What is the difference between CommonJS and ES modules in Node?

CommonJS was Node's original module system. require runs during evaluation, so it can be conditional and synchronous. ES modules are the language standard, with static imports resolved before evaluation, which enables tree shaking and better tooling.

import { readFile } from "node:fs/promises";

export const config = { retries: 3 };

export async function loadConfig(path) {
  const raw = await readFile(path, "utf8");
  return JSON.parse(raw);
}

Practical differences

• CommonJS exports a value snapshot at require time; ESM bindings stay live
• ESM resolves imports statically, which helps bundlers and circular-dependency analysis
• CommonJS has __dirname and __filename; ESM uses import.meta.url
• ESM requires file extensions in relative specifiers unless a loader resolves them
• Setting type to module in package.json makes .js files ESM

Interoperability is the awkward part. ESM can import a CommonJS default export, but named imports depend on static analysis. CommonJS cannot require an ES module in every Node version, and top-level await makes it impossible synchronously. In modern services I pick one system per package and keep boundaries explicit.