Store UTM params & referrer UTL as subscriber fields on MailChimp form

lead_form_utm_params.js

// $(document).ready() replacement.
function ready(fn) {
  if (document.readyState != 'loading'){
    fn();
  } else {
    document.addEventListener('DOMContentLoaded', fn);
  }
}

// Get UTM params from URL to pass to Mailchimp. Store in localStorage so they follow the user around the site.
function getParameterByName(name) {
  // First try url params.
  name = name.replace(/[\[]/, "\\\[").replace(/[\]]/, "\\\]");
  var regexS = "[\\?&]" + name + "=([^&#]*)";
  var regex = new RegExp(regexS);
  var results = regex.exec(window.location.search);
  if (results == null) {
    // Try localStorage if no url params.
    var storedParam = localStorage.getItem(name);
    if (storedParam) {
      return storedParam;
    }
    return "";
  } else {
    return decodeURIComponent(results[1].replace(/\+/g, " "));
  }
}

function storeUTMParams(name) {
  var utmParam = getParameterByName(name);
  if (utmParam) {
    localStorage.setItem(name, utmParam);
  }
}

ready(function () {
  var formTarget = document.getElementById("mc-embedded-subscribe-form");

  function addFormElem(paramName, fieldName) {
    var paramValue = getParameterByName(paramName);
    var utmEl = "<input type='hidden' name='" + fieldName + "' value='" + paramValue + "' />";
    if (paramValue != "" && formTarget) {
      formTarget.insertAdjacentHTML('afterbegin', utmEl)
    }
  }

  // Note: Only change the values after ":" if they are different for you
  // They are the same one as you added in your email marketing provider
  var utmParams = {
    "utm_source"   : "USOURCE",
    "utm_medium"   : "UMEDIUM",
    "utm_campaign" : "UCAMPAIGN",
    "utm_content"  : "UCONTENT",
    "utm_term"     : "UTERM"
  };

  for (var param in utmParams) {
    addFormElem(param, utmParams[param]);
    storeUTMParams(param);
  }

  // Also add referrer.
  if (formTarget) {
    var referredElm = "<input type='hidden' name='REFERRER' value='" + window.location.href.split('?')[0] + "' />";
    formTarget.insertAdjacentHTML('afterbegin', referredElm);
  }