app.js

const timeout = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

const server_side_call = async (method, ...args) => {
  if (!["foo", "bar", "baz"].includes(method)) {
    throw new Error(`invalid server-side method ${method}`);
  }

  console.log(`${method} successfully executed server-side`);
};

const create_api = async () => {
  let api = new Proxy({
    foo: async (...args) => {
      console.log(`foo successfully executed client-side`);
    },
  }, {
    get: (target, name) => {
      if (target.hasOwnProperty(name)) {
        return target[name];
      }

      return async (...args) => {
        return await server_side_call(name, ...args);
      };
    },
  });

  return timeout(100).then(() => api);
};

(async () => {
  await timeout(10);
  const api = await create_api();
  await timeout(10);
  api.foo();
  await timeout(10);
  api.bar();
  await timeout(10);
  api.baz();
})();