title: ionic v4 back button canGoBack alternative
body: <p>I have an ionic application that is currently based on version 4. In version 4 they switched to angular routing model and basically parts of the <code>NavController</code> were deprecated and I'm looking for the proper replacement for <code>NavController.canGoBack</code>.</p>
  
  <p>To give some context I basically have an app that requires a pin code unlock prior to showing application contents, on my pin-code page I already implemented a guard to prevent deactivation (basically prevent). However, while implementing the back button I noticed that when I press the back button on the very first page. It seems to bypass the guard, and instead the screen turns blanc and then shows the pin code page again.</p>
  
  <p>I have my suspicions that when I issue an <code>this.navCtrl.back();</code> on it refreshes the page and it doesn't actually use the javascript based routing. So to that end I was hoping I would be able <code>NavController.canGoBack</code> to give me some indication if there's anything to navigate back to.</p>