bare-bundle-transform-plugin-babel/index.js

const babel = require("@babel/core");

async function plugin(bundle, { name, data, mode }) {
  return new Promise((resolve, reject) => {
    // only transform javascript files
    if (name.match(/\.js$|\.cjs$|\.mjs$/)) {
      babel.transform(data.toString(), (err, result) => {
        if (err) reject(err);
        // overwrite the old code with the transpiled code in the bundle
        bundle.write(name, result.code, { mode });
        resolve();
      });
    } else {
      resolve();
    }
  });
}

module.exports = plugin;