Working with the History API

HTML5 introduced the pushState() and replaceState() methods for add and modifying history entries, respectively. These methods work in conjunction with the onpopstate event.

Adding and modifying history entries

Using pushState() changes the referrer that gets used in the HTTP header for XMLHttpRequest objects created after you change the state. The referrer will be the URL of the document whose window is this at the time of creation of the XMLHttpRequest object.

Example of pushState() method

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

let stateObj = {
    foo: "bar",
}

history.pushState(stateObj, "page 2", "bar.html")

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

Suppose now that the user navigates to https://google.com, then clicks the Back button. At this point, the URL bar will display https://mozilla.org/bar.html and history.state will contain the stateObj. The popstate event won't be fired because the page has been reloaded. The page itself will look like bar.html.

If the user clicks Back once again, the URL will change to https://mozilla.org/foo.html, and the document will get a popstate event, this time with a null state object. Here too, going back doesn't change the document's contents from what they were in the previous step, although the document might update its contents manually upon receiving the popstate event.

The pushState() method

pushState() takes three parameters: a state object; a title (currently ignored); and (optionally), a URL.

Let's examine each of these three parameters in more detail.

state object

The state object is a JavaScript object which is associated with the new history entry created by pushState(). Whenever the user navigates to the new state, a popstate event is fired, and the state property of the event contains a copy of the history entry's state object. The state object can be anything that can be serialized. Because Firefox saves state objects to the user's disk so they can be restored after the user restarts the browser, we impose a size limit of 640k characters on the serialized representation of a state object. If you pass a state object whose serialized representation is larger than this to pushState(), the method will throw an exception. If you need more space than this, you're encouraged to use sessionStorage and/or localStorage.

title

All browsers but Safari currently ignore this parameter, although they may use it in the future. Passing the empty string here should be safe against future changes to the method. Alternatively, you could pass a short title for the state to which you're moving.

URL

The new history entry's URL is given by this parameter. Note that the browser won't attempt to load this URL after a call to pushState(), but it might attempt to load the URL later, for instance after the user restarts the browser. The new URL does not need to be absolute; if it's relative, it's resolved relative to the current URL. The new URL must be of the same origin as the current URL; otherwise, pushState() will throw an exception. This parameter is optional; if it isn't specified, it's set to the document's current URL.

In a sense, calling pushState() is similar to setting window.location = "#foo", in that both will also create and activate another history entry associated with the current document.

But pushState() has a few advantages:

• The new URL can be any URL in the same origin as the current URL. In contrast, setting window.location keeps you at the same document only if you modify only the hash.
• You don't have to change the URL if you don't want to. In contrast, setting window.location = "#foo"; creates a new history entry only if the current hash isn't #foo.
• You can associate arbitrary data with your new history entry. With the hash-based approach, you need to encode all of the relevant data into a short string.
• If title is subsequently used by browsers, this data can be utilized (independent of, say, the hash).

Note that pushState() never causes a hashchange event to be fired, even if the new URL differs from the old URL only in its hash.

In other documents, it creates an element with a null namespace URI.

The replaceState() method

history.replaceState() operates exactly like history.pushState(), except that replaceState() modifies the current history entry instead of creating a new one. Note that this doesn't prevent the creation of a new entry in the global browser history.

replaceState() is particularly useful when you want to update the state object or URL of the current history entry in response to some user action.

Example of replaceState() method

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

let stateObj = { foo: "bar" }
history.pushState(stateObj, "page 2", "bar.html")

The explanation of these two lines above can be found at the above section Example of pushState() method section.

Next, suppose https://mozilla.org/bar.html executes the following JavaScript:

history.replaceState(stateObj, "page 3", "bar2.html")

This will cause the URL bar to display https://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://mozilla.org/bar2.html. If the user now clicks Back again, the URL bar will display https://mozilla.org/foo.html, and totally bypass bar.html.

The popstate event

A popstate event is dispatched to the window every time the active history entry changes. If the history entry being activated was created by a call to pushState or affected by a call to replaceState, the popstate event's state property contains a copy of the history entry's state object.

See WindowEventHandlers.onpopstate for sample usage.

Reading the current state

When your page loads, it might have a non-null state object. This can happen, for example, if the page sets a state object (using pushState() or replaceState()) and then the user restarts their browser. When the page reloads, the page will receive an onload event, but no popstate event. However, if you read the history.state property, you'll get back the state object you would have gotten if a popstate had fired.

You can read the state of the current history entry without waiting for a popstate event using the history.state property like this:

let currentState = history.state

See also

• History API
• Ajax navigation example
• window.history