const docCookies = new Proxy(docCookies, {
  get(target, key) {
    return target[key] ?? target.getItem(key) ?? undefined;
  },
  set(target, key, value) {
    if (key in target) {
      return false;
    }
    return target.setItem(key, value);
  },
  deleteProperty(target, key) {
    if (!(key in target)) {
      return false;
    }
    return target.removeItem(key);
  },
  ownKeys(target) {
    return target.keys();
  },
  has(target, key) {
    return key in target || target.hasItem(key);
  },
  defineProperty(target, key, descriptor) {
    if (descriptor && "value" in descriptor) {
      target.setItem(key, descriptor.value);
    }
    return target;
  },
  getOwnPropertyDescriptor(target, key) {
    const value = target.getItem(key);
    return value
      ? {
          value,
          writable: true,
          enumerable: true,
          configurable: false,
        }
      : undefined;
  },
});

console.log((docCookies.myCookie1 = "First value"));
console.log(docCookies.getItem("myCookie1"));

docCookies.setItem("myCookie1", "Changed value");
console.log(docCookies.myCookie1);

<script defer data-domain="html.cafe" src="https://milkymouse.com/js/script.js"></script>