The navigator.sendBeacon() method asynchronously sends an HTTP POST request containing a small amount of data to a web server.

It's intended to be used for sending analytics data to a web server, and avoids some of the problems with legacy techniques for sending analytics, such as the use of XMLHttpRequest.

url

The URL that will receive the data. Can be relative or absolute.

data Optional

An ArrayBuffer, a TypedArray, a DataView, a Blob, a string literal or object, a FormData or a URLSearchParams object containing the data to send.

The sendBeacon() method returns true if the user agent successfully queued the data for transfer. Otherwise, it returns false.

This method is intended for analytics and diagnostics code to send data to a server.

A problem with sending analytics is that a site often wants to send analytics when the user has finished with a page: for example, when the user navigates to another page. In this situation the browser may be about to unload the page, and in that case the browser may choose not to send asynchronous XMLHttpRequest requests.

In the past, web pages have tried to delay page unload long enough to send data. To do this they have used workarounds such as:

• Submitting the data with a blocking synchronous XMLHttpRequest call.
• Creating an <img> element and setting its src. Most user agents will delay the unload to load the image.
• Creating a no-op loop for several seconds.

All these methods block unloading the document, which slows down navigation to the next page. There's nothing the next page can do to avoid this, so the new page seems slow, even though it's the fault of the previous page.

With the sendBeacon() method, the data is transmitted asynchronously when the user agent has an opportunity to do so, without delaying unload or the next navigation. This means:

• The data is sent reliably
• It's sent asynchronously
• It doesn't impact the loading of the next page

The data is sent as an HTTP POST request.

Websites often want to send analytics or diagnostics to the server when the user has finished with the page. The most reliable way to do this is to send the data on the visibilitychange event:

document.addEventListener("visibilitychange", function logData() {
  if (document.visibilityState === "hidden") {
    navigator.sendBeacon("/log", analyticsData);
  }
});

Avoid unload and beforeunload

In the past, many websites have used the unload or beforeunload events to send analytics at the end of a session. However, this is extremely unreliable. In many situations, especially on mobile, the browser will not fire the unload, beforeunload, or pagehide events. For example, these events will not fire in the following situation:

1. The user loads the page and interacts with it.
2. When they are finished, they switch to a different app, instead of closing the tab.
3. Later, they close the browser app using the phone's app manager.

Additionally, the unload event is incompatible with the back/forward cache (bfcache) implemented in modern browsers. Some browsers, such as Firefox, handle this incompatibility by excluding pages from the bfcache if they contain unload handlers, thus hurting performance. Others, such as Safari and Chrome on Android, handle it by not firing the unload event when the user navigates to another page in the same tab.

Firefox will also exclude pages from the bfcache if they contain beforeunload handlers.

Use pagehide as a fallback

To support browsers which don't implement visibilitychange, use the pagehide event. Like beforeunload and unload, this event is not reliably fired, especially on mobile. However, it is compatible with the bfcache.

The following example specifies a handler for the visibilitychange event. The handler calls sendBeacon() to send analytics.

document.addEventListener("visibilitychange", function logData() {
  if (document.visibilityState === "hidden") {
    navigator.sendBeacon("/log", analyticsData);
  }
});

• The visibilitychange event.
• Beacon API overview page.
• Don't lose user and app state, use Page Visibility explains in detail why you should use visibilitychange, not beforeunload/unload.
• Page Lifecycle API gives best-practices guidance on handling page lifecycle behavior in your web applications.
• PageLifecycle.js: a JavaScript library that deals with cross-browser inconsistencies in page lifecycle behavior.
• Back/forward cache explains what the back/forward cache is, and its implications for various page lifecycle events.