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