第133页 Beacons

涨姿势
If you only care about sending data to the server (with possibly a very simple response), use image beacons.

Beacons are the fastest and most efficient way to send data back to the server. The server doesn’t have to send back any response body at all, so you don’t have to worry about downloading data to the client. The only downside is that it you are limited in the type of responses you can receive. If you need to pass large amounts of data back to the client, use XHR.
var url = '/status_tracker.php';
var params = [
    'step=2',
    'time=1248027314'
];
(new Image()).src = url + '?' + params.join('&');