Neat little Rollup script for generating a CJS version of an ESM module.

rollup.config.js

import fs from "fs";
import { builtinModules as builtin } from "module";

const pkg = JSON.parse(fs.readFileSync("./package.json"));

export default {
  input: "./src/index.js",
  output: {
    file: "build/index.cjs",
    format: "cjs",
  },
  external: [
    ...Object.keys(pkg.dependencies),
    ...builtin,
  ]
};