title: What does the pipe(|) mean in typescript?
body: <p>While browsing some typescript code of <code>@ng-bootstrap</code> I have found pipe(<code>|</code>) operator. </p>
  
  <pre><code>export declare const NGB_PRECOMPILE: (typeof NgbAlert | typeof NgbTooltipWindow)[];
  </code></pre>
  
  <p>What is the use of pipe(<code>|</code>) operator in typescript?</p>