History.replaceState()

stateObj

The state object is a JavaScript object which is associated with the history entry passed to the replaceState method. The state object can be null.

unused

This parameter exists for historical reasons, and cannot be omitted; passing the empty string is traditional, and safe against future changes to the method.

url Optional

The URL of the history entry. The new URL must be of the same origin as the current URL; otherwise replaceState throws an exception.

None (undefined).

Suppose https://www.mozilla.org/foo.html executes the following JavaScript:

const stateObj = { foo: 'bar' };
history.pushState(stateObj, '', 'bar.html');

On the next page you could then use history.state to access the stateObj that was just added.

The explanation of these two lines above can be found in the Example of pushState() method section of the Working with the History API article. Then suppose https://www.mozilla.org/bar.html executes the following JavaScript:

history.replaceState(stateObj, '', 'bar2.html');

This will cause the URL bar to display https://www.mozilla.org/bar2.html, but won't cause the browser to load bar2.html or even check that bar2.html exists.

Suppose now that the user navigates to https://www.microsoft.com, then clicks the Back button. At this point, the URL bar will display https://www.mozilla.org/bar2.html. If the user now clicks Back again, the URL bar will display https://www.mozilla.org/foo.html, and totally bypass bar.html.