title: Angular 5 : conbination of multiple observable
body: <p>I have an array on Observable, and I'd like to get one unique Observable which will return true if all the observables of my array will return true.</p>
  
  <p>I tried this, but does not seem to work:</p>
  
  <pre class="lang-js prettyprint-override"><code>const t = [Observable.of(true), Observable.of(false)];
  let obs = Observable.of(true);
  t.forEach(o =&gt; obs = obs.concat(o));
  
  const obsResult = obs.pipe(
    takeWhile(val =&gt; val !== false), 
    reduce((acc, val) =&gt; acc &amp;&amp; val)
  );
  obsResult.subscribe(r =&gt; console.log('result', r));
  </code></pre>
  
  <p><code>t[1]</code> is false so I expect obsResult to send <code>false</code>, but instead I get <code>true</code></p>