title: rxjs concatMap with array
body: <p>There is Higher Order Observable observable</p>
  
  <pre><code>const obs1 = interval(1000).pipe(take(5));
  const obs2 = interval(500).pipe(take(2));
  const obs3 = interval(2000).pipe(take(1));
  //emit three observables
  const source = of(obs1, obs2, obs3);
  </code></pre>
  
  <p>How to use concatMap in order to as soon as the previous observable is completed, the next one immediately begins. After it will return result array of Observable.Need to use concatMap.</p>