function search(keyword) {

  return new Promise((resolve, reject) => {

    setTimeout(() => {

      resolve('Result of ' + keyword);

    }, Math.random() * 1000);

  });

}

​

function withoutSwitchMap() {

  const keyword = document.getElementById('keyword-without');

  const result = document.getElementById('result-without');

​

  keyword.addEventListener('keyup', e => {

    const value = e.target.value;

    search(value)

      .then(data => result.textContent = data);

  });

}

​

function withSwitchMap() {

  const keyword = document.getElementById('keyword-with');

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

​

  const keyword$ = Rx.Observable.fromEvent(keyword, 'keyup')

    .map(e => e.target.value);

  keyword$

    .switchMap(search)

    .subscribe(data => result.textContent = data);

}

​

withoutSwitchMap();

withSwitchMap();