///// code.gs: the server side /////

var serverService = (function() {
  // create and initialise the service
  var service = new ServerClass(42); // new instance of something
  service.name = 'foo';
  // return the service
  return function() {
    var args = Array.prototype.slice.call(arguments); // because the server doesn't support the spread operator
    return service[args.shift()].apply(service, args);
  };
})();


///// bundle.js: the client side /////

var clientService = (function() {
  return function(...args) {
    return new Promise(function (resolve, reject) {
      google.script.run
        .withSuccessHandler(resolve)
        .withFailureHandler(reject)
        serverService(...args);
    });
  }; // async services promise wrapper
})();

clientService('set', 777)
  .then(console.log)
  .catch(console.warn);

var serverService = (function serverService() {...})();