Beacon API

The Beacon API is used to send an asynchronous and non-blocking request to a web server. The request does not expect a response. Unlike requests made using XMLHttpRequest or the Fetch API, the browser guarantees to initiate beacon requests before the page is unloaded and to run them to completion.

The main use case for the Beacon API is to send analytics such as client-side events or session data to the server. Historically, websites have used XMLHttpRequest for this, but browsers do not guarantee to send these asynchronous requests in some circumstances (for example, if the page is about to be unloaded). To combat this, websites have resorted to various techniques, such as making the request synchronous, that have a bad effect on responsiveness. Because beacon requests are both asynchronous and guaranteed to be sent, they combine good performance characteristics and reliability.

For more details about the motivation for and usage of this API, see the documentation for the navigator.sendBeacon() method.