While pushState() adds a new entry to the history, replaceState() replaces the current history entry without adding one.

history.pushState({}, '', '/posts')
const state = { post: 'first' }
history.pushState(state, '', '/post/first')
const state = { post: 'second' }
history.replaceState(state, '', '/post/second')

If you then call:

history.back()

the browser navigates to /posts, because /post/first was replaced by /post/second and no longer exists as a separate entry..

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
7:	Bfcache and page restoration