const select = document.querySelector('select');

const result = document.getElementById('result');

​

const id$ = Rx.Observable.fromEvent(select, 'change').map(e => e.target.value);

​

const fetchPost = id => {

  const url = `https://jsonplaceholder.typicode.com/posts/${id}`;

  return Rx.Observable.ajax(url).map(res => res.response);

};

​

const render = post => {

  result.innerHTML = post.title;

};

​

id$.switchMap(fetchPost).subscribe(render);