ES modules use `import`/`export`, are always strict mode, and are evaluated once and cached. Node's older CommonJS uses `require`/`module.exports`.
// math.js
export function add(a, b) { return a + b; }
export default class Client {}
// app.js
import Client, { add } from "./math.js";Named exports are easier to refactor and tree-shake; prefer them over default exports.