e";
        var worker_url = "worker.js";

        return navigator.serviceWorker.register(worker_url, {scope: scope})
            .then(
                function(registration) {
                    return registration.unregister();
                })
            .then(
                function() {
                    return navigator.serviceWorker.register(worker_url, {scope: scope});
                })
            .then(
                function(registration) {
                    add_completion_callback(