Mailchimp client-side ajax subscribe request, returns promise

mailchimpSubscribeRequest.js

/**
 * Mailchimp client-side ajax subscribe request promise
 * @param {object} options
 * @param {string} options.action Mailchimp subscribe form action url <form action="URL">
 * @param {string} options.email email address
 * @returns {Promise.<string, Error>} promise of a success message or an Error containing message
 * @example
    mailchimpSubscribeRequest({
      action: 'https://blabla.us6.list-manage.com/subscribe/post?u=61f5z352da53ba442da9cb35&amp;id=02435aaa99',
      email: '[email protected]'
    })
    .then(res => {
      alert(res) // success message (or please confirm on opt in)
    })
    .catch(err => {
      alert(`Error\n${err.message}`)
    })
 */
const mailchimpSubscribeRequest = ({ action, email }) => {
  return Promise.resolve(action)
    .then(parseUrl)
    .then(src => {
      return jsonpRequest(src, 'c', {
        EMAIL: email,
        // FNAME
      })
    })
    .then(res => {
      if(res.result === 'success') {
        return res.msg;
      } else if(res.msg) {
        /*
          based on: https://github.com/scdoshi/jquery-ajaxchimp/blob/master/jquery.ajaxchimp.js

          Codes:
          1 - Please enter a value
          2 - An email address must contain a single @
          3 - The domain portion of the email address is invalid (the portion after the @: )
          4 - The username portion of the email address is invalid (the portion before the @: )
          5 - This email address looks fake or invalid. Please enter a real email address
        */
        const [ code, message ] = res.msg.split(' - ')

        throw Object.assign(new Error(message), { code })
      } else {
        throw Object.assign('Unknown Response', { code: 0 })
      }
    })
}

// Converts embedded form action endpoint to jsonp endpoint
// https://blabla.us6.list-manage.com/subscribe/post?u=61f5z352da53ba442da9cb35&amp;id=02435aaa99
// Get url at: audience > signup forms > embedded forms
const parseUrl = mailchimpActionUrl => {
  const url = new URL(mailchimpActionUrl);

  url.protocol = 'https:'

  url.pathname = url.pathname.replace('/post', '/post-json')

  return url;
}

const jsonpRequest = (url, callbackParam = 'callback', params = {}) => new Promise((res, rej) => {
  const objectToParams = obj => Object.keys(obj).map(k => [k, obj[k]].map(encodeURIComponent).join('=')).join('&')

  const teardown = () => {
    delete window[globalCallbackName]

    script.remove()
  }

  const globalCallbackName = `cb${Date.now()}`

  window[globalCallbackName] = response => {
    res(response);

    teardown()
  }

  const paramString = objectToParams({
    ...params,
    [callbackParam]: globalCallbackName,
    _: Date.now() // cache bust
  })

  const script = document.createElement('script')

  script.onerror = () => {
    rej(new Error('Request Error'))

    teardown()
  }

  const src = new URL(url);
  src.search = src.search ? `${src.search}&${paramString}` : paramString
  script.src = src.toString()

  document.head.appendChild(script)
})

export default mailchimpSubscribeRequest