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();
})();