The index read-only property of the NavigationHistoryEntry interface returns the index of the history entry in the history entries list (that is, the list returned by Navigation.entries()), or -1 if the entry does not appear in the list or if current document is not fully active.
A number representing the index of the entry in the history entries list, or -1 if this item does not appear in the list.
const current = navigation.currentEntry;
console.log(current.index);