title: How to handle if it is not filtered
body: <p>Is there a way to handle if a router event is not filtered when using it?</p>
  
  <p>I want to process it when it is not filtered, but I do not know what to do.</p>
  
  <pre><code>this.router.events
        .filter(e =&gt; e instanceof NavigationStart)
        .pairwise()
        .subscribe((event: any[]) =&gt; {
              // If...
        }
  );
              // I want to Else { }
  </code></pre>
  
  <p>Thank you.</p>