mailchimp-ajax.js

function addSubscriberToMailChimp ({email, firstName, formAction, onSuccess, onError}) {
  // Get url for mailchimp
  let url = (formAction || "").replace('/post?', '/post-json?');

  // add email and first name
  url += `&EMAIL=${encodeURIComponent(email)}&FNAME=${encodeURIComponent(firstName)}`;

  // Create & add post script to the DOM
  let script = document.createElement('script');
  script.src = url + "&c=mailchimpCallback";
  document.body.appendChild(script);

  // Callback function
  let callback = 'mailchimpCallback';
  window[callback] = function(res) {

    // Remove script from the DOM after it's called
    delete window[callback];
    document.body.removeChild(script);

    // Display notices
    if (res.result === "success") {
      onSuccess(res);
    } else {
      onError(res);
    } 
  };
}

addSubscriberToMailChimp({
  firstName: "xxxxxxx",
  email: "xxxxxx@example.com",
  // create an "embedded form" in mailchimp, then paste the form's action here
  formAction: "https://xxxxxxxx.us8.list-manage.com/subscribe/post?u=xxxxxxxxxxx&id=xxxxxxxxxx",
  onSuccess: (res) => console.log("success", res),
  onError: (res) => console.log("error", res)
});