What modularity systems exist in Node.js?

CommonJS and ECMAScript Modules (ESM).

CommonJS:

• Uses require() for importing modules and module.exports or exports for exporting.
• Loads modules synchronously.
• Was the standard for Node.js versions before 12.0.0.

// CommonJS import
const fs = require('fs');

// CommonJS export
module.exports = {
  readFile: fs.readFileSync
};

ECMAScript Modules (ESM):

• Uses import and export statements.
• Loads modules asynchronously.
• Is the standard for newer Node.js versions (starting from 12.0.0 with --experimental-modules flag, fully from 14.0.0 by default with .mjs extension or type: "module" in package.json).

// ESM import
import fs from 'fs';

// ESM export
export const readFile = fs.readFileSync;

Comparison:

Feature	CommonJS	ESM
Import	require()	import
Export	module.exports, exports	export
Loading	Synchronous	Asynchronous
Syntax	Dynamic (can be inside conditions)	Static (top-level only)

Note: ESM support in Node.js is evolving, and some features may vary depending on version and configuration.