title: Using Promise inside SwitchMap operator
body: <pre><code>source$.switchMap(x =&gt; promise(x))
  .subscribe(x =&gt; console.log(x));
  
  source$.switchMap(x =&gt; from(promise(x)))
  .subscribe(x =&gt; console.log(x));
  </code></pre>
  
  <p>both the lines of code resolves the promise but can someone really point out what's the different in between these two lines of code ?</p>