Node.js modules: CommonJS (require) versus ES modules (import)

Node.js supports two module systems, and most confusing errors in student projects come from mixing them.

• CommonJS (the older default): const fs = require('fs') and module.exports = something. Loaded synchronously.
• ES modules (the JavaScript standard): import fs from 'fs' and export default something. Supports top-level await.

How Node decides

• Files ending in .mjs are ES modules; files ending in .cjs are CommonJS.
• For .js files, Node looks at the nearest package.json: "type": "module" means ES modules; no type field means CommonJS.

import { readFile } from 'node:fs/promises';
import { fileURLToPath } from 'node:url';
const me = fileURLToPath(import.meta.url);
const text = await readFile(me, 'utf8');
console.log('top-level await works here:', text.includes('top-level'));

top-level await works here: true

Errors that mean you mixed them

• "require is not defined in ES module scope": you used require in an ES module. Use import, or rename the file to .cjs.
• "Cannot use import statement outside a module": you used import in a CommonJS file. Add "type": "module" to package.json or rename the file to .mjs.
• __dirname and __filename do not exist in ES modules; build them from import.meta.url as above (newer Node versions also offer import.meta.dirname).

For new projects, ES modules are the direction the ecosystem is moving. Prefix built-in modules with node: (as in node:fs) so it is obvious they are not npm packages.