CommonJS vs ES Modules in Node.js

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Identify the module system

   Check package.json "type" field and the file extension (.mjs = ESM, .cjs = CommonJS).

2. Step 2

   Pick the syntax

   Use require()/module.exports for CommonJS and import/export for ES Modules — never mix them in one file.

3. Step 3

   Understand loading

   require() loads synchronously at call time; ESM imports are hoisted and resolved statically before execution.

4. Step 4

   Handle interop

   In ESM use import() or createRequire() to pull in a CommonJS module; a default import gives you module.exports.

5. Step 5

   Use ESM-only features

   Reach for top-level await and static analysis benefits like tree-shaking only in ESM.

Code Example

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

// app.cjs
const { add } = require('./math.cjs');
console.log(add(2, 3)); // 5

// math.mjs
export function add(a, b) {
  return a + b;
}

// app.mjs
import { add } from './math.mjs';
console.log(add(2, 3)); // 5

// top-level await works in ESM
const data = await Promise.resolve(42);

Follow-up Questions

• How do you import a CommonJS module from an ES Module?
• Why can't you use __dirname directly in ES Modules?
• What does "type": "module" do in package.json?
• What is top-level await and where is it allowed?
• How does static analysis enable tree-shaking in ESM?