NavigationDestination: key property

The key read-only property of the NavigationDestination interface returns the key value of the destination NavigationHistoryEntry if the NavigateEvent.navigationType is traverse, or null otherwise.

The key is a unique, UA-generated value that represents the history entry's slot in the history entries list, used to navigate to this place in the history via Navigation.traverseTo(). It will be reused by other entries that replace the entry in the list (i.e. if the NavigateEvent.navigationType is replace).

navigation.addEventListener("navigate", (event) => {
  console.log(event.destination.key);
});