key
The key read-only property of the NavigationHistoryEntry interface returns the key of the history entry, or an empty string if current document is not fully active. This is a unique, UA-generated value that represents the history entry's slot in the entries list. It is used to navigate that particular slot via Navigation.traverseTo(). The key will be reused by other entries that replace the entry in the list (that is, if the NavigateEvent.navigationType is replace).