improve push notifications code

static/sw.js

@@ -1,32 +1,29 @@
 /* eslint-env serviceworker */
-self.addEventListener('push', function (event) {
+
+function getWindowClients () {
+  return clients.matchAll({ includeUncontrolled: true })
+    .then((clientList) => clientList.filter(({ type }) => type === 'window'))
+}
+
+self.addEventListener('push', (event) => {
   if (event.data) {
-    const data = event.data.json()
+    event.waitUntil(getWindowClients().then((list) => {
+      const data = event.data.json()
 
-    const promiseChain = clients.matchAll({
-      includeUncontrolled: true
-    }).then(function (clientList) {
-      const list = clientList.filter((item) => item.type === 'window')
-      if (list.length) return
-      return self.registration.showNotification(data.title, data)
-    })
-
-    event.waitUntil(promiseChain)
+      if (list.length === 0) return self.registration.showNotification(data.title, data)
+    }))
   }
 })
 
-self.addEventListener('notificationclick', function (event) {
+self.addEventListener('notificationclick', (event) => {
   event.notification.close()
 
-  event.waitUntil(clients.matchAll({
-    includeUncontrolled: true
-  }).then(function (clientList) {
-    const list = clientList.filter((item) => item.type === 'window')
-
+  event.waitUntil(getWindowClients().then((list) => {
     for (var i = 0; i < list.length; i++) {
       var client = list[i]
       if (client.url === '/' && 'focus' in client) { return client.focus() }
     }
-    if (clients.openWindow) { return clients.openWindow('/') }
+
+    if (clients.openWindow) return clients.openWindow('/')
   }))
 })