This event is called on window every time the fragment identifier of the URL changes:

window.onhashchange = event => {
  console.log(location.hash)
}

or

window.addEventListener('hashchange', event => {
  console.log(location.hash)
})

Lessons in this unit:

0:	Introduction
1:	Navigating the history
2:	Add an entry to the history
3:	Modify history entries
4:	Access the current history entry state
5:	The `popstate` event
6:	▶︎ The `hashchange` event