fix(auth,web): convert Auth errors that omit customData without throwing (#18686)

JS Auth errors sometimes omit customData, which caused a TypeError in
getFirebaseAuthException after the sign-in failure was already handled.

Fixes #18680

Author: SelaseKay

packages/firebase_auth/firebase_auth_web/lib/src/interop/auth_interop.dart

@@ -630,7 +630,7 @@ extension AuthErrorExtension on AuthError {
   external set tenantId(JSString s);
   external JSString get phoneNumber;
   external set phoneNumber(JSString s);
-  external JSObject get customData;
+  external JSObject? get customData;
 }
 
 extension type AuthErrorCustomData._(JSObject _) implements JSObject {

packages/firebase_auth/firebase_auth_web/lib/src/utils/web_utils.dart

@@ -92,7 +92,8 @@ FirebaseAuthException getFirebaseAuthException(
       .replaceFirst('Firebase: ', '');
 
   // "customData" - see Firebase AuthError docs: https://firebase.google.com/docs/reference/js/auth.autherror
-  final customData = exception.customData as auth_interop.AuthErrorCustomData;
+  // Some Auth error shapes omit customData; casting a missing value throws TypeError.
+  final customData = exception.customData as auth_interop.AuthErrorCustomData?;
 
   if (code == 'multi-factor-auth-required') {
     final _auth = auth;
@@ -111,9 +112,9 @@ FirebaseAuthException getFirebaseAuthException(
     return FirebaseAuthMultiFactorExceptionPlatform(
       code: code,
       message: message,
-      email: customData.email?.toDart,
-      phoneNumber: customData.phoneNumber?.toDart,
-      tenantId: customData.tenantId?.toDart,
+      email: customData?.email?.toDart,
+      phoneNumber: customData?.phoneNumber?.toDart,
+      tenantId: customData?.tenantId?.toDart,
       resolver: MultiFactorResolverWeb(
         resolverWeb.hints.map(fromInteropMultiFactorInfo).toList(),
         MultiFactorSessionWeb('web', resolverWeb.session),
@@ -127,9 +128,9 @@ FirebaseAuthException getFirebaseAuthException(
   return FirebaseAuthException(
     code: code,
     message: message,
-    email: customData.email?.toDart,
-    phoneNumber: customData.phoneNumber?.toDart,
-    tenantId: customData.tenantId?.toDart,
+    email: customData?.email?.toDart,
+    phoneNumber: customData?.phoneNumber?.toDart,
+    tenantId: customData?.tenantId?.toDart,
     credential: credential,
   );
 }

packages/firebase_auth/firebase_auth_web/test/get_firebase_auth_exception_test.dart

@@ -0,0 +1,64 @@
+// Copyright 2020, the Chromium project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+@TestOn('browser')
+library;
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import 'package:firebase_auth_platform_interface/firebase_auth_platform_interface.dart';
+import 'package:firebase_auth_web/src/utils/web_utils.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  setUp(() {
+    // getFirebaseAuthException always calls OAuthProvider.credentialFromError.
+    // Stub it so this test does not need the full Auth JS SDK.
+    final oauthProvider = JSObject();
+    oauthProvider['credentialFromError'] = ((JSAny _) => null).toJS;
+    final firebaseAuth = JSObject();
+    firebaseAuth['OAuthProvider'] = oauthProvider;
+    globalContext['firebase_auth'] = firebaseAuth;
+  });
+
+  JSObject authError({required bool includeCustomData}) {
+    final error = JSObject();
+    error['name'] = 'FirebaseError'.toJS;
+    error['code'] = 'auth/invalid-credential'.toJS;
+    error['message'] =
+        'Firebase: INVALID_LOGIN_CREDENTIALS (auth/invalid-credential).'.toJS;
+    if (includeCustomData) {
+      final customData = JSObject();
+      customData['appName'] = '[DEFAULT]'.toJS;
+      customData['email'] = 'user@example.com'.toJS;
+      error['customData'] = customData;
+    }
+    return error;
+  }
+
+  test(
+    'converts an Auth error that omits customData without throwing',
+    () {
+      final exception = getFirebaseAuthException(
+        authError(includeCustomData: false),
+      );
+
+      expect(exception, isA<FirebaseAuthException>());
+      expect(exception.code, 'invalid-credential');
+      expect(exception.email, isNull);
+      expect(exception.phoneNumber, isNull);
+      expect(exception.tenantId, isNull);
+    },
+  );
+
+  test('still reads email from customData when it is present', () {
+    final exception = getFirebaseAuthException(
+      authError(includeCustomData: true),
+    );
+
+    expect(exception.code, 'invalid-credential');
+    expect(exception.email, 'user@example.com');
+  });
+}

packages/firebase_core/firebase_core_web/lib/src/interop/utils/es6_interop.dart

@@ -22,5 +22,6 @@ extension JSErrorExtension on JSError {
   external JSString? get stack;
 
   // "customData" - see Firebase AuthError docs: https://firebase.google.com/docs/reference/js/auth.autherror
-  external JSAny get customData;
+  // Auth errors from the JS SDK usually include this, but some error shapes omit it.
+  external JSAny? get customData;
 }