CommonJS vs ESM

TL;DR

Node.js traditionally used CommonJS (require(), module.exports), which is synchronous. Modern Node.js also supports ECMAScript Modules (ESM) (import, export), which is asynchronous, standard in browsers, and supports Top-Level Await.

Mental Model

How It Works

CommonJS

• Loads modules synchronously at runtime.
• Files are wrapped in an IIFE (Immediately Invoked Function Expression) before execution, providing them with __dirname, __filename, require, and module.
• Uses require() to load and module.exports to export.

ECMAScript Modules (ESM)

• Loads and parses modules asynchronously before executing the code (parsing phase vs execution phase).
• Does not have __dirname, __filename, or require built-in (you must use import.meta.url).
• Supports Top-Level Await (using await outside of an async function).

Example

// --- CommonJS ---
const fs = require('fs');
module.exports = function doSomething() { /* ... */ };

// --- ESM ---
// (Requires "type": "module" in package.json OR .mjs extension)
import fs from 'fs';
export function doSomething() { /* ... */ };

// Top-level await is valid in ESM!
const data = await fs.promises.readFile('config.json');

Common Interview Questions

Can you use require() inside an ESM file?

By default, no. require is not defined in an ES module. If you need to import a CommonJS module into an ESM file, you can just import it, or you can construct require using module.createRequire(import.meta.url).

How does Node.js know if a file is CJS or ESM?

Node.js treats .cjs files as CommonJS and .mjs files as ESM. For .js files, it checks the nearest package.json for the "type" field. If "type": "module", it treats .js as ESM. If "type": "commonjs" (or missing), it defaults to CommonJS.