rollup3.js

// rollup.config.js
import resolve from 'rollup-plugin-node-resolve'
import commonjs from 'rollup-plugin-commonjs'

const esm = process.env.BABEL_ENV === 'esm'

export default {
  input: 'src/index.js',
  output: {
    format: esm ? 'esm' : 'cjs',
    file: `bundle.js`,
    dir: esm ? 'esm' : 'lib'
  },
  plugins: [
    resolve(),
    commonjs()
  ]
}