Messaging: Convert from var to const (#354)

* update firebase SDK version in service worker

* Convert from var to const for consistency

.eslintrc.json

@@ -1,4 +1,7 @@
 {
+    "parserOptions": {
+        "ecmaVersion": 2017
+    },
     "env": {
         "browser": true
     },

messaging/firebase-messaging-sw.js

@@ -5,7 +5,7 @@ importScripts('/__/firebase/6.3.4/firebase-app.js');
 importScripts('/__/firebase/6.3.4/firebase-messaging.js');
 importScripts('/__/firebase/init.js');
 
-var messaging = firebase.messaging();
+const messaging = firebase.messaging();
 
 /**
  * Here is is the code snippet to initialize Firebase Messaging in the Service
@@ -38,8 +38,8 @@ var messaging = firebase.messaging();
 messaging.setBackgroundMessageHandler(function(payload) {
   console.log('[firebase-messaging-sw.js] Received background message ', payload);
   // Customize notification here
-  var notificationTitle = 'Background Message Title';
-  var notificationOptions = {
+  const notificationTitle = 'Background Message Title';
+  const notificationOptions = {
     body: 'Background Message body.',
     icon: '/firebase-logo.png'
   };

messaging/index.html

@@ -156,7 +156,7 @@ <h4>Needs Permission</h4>
 
   function showToken(currentToken) {
     // Show token in console and UI.
-    var tokenElement = document.querySelector('#token');
+    const tokenElement = document.querySelector('#token');
     tokenElement.textContent = currentToken;
   }