Implement isSupported static method

Author: mmermerkaya

packages/messaging/index.ts

@@ -26,23 +26,27 @@ import { ERROR_CODES, errorFactory } from './src/models/errors';
 
 import * as types from '@firebase/messaging-types';
 
+export interface MessagingServiceFactory extends FirebaseServiceFactory {
+  isSupported?(): boolean;
+}
+
 export function registerMessaging(instance: _FirebaseNamespace): void {
   const messagingName = 'messaging';
-  const factoryMethod: FirebaseServiceFactory = app => {
+
+  const factoryMethod: MessagingServiceFactory = app => {
+    if (!isSupported()) {
+      throw errorFactory.create(ERROR_CODES.UNSUPPORTED_BROWSER);
+    }
+
     if (self && 'ServiceWorkerGlobalScope' in self) {
       // Running in ServiceWorker context
-      if (isSWControllerSupported()) {
-        return new SWController(app);
-      }
+      return new SWController(app);
     } else {
       // Assume we are in the window context.
-      if (isWindowControllerSupported()) {
-        return new WindowController(app);
-      }
+      return new WindowController(app);
     }
-
-    throw errorFactory.create(ERROR_CODES.UNSUPPORTED_BROWSER);
   };
+  factoryMethod.isSupported = isSupported;
 
   const namespaceExports = {
     // no-inline
@@ -73,6 +77,16 @@ declare module '@firebase/app-types' {
   }
 }
 
+function isSupported(): boolean {
+  if (self && 'ServiceWorkerGlobalScope' in self) {
+    // Running in ServiceWorker context
+    return isSWControllerSupported();
+  } else {
+    // Assume we are in the window context.
+    return isWindowControllerSupported();
+  }
+}
+
 /**
  * Checks to see if the required APIs exist.
  */

packages/messaging/test/index.test.ts

@@ -20,10 +20,9 @@ import { sandbox, SinonSandbox, SinonStub } from 'sinon';
 import { FirebaseApp } from '@firebase/app-types';
 import {
   _FirebaseNamespace,
-  FirebaseServiceFactory
 } from '@firebase/app-types/private';
 
-import { registerMessaging } from '../index';
+import { MessagingServiceFactory, registerMessaging } from '../index';
 import { ERROR_CODES } from '../src/models/errors';
 
 import { SWController } from '../src/controllers/sw-controller';
@@ -54,7 +53,7 @@ describe('Firebase Messaging > registerMessaging', () => {
   });
 
   describe('factoryMethod', () => {
-    let factoryMethod: FirebaseServiceFactory;
+    let factoryMethod: MessagingServiceFactory;
     let fakeApp: FirebaseApp;
 
     beforeEach(() => {
@@ -66,6 +65,12 @@ describe('Firebase Messaging > registerMessaging', () => {
       });
     });
 
+    describe('isSupported', () => {
+      it('is a static method on factoryMethod', () => {
+        expect(factoryMethod.isSupported).to.be.a('function');
+      });
+    });
+
     describe('in Service Worker context', () => {
       beforeEach(() => {
         // self.ServiceWorkerGlobalScope exists