .js files use ESM:

// math.js (with "type": "module")
export function add(a, b) {
  return a + b;
}

Named Exports

// 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';

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

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

Default Export

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

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

const user = new User('Alice');

Mixed Exports

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

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

Import Everything

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

// 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:

// ❌ 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!