How do ES Modules differ from CommonJS in Node.js, and what are the migration challenges?

Key Differences

Feature	CommonJS (CJS)	ES Modules (ESM)
Syntax	require() / module.exports	import / export
Loading	Synchronous	Asynchronous
Tree-shaking	❌ Not possible	✅ Static analysis
Top-level await	❌ No	✅ Yes
Execution	Runs at require time	Parsed, then executed
Mutability	Exports are copies	Exports are live bindings
this at top	module.exports	undefined

Interoperability

// CJS importing ESM (dynamic import only)
const { default: esmModule } = await import("./esm-module.mjs");

// ESM importing CJS (named imports may not work)
import cjsModule from "./cjs-module.cjs"; // ✅ default import works
import { specific } from "./cjs-module.cjs"; // ⚠️ May fail

Migration Challenges

1. __dirname and __filename not available in ESM:

// ESM replacement
import { fileURLToPath } from "url";
import { dirname } from "path";
const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);

2. JSON imports require assertion:

import pkg from "./package.json" with { type: "json" };

3. Jest ESM support: Use --experimental-vm-modules or switch to Vitest
4. Package.json: "type": "module" makes all .js files ESM by default

Best Practice for New Projects

// package.json
{ "type": "module" }

Use .mts/.mtsextensions for explicit ESM,.cts` for CJS.