title: Get typed information from subscribe callback for Observable.fromEvent()
body: <p>I have this:</p>
  
  <pre class="lang-js prettyprint-override"><code>this.keydownSub = Observable.fromEvent(document, 'keyup').subscribe(v =&gt; {
  
    if(v){
      if(v.keyCode == '37'){
        this.goLeft();
      }
      if(v.keyCode == '39'){
        this.goRight();
      }
    }
  });
  </code></pre>
  
  <p>the problem is that there is no type information for the variable <code>v</code>, and that's because the event type is dynamic ('keyup' is used).</p>
  
  <p>Is there some way to use something like this:</p>
  
  <pre class="lang-js prettyprint-override"><code>this.keydownSub = Observable.fromEvent(document).keyUp().subscribe(v =&gt; {
  
    if(v){
      if(v.keyCode == '37'){
        this.goLeft();
      }
      if(v.keyCode == '39'){
        this.goRight();
      }
    }
  });
  </code></pre>
  
  <p>so that static typing information is possible/available?</p>