tsdx.config.base.js

module.exports = {
  rollup(config, opts) {
    if (opts.format === 'esm') {
      config = { ...config, preserveModules: true }
      config.output = {
        ...config.output,
        dir: 'dist/',
        entryFileNames: '[name].mjs',
      }
      delete config.output.file
    }

    return config
  },
}