CommonJS vs ES modules: require, import and making them work together

Sana’s Node.js server breaks after she upgrades a library: Error [ERR_REQUIRE_ESM] on older Node.js, or a missing default export on newer ones. The library moved from CommonJS to ES modules.

CommonJS is Node’s original module system: require() loads and runs a file synchronously and returns its module.exports object. The values you read from it are plain property reads, not live bindings.

ES modules are static and may load asynchronously: the graph is built before code runs (JS.10.01). Interop means making the two systems load each other.

cat > counter.cjs <<'JS'
let count = 0;
module.exports = { count, increment() { count++; } };
JS
cat > price.mjs <<'JS'
export default function price(n) { return "₹" + n; }
export const currency = "INR";
JS
cat > main.cjs <<'JS'
const counter = require("./counter.cjs");
counter.increment();
console.log("CommonJS count:", counter.count);
const esm = require("./price.mjs");
console.log(esm.default(99), esm.currency);
JS
node main.cjs

CommonJS count: 0
₹99 INR

count stayed 0: it was copied into the exports object once. And require of an ES module returned its namespace, with the default under .default.