Window: popstate event

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("popstate", (event) => {});
onpopstate = (event) => {};

A PopStateEvent. Inherits from Event.

PopStateEvent.state Read only

Returns a copy of the information that was provided to pushState() or replaceState().

In addition to the Window interface, the event handler property onpopstate is also available on the following elements:

• HTMLBodyElement
• HTMLFrameSetElement
• SVGSVGElement

If the history entry being activated was created by a call to history.pushState() or was affected by a call to history.replaceState(), the popstate event's state property contains a copy of the history entry's state object.

These methods and their corresponding events can be used to add data to the history stack which can be used to reconstruct a dynamically generated page, or to otherwise alter the state of the content being presented while remaining on the same Document.

Note that just calling history.pushState() or history.replaceState() won't trigger a popstate event. The popstate event will be triggered by doing a browser action such as a click on the back or forward button (or calling history.back() or history.forward() in JavaScript).

Browsers tend to handle the popstate event differently on page load. Chrome (prior to v34) and Safari always emit a popstate event on page load, but Firefox doesn't. chrome 实验不成功，也许v34之后都不会触发

It's important to first understand that — to combat unwanted pop-ups — browsers may not fire the popstate event at all unless the page has been interacted with.

This section describes the steps that browsers follow in the cases where they do potentially fire the popstate event (that is, in the cases where the page has been interacted with).

When a navigation occurs — either due to the user triggering the browser's Back button or otherwise — the popstate event is near the end of the process to navigate to the new location. It happens after the new location has loaded (if needed), displayed, made visible, and so on — after the pageshow event is sent, but before the persisted user state information is restored and the hashchange event is sent.

To better understand when the popstate event is fired, consider this simplified sequence of events that occurs when the current history entry changes due to either the user navigating the site or the history being traversed programmatically. Here, the transition is changing the current history entry to one we'll refer to as new-entry. The current page's session history stack entry will be referred to as current-entry.

1. If new-entry doesn't currently contain an existing Document, fetch the content and create its Document before continuing. This will eventually send events such as DOMContentLoaded and load to the Window containing the document, but the steps below will continue to execute in the meantime.
2. If current-entry's title wasn't set using one of the History API methods (pushState() or replaceState()), set the entry's title to the string returned by its document.title attribute.
3. If the browser has state information it wishes to store with the current-entry before navigating away from it, it then does so. The entry is now said to have "persisted user state." This information the browser might add to the history session entry may include, for instance, the document's scroll position, the values of form inputs, and other such data.
4. If new-entry has a different Document object than current-entry, the browsing context is updated so that its document property refers to the document referred to by new-entry, and the context's name is updated to match the context name of the now-current document.
5. Each form control within new-entry's Document that has autocomplete configured with its autofill field name set to off is reset. See The HTML autocomplete attribute for more about the autocomplete field names and how autocomplete works.
6. If new-entry's document is already fully loaded and ready—that is, its readyState is complete—and the document is not already visible, it's made visible and the pageshow event is fired at the document with the PageTransitionEvent's persisted attribute set to true.
7. The document's URL is set to that of new-entry.
8. If the history traversal is being performed with replacement enabled, the entry immediately prior to the destination entry (taking into account the delta parameter on methods such as go()) is removed from the history stack.
9. If the new-entry doesn't have persisted user state and its URL's fragment is non-null, the document is scrolled to that fragment.
10. Next, current-entry is set to new-entry. The destination entry is now considered to be current.
11. If new-entry has serialized state information saved with it, that information is deserialized into History.state; otherwise, state is null.
12. If the value of state changed, the popstate event is sent to the document. 不是根据 state 来判断是否 send！！
13. Any persisted user state is restored, if the browser chooses to do so.
14. If the original and new entries shared the same document, but had different fragments in their URLs, send the hashchange event to the window.

As you can see, the popstate event is nearly the last thing done in the process of navigating pages in this way.

A page at http://example.com/example.html running the following code will generate logs as indicated:

window.addEventListener("popstate", (event) => {
  console.log(
    `location: ${document.location}, state: ${JSON.stringify(event.state)}`,
  );
});
history.pushState({ page: 1 }, "title 1", "?page=1");
history.pushState({ page: 2 }, "title 2", "?page=2");
history.replaceState({ page: 3 }, "title 3", "?page=3");
history.back(); // Logs "location: http://example.com/example.html?page=1, state: {"page":1}"
history.back(); // Logs "location: http://example.com/example.html, state: null"
history.go(2); // Logs "location: http://example.com/example.html?page=3, state: {"page":3}"

The same example using the onpopstate event handler property:

window.onpopstate = (event) => {
  console.log(
    `location: ${document.location}, state: ${JSON.stringify(event.state)}`,
  );
};
history.pushState({ page: 1 }, "title 1", "?page=1");
history.pushState({ page: 2 }, "title 2", "?page=2");
history.replaceState({ page: 3 }, "title 3", "?page=3");
history.back(); // Logs "location: http://example.com/example.html?page=1, state: {"page":1}"
history.back(); // Logs "location: http://example.com/example.html, state: null"
history.go(2); // Logs "location: http://example.com/example.html?page=3, state: {"page":3}"

Note that even though the original history entry (for http://example.com/example.html) has no state object associated with it, a popstate event is still fired when we activate that entry after the second call to history.back().

• Manipulating the browser history (the History API)
• Window: hashchange event