title: Fire async request in parallel but get result in order using rxjs
body: <p>For example:</p>
  
  <p>Get 5 pages in parrallel using jquery ajax. When page2 return, do nothing. When page1 return, do something with page1 and page2.</p>
  
  <pre><code>// assume there is some operator that can do this, 
  // then it might look like this?
  Rx.Observable.range(1, 5).
  someOperator(function(page) {
    return Rx.Observable.defer( () =&gt; $.get(page) );
  }).scan(function(preVal, curItem) {
    preVal.push(curItem);
    return preVal;
  }, []);
  </code></pre>