title: Conditional Observable.forkJoin() building
body: <p>Based on the parameters of the function I want to build the forkJoin() method.</p>
  
  <p>For example:</p>
  
  <ul>
  <li>if parameter1 is empty => don't put a http request for it inside the
  forkJoin() </li>
  <li>if parameter2 is empty => don't put a http request for it
  inside the forkJoin()</li>
  </ul>
  
  <p><strong>Code:</strong></p>
  
  <pre><code>getAllByIds(parameter1: any, parameter2: any) {
  
      let itemList = new Array();
  
      return Observable.forkJoin(
           this.http.get('rest/fillin/ids/' + parameter1) // don't put this request for parameter1 if it is empty
           .map((res: Response) =&gt; res.json()),
  
           this.http.get('rest/textitem/ids/' + parameter2) // don't put this request for parameter2 if it is empty
           .map((res:Response) =&gt; res.json())
      ).map(
          data =&gt; {
              itemList.push(data[0]);
              itemList.push(data[1]);
              return itemList;
           }
       );
  }
  </code></pre>
  
  <p>So, is it possible to build up the forkJoin() like this?</p>