The s.useBeacon variable forces the next request to use the browser's sendBeacon API . Using s.sendBeacon allows an HTTP request to be sent outside of the context of a page. It is useful for exit links and other situations where you want to send off information before the page unloads.
Setting this value only applies to the next request that AppMeasurement fires. After the request fires, s.useBeacon resets to false. This variable applies to both s.t() and s.tl() image requests.
Using the s.useBeacon variable requires AppMeasurement 2.17.0 or higher.
ExitLinks uses this variable automatically without any additonal configuration.
s.useBeacon = true;