title: RxJs operator that behaves like withLatestFrom but waits for value of second stream
body: <p>Hi I'm looking for an RxJs operator that behaves similar to withLatestFrom, with the exception that it would wait for the second stream to emit a value instead of skipping it. To be claer: I only want emissions when the first stream emits a new value.</p>
  
  <p>So instead of:</p>
  
  <pre><code>---A-----B----C-----D-|
  ------1----------2----|
     withLatestFrom
  ---------B1---C1----D2|
  </code></pre>
  
  <p>I want this behavior:</p>
  
  <pre><code>---A-----B----C-----D-|
  ------1----------2----|
     ?????????????
  ------A1-B1---C1----D2|
  </code></pre>
  
  <p>Is there an operator for this?</p>