Historically, when sending data from browsers to server before the page is closed (e.g. through the beforeunload event), XHR synchronous requests have been used.

This is now deprecated in Chrome, with likely more browsers to follow. The prescribed solution is either navigator.sendBeacon (which only works for POST) or fetch with a keepalive flag.

It's not possible to properly polyfill the keepalive flag of course, but sending a synchronous XHR request is a good alternative for older browsers. Would you guys accept a PR that makes synchronous requests when the keepalive flag is set to true, with corresponding documentation in the README? This would make it so that people could use a single API call to support keepalive requests on beforeunload, instead of having to do lots of code branching.