What is the difference between CommonJS and ES Modules in Node?

Question

Accepted Answer

Node supports **two module systems**: **CommonJS (CJS)** — the original, using `require`/`module.exports` — and **ES Modules (ESM)** — the standard, using `import`/`export`. They differ in syntax, loading behavior, and interop.

## CommonJS — Node's original

```js
// math.js
function add(a, b) { return a + b; }
module.exports = { add };

// app.js
const { add } = require("./math"); // synchronous, runtime
```

## ES Modules — the standard

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

// app.mjs
import { add } from "./math.mjs"; // static, hoisted
```

## Key differences

```text
                 CommonJS              ES Modules
Syntax           require/module.exports import/export
Loading          synchronous, runtime  static, hoisted
Top-level await  no                    yes
Tree-shaking     hard                  yes (statically analyzable)
File extension   .cjs / .js (default)  .mjs / .js with "type":"module"
__dirname/__filename available         not available (use import.meta.url)
```

## Enabling ESM

```json
// package.json
{ "type": "module" }  // now .js files are treated as ESM
```

Or use the `.mjs` extension explicitly. Without this, Node defaults `.js` to CommonJS.

## Interop gotchas

```js
// ESM can import CommonJS:
import pkg from "some-cjs-lib"; // works (default import)

// CommonJS CANNOT require() a pure-ESM package (it's async):
const esm = require("esm-only-pkg"); // ❌ ERR_REQUIRE_ESM
// must use dynamic import: const esm = await import("esm-only-pkg");
```

The rough edge: CJS can't `require()` ESM-only packages — a common migration pain. Also note ESM lacks `__dirname` (use `import.meta.url`).

## Why it matters

Knowing both systems is essential for real Node work: ESM is the future (standard, tree-shakeable, top-level await, browser-aligned) and the right choice for new projects, but a vast amount of the npm ecosystem and existing code is CommonJS.

Understanding the syntax differences, how to enable ESM (`"type": "module"`), and the interop limitations (especially CJS-can't-require-ESM) prevents the frequent confusion and errors that arise when mixing them.