title: Convert return Promise into return Observable
body: <p>I don't know how I can convert this expression</p>
  <pre><code>return new Promise((resolve, reject) =&gt;
    result.length === 0 ? resolve() : reject(result)
  );
  </code></pre>
  <p>using Observable in the place of Promise. 'result' is the variable that I would return in case of error.</p>