Modules & npm

Two module systems, one ecosystem

Node.js ships with two module systems. CommonJS (CJS) is the original system and uses require() to load modules synchronously. ECMAScript Modules (ESM) is the modern standard, shared with browsers, and uses import / export statements.

Both systems are fully supported in current Node.js versions. Which one a file uses depends on the file extension and the "type" field in the nearest package.json.

CJS vs ESM at a glance

	CommonJS	ESM
Syntax	`require()` / `module.exports`	`import` / `export`
File extension	`.js` (default) or `.cjs`	`.mjs`, or `.js` when `"type": "module"`
`"type"` in package.json	`"commonjs"` (or absent)	`"module"`
Loading	Synchronous	Asynchronous (static analysis)
Top-level `await`	Not supported	Supported

What this module covers

Lesson	Topic
Overview (this page)	CJS vs ESM comparison and module system basics
package.json Deep Dive	Key fields, semver ranges, and `npm init`
Module Resolution	How Node finds files, the `exports` map, and bare specifiers
Managing Dependencies	Installing, updating, and auditing packages with npm
Publishing Packages	Preparing and publishing a package to the npm registry

ESM in action

The snippet below uses ESM import to load the built-in path module and print the platform path separator — the character Node uses between directory components (/ on Unix, \ on Windows):

Node.js

import path from 'node:path';

console.log('path.sep:', path.sep);
console.log('Joined path:', path.join('src', 'content', 'index.mdx'));

Needs the Node.js runtime — open in StackBlitz to run.