How to implement Back button functionality in ionic 4?

In your HTML, go to ion-toolbar tag inside ion-header tag and add the back button as

<ion-buttons slot="start">
   <ion-back-button></ion-back-button>
</ion-buttons>

This will add a back button at top left corner of your page and clicking on it will take you to previous route.

For more configs of back-button check the official Ionic v4.0 docs at https://ionicframework.com/docs/api/back-button/

Looks like there may be an issue where you need to supply a defaultHref attribute to get this to show up. On the surface this seems like annoying overhead, but I guess it does make you think about navigation paths for each page. Not ideal, but not horrible.

https://forum.ionicframework.com/t/ionic-4-back-button-angular/137866/18

could you please trying to add back button in ion-toolbar .

 <ion-buttons slot="start">
      <ion-back-button defaultHref=""></ion-back-button>
  </ion-buttons>