CommonJS modules

Node historically used CommonJS: require() loads modules synchronously; module.exports defines what they expose. Millions of npm packages still use this style.

Export and import (CommonJS)

// greeter.cjs
module.exports = function greet(name) {
  return `Hello, ${name}`;
};

// app.cjs
const greet = require('./greeter.cjs');
console.log(greet('Ada'));

Built-in require

const path = require('node:path');
const fs = require('node:fs');

No import at top level in pure CommonJS files unless using dynamic import().

Playground note

Runnable snippets use ESM. To compare CommonJS in the runner, use createRequire from node:module (see editor code).

Important interview questions and answers

1. Q: require vs import?
   A: require is sync CommonJS; import is ESM (static, async loading at graph build). Node supports both with configuration.
2. Q: __dirname in ESM?
   A: Not defined—derive from import.meta.url with fileURLToPath and path.dirname.

Self-check

1. What does module.exports do?
2. Why is require synchronous?

Pitfall: Mixing require and top-level await in the same file causes confusion—pick ESM (import) or CommonJS (require) per file in new projects.