Fun with proxies :)

gistfile1.txt

const HeaderAPI = {};

HeaderAPI.updateAuthenticated = function() {
  console.log('UPDATING!');
}

class Header {
  constructor() {
  }
}

Header.prototype = new Proxy(Header.prototype, {
  get(target, name) {
    const proxiedMethods = ['updateAuthenticated'];
    if(proxiedMethods.some((method) => method === name)) {
      if(!window.HeaderAPI) {
        return () => { console.log('This method is not defined!') }
      }
      return window.HeaderAPI[name];
    }
    return target[name];
  }
})

const proxyHeader = new Header();
proxyHeader.updateAuthenticated();