What is the purpose of the popstate event in the History API?

The popstate event is specifically designed to signify when the active history entry changes within the browser's session history. This event is triggered whenever the user navigates to a different state using the History API, typically through browser navigation, such as clicking the back or forward button. When the popstate event is fired, it allows developers to respond accordingly and modify the displayed content of the web application based on the new state.

The other options do not accurately reflect the function of the popstate event. For instance, although a change in history entries might involve new entries being added, the popstate event does not indicate this; rather, it focuses on changes to the current history entry. Additionally, while the document may change in response to various events, the popstate event is not specifically for notifying users of such changes. Triggering the display of previous history entries can happen as a result of the popstate event but isn't its primary purpose; the event rather signifies the change itself.