title: Typescripts Strict Null Checking and Observable Filters
body: <p>I would expect that value.length to be always defined by the time it reaches the .map operator. But using strictNullChecks with typescript throws the error "Object is possibly 'undefined'." Am I wrong, or is this a known issue?</p>
  
  <pre><code>this.count$ = this.store.select&lt;Model&gt;('state')
              .filter(value =&gt; !!value &amp;&amp; !!value.prop)
              .map(value =&gt; value.prop)
  </code></pre>