JavaScript: Immediately active Service Worker #JavaScript

serviceworker.js

self.addEventListener('install', function(event) {
	return self.skipWaiting();
});

self.addEventListener('activate', function(event) {
	return event.waitUntil(self.clients.claim());
});

wait.js

function init() {
	// do something after Service Worker is active
}

if('serviceWorker' in navigator && 'Promise' in global) {
	app.worker = new Promise(function(resolve, reject) {
		navigator.serviceWorker.register('path/to/serviceworker.js')
			.then(function(registration) {
				var serviceWorker;

				if(serviceWorker = registration.installing || registration.waiting) {
					serviceWorker.addEventListener('statechange', function(event) {
						if(event.target.state === 'activated') {
							resolve();
						}
					});
				}

				if(registration.active) {
					resolve();
				}
			})
			.catch(function() {
				reject();
			});
	})
	.finally(init);
} else {
	init();
}