title: RxJS: How to not subscribe to initial value and/or undefined?
body: <p>Being new to RxJS I often create a subject which holds values in the future, but is initially <code>undefined</code>. It can only be <code>undefined</code> the first time. I currently use a <code>filter</code> to skip <code>undefined</code> values, but this is quite cumbersome as I do it <em>everywhere</em> as I need only once. (Maybe I do something wrong here?) Can I somehow subscribe to <code>mySubject</code> only after it got its first value via <code>onNext</code>?</p>
  
  <pre><code>var mySubject = new Rx.BehaviorSubject(undefined);
  
  mySubject.filter(function(value) {
    return value !== undefined;
  }).subscribe(function(value) {
    // do something with the value
  });
  </code></pre>