Ajax

beacons.js

// Beacons
// This technique is very similar to dynamic script tag insertion. JavaScript is used to create
// a new Image object, with the src set to the URL of a script on your server. This URL
// contains the data we want to send back in the GET format of key-value pairs. Note that
// no img element has to be created or inserted into the DOM.

var url = '/status_tracker.php';
var params = [
    'step=2',
    'time=1248027314'
];
var beacon = new Image();
beacon.src = url + '?' + params.join('&');
beacon.onload = function() {
    if (this.width == 1) {
        // Success.
    }
    else if (this.width == 2) {
       // Failure; create another beacon and try again.
    }
};
beacon.onerror = function() {
 // Error; wait a bit, then create another beacon and try again.
};

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