Beacon Transport

navigator.sendBeacon is a browser mechanism that lets a small data packet be sent reliably to the server even as the page is closing or being left, without slowing the user's departure.

The problem this mechanism solves is that analytics tools must send collected data exactly as the user is closing the page, yet an ordinary request at that moment is often canceled because the browser will not wait for it. sendBeacon solves this with a “fire and forget” send: it hands the packet to the system queue and guarantees it is dispatched even after the user leaves, without blocking navigation. Its limits are worth knowing: it suits only small POST packets, returns no server response, and is absent in very old browsers, where a fallback method is needed. This is why mature tools make sendBeacon the primary path and keep an alternate route for older browsers.