title: Rxjs execute the handler only once
body: <p>I have two observable that I am listening too with the merge operator when one of them is fire, or both I need to execute the handler only once, How can I do this with rx?</p>
  
  <pre><code>var source1 = Rx.Observable.interval(1000);
  
  var source2 = Rx.Observable.interval(1000);
  
  var source = Rx.Observable.merge(
      source1,
      source2)
      .subscribe(() =&gt; console.log('This needs to run only once and not kill the stream'))
  </code></pre>