How to migrate Flutter project to Firebase version-9 modern web modular style


I am facing problem with initializing the Firebase and Flutter project, while I just updated my firebase-tools and firebase to version 9 which is modern web modular style and its been a week nothing has helped the documentation of flutter is still according to version 8 which is old style compat type and when I follow firebase documentation which is updated its throwing errors with flutter. I need your help firebase and flutter experts please provide some solution.

The two error which I get are,

core.dart:56 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'app')
    at$ [as app] (core.dart:56)
    at initializeApp (firebase_core_web.dart:68)
    at (<anonymous>)
    at runBody (async_patch.dart:84)
    at Object._async [as async] (async_patch.dart:123)
    at (firebase_core_web.dart:28)
    at initializeApp (firebase.dart:42)
    at (<anonymous>)
    at runBody (async_patch.dart:84)
    at Object._async [as async] (async_patch.dart:123)
    at Function.initializeApp (firebase.dart:38)
    at main$ (main.dart:9)
    at main$.next (<anonymous>)
    at runBody (async_patch.dart:84)
    at Object._async [as async] (async_patch.dart:123)
    at main$ (main.dart:7)
    at main (web_entrypoint.dart:19)
    at (<anonymous>)
    at async_patch.dart:45
    at _RootZone.runUnary (zone.dart:1613)
    at _FutureListener.thenAwait.handleValue (future_impl.dart:155)
    at handleValueCallback (future_impl.dart:707)
    at Function._propagateToListeners (future_impl.dart:736)
    at[_completeWithValue] (future_impl.dart:542)
    at future.dart:401
    at _RootZone.runUnary (zone.dart:1613)
    at _FutureListener.then.handleValue (future_impl.dart:155)
    at handleValueCallback (future_impl.dart:707)
    at Function._propagateToListeners (future_impl.dart:736)
    at[_completeWithValue] (future_impl.dart:542)
    at (future_impl.dart:580)
    at Object._microtaskLoop (schedule_microtask.dart:40)
    at _startMicrotaskLoop (schedule_microtask.dart:49)
    at async_patch.dart:166


══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
js_primitives.dart:30 The following JSNoSuchMethodError was thrown building MyApp(dirty):
js_primitives.dart:30 TypeError: Cannot read properties of undefined (reading 'app')
js_primitives.dart:30 The relevant error-causing widget was:
js_primitives.dart:30   MyApp file:///C:/Users/Neha/AndroidStudioProjects/devcom/lib/main.dart:10:10
js_primitives.dart:30 When the exception was thrown, this was the stack:
js_primitives.dart:30 C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 236:49  throw_
js_primitives.dart:30 packages/firebase_core_web/firebase_core_web.dart 73:3                                                                     _catchJSError
js_primitives.dart:30 packages/firebase_core_web/src/firebase_core_web.dart 138:13                                                               app
js_primitives.dart:30 packages/firebase_core/src/firebase.dart 55:41                                                                             app
js_primitives.dart:30 packages/firebase_auth/src/firebase_auth.dart 38:47                                                                        get instance
js_primitives.dart:30 packages/devcom/auth.dart 12:51                                                                                            new
js_primitives.dart:30 packages/devcom/main.dart 17:13                                                                                            build
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 4648:28                                                                        build
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 4574:15                                                                        performRebuild
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 4267:5                                                                         rebuild
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 4553:5                                                                         [_firstBuild]
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 4548:5                                                                         mount
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 3611:13                                                                        inflateWidget
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 3363:18                                                                        updateChild
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 1189:16                                                                          [_rebuild]
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 1159:5                                                                           mount
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 1104:16                                                                          <fn>
js_primitives.dart:30 packages/flutter/src/widgets/framework.dart 2535:19                                                                        buildScope
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 1103:12                                                                          attachToRenderTree
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 936:24                                                                           attachRootWidget
js_primitives.dart:30 packages/flutter/src/widgets/binding.dart 917:7                                                                            <fn>
js_primitives.dart:30 C:/b/s/w/ir/cache/builder/src/out/host_debug/dart-sdk/lib/_internal/js_dev_runtime/private/isolate_helper.dart 48:19       internalCallback
js_primitives.dart:30 ════════════════════════════════════════════════════════════════════════════════════════════════════


import 'package:flutter/material.dart';
import 'package:devcom/auth.dart';
import 'package:devcom/auth_provider.dart';
import 'package:devcom/root_page.dart';
import 'package:firebase_core/firebase_core.dart';

void main() async {

class MyApp extends StatelessWidget {
  Widget build(BuildContext context) {
    return AuthProvider(
      auth: Auth(),
      child: MaterialApp(
        title: 'Flutter login demo',
        theme: ThemeData(
        home: RootPage(),

index.html :

<!DOCTYPE html>
    <!-- <script src="" type = "module"></script>
    <script src="" type = "module"> </script>
    <script src="" type = "module"></script> -->
    <meta charset="UTF-8">
    <meta content="IE=Edge" http-equiv="X-UA-Compatible">
    <meta name="description" content="A new Flutter project.">
    <!-- iOS meta tags & icons -->
    <meta name="apple-mobile-web-app-capable" content="yes">
    <meta name="apple-mobile-web-app-status-bar-style" content="black">
    <meta name="apple-mobile-web-app-title" content="devcom">
    <link rel="apple-touch-icon" href="icons/Icon-192.png">
    <!-- Favicon -->
    <link rel="shortcut icon" type="image/png" href="favicon.png" />
    <link rel="manifest" href="manifest.json">
<body >
  <!-- The core Firebase JS SDK is always required and must be listed first -->
<script type="module"> 
  import { initializeApp } from '';
  import {  } from '';
  import { getAnalytics } from "";

    const firebaseConfig = {
     apiKey: "...",
      authDomain: "......",
      databaseURL: ".....",
      projectId: "...",
      storageBucket: "",
      messagingSenderId: "",
      measurementId: "",
      appId: "1:.........:web:...........",
    const app = initializeApp({ firebaseConfig });
// const app = getAnalytics({ firebaseConfig })    
    var serviceWorkerVersion = null;
    var scriptLoaded = false;
    function loadMainDartJs() {
      if (scriptLoaded) {
      scriptLoaded = true;
      var scriptTag = document.createElement('script');
      scriptTag.src = 'main.dart.js';
      scriptTag.type = 'application/javascript';

    if ('serviceWorker' in navigator) {
      // Service workers are supported. Use them.
      window.addEventListener('load', function () {
        // Wait for registration to finish before dropping the <script> tag.
        // Otherwise, the browser will load the script multiple times,
        // potentially different versions.
        var serviceWorkerUrl = 'flutter_service_worker.js?v=' + serviceWorkerVersion;
          .then((reg) => {
            function waitForActivation(serviceWorker) {
              serviceWorker.addEventListener('statechange', () => {
                if (serviceWorker.state == 'activated') {
                  console.log('Installed new service worker.');
            if (! && (reg.installing || reg.waiting)) {
              // No active web worker and we have installed or are installing
              // one for the first time. Simply wait for it to activate.
              waitForActivation(reg.installing ?? reg.waiting);
            } else if (! {
              // When the app updates the serviceWorkerVersion changes, so we
              // need to ask the service worker to update.
              console.log('New service worker available.');
            } else {
              // Existing service worker is still good.
              console.log('Loading app from service worker.');

        // If service worker doesn't succeed in a reasonable amount of time,
        // fallback to plaint <script> tag.
        setTimeout(() => {
          if (!scriptLoaded) {
              'Failed to load app from service worker. Falling back to plain <script> tag.',
        }, 4000);
    } else {
      // Service workers not supported. Just drop the <script> tag.


Your flutter code does not follow what the docs suggest, especially where and how Firebase.initializeApp(); is called.

Here’s a snippet directly from the docs:

import 'package:flutter/material.dart';

// Import the firebase_core plugin
import 'package:firebase_core/firebase_core.dart';

void main() {

/// We are using a StatefulWidget such that we only create the [Future] once,
/// no matter how many times our widget rebuild.
/// If we used a [StatelessWidget], in the event where [App] is rebuilt, that
/// would re-initialize FlutterFire and make our application re-enter loading state,
/// which is undesired.
class App extends StatefulWidget {
  // Create the initialization Future outside of `build`:
  _AppState createState() => _AppState();

class _AppState extends State<App> {
  /// The future is part of the state of our widget. We should not call `initializeApp`
  /// directly inside [build].
  final Future<FirebaseApp> _initialization = Firebase.initializeApp();

  Widget build(BuildContext context) {
    return FutureBuilder(
      // Initialize FlutterFire:
      future: _initialization,
      builder: (context, snapshot) {
        // Check for errors
        if (snapshot.hasError) {
          return SomethingWentWrong();

        // Once complete, show your application
        if (snapshot.connectionState == ConnectionState.done) {
          return MyAwesomeApp();

        // Otherwise, show something whilst waiting for initialization to complete
        return Loading();

As you can see, that function is called after runApp(App()); has been called.
Not only does Firebase.initializeApp(); return a Future<FirebaseApp> (which you do not await in your code), it is part of the main widget’s state.

Answered By – Mat

Answer Checked By – Marie Seifert (FlutterFixes Admin)

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