Generate both an mjs (es6 module) and js file using TypeScript

prepare.js

const fs = require("fs");

const { exec } = require("child_process");

/**
 * Response from exec
 * @typedef ExecResponse
 * @type {object}
 * @property {?Error} error - If the operation resulted in an error, this will have a value.
 * @property {string} stdout - Text written to stdout
 * @property {string} stderr - Text written to stderr
 */

/**
 * Wrapper around fs.exec that returns a Promise instead of using callback parameter.
 * @param {string} command - Command to execute.
 * @returns {Promise.<ExecResponse>}
 */
function execPromise(command) {
  return new Promise((resolve, reject) => {
    exec(command, (error, stdout, stderr) => {
      if (error) {
        reject({ error, stdout, stderr });
      } else {
        resolve({ error, stdout, stderr });
      }
    });
  });
}

(async () => {
  await execPromise("tsc --target es2015 --module es2015");
  await fs.promises.rename("index.js", "index.mjs");
  await execPromise("tsc --target es5 --module commonjs");
})();