This specification defines an interoperable means for site developers to asynchronously transfer small HTTP data from the User Agent to a web server. Initiated by the navigator.sendBeacon() method, the 'beacon' data will be transmitted by the User Agent as soon as possible, but independent of document navigation. The sendBeacon method returns true if the user agent is able to successfully queue the data for transfer. Otherwise it returns false.
Docs: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/sendBeacon http://updates.html5rocks.com/2014/10/Send-beacon-data-in-Chrome-39
Samples: https://github.com/GoogleChrome/samples/tree/gh-pages/beacon