Fix JsBackedMap sometimes returning undefined instead of null

lib/react_client/js_backed_map.dart

@@ -59,7 +59,9 @@ class JsBackedMap extends MapBase<dynamic, dynamic> {
 
  @override
  dynamic /*?*/ operator [](Object? key) {
- return DartValueWrapper.unwrapIfNeeded(js_util.getProperty(jsObject, nonNullableJsPropertyName(key)));
+ // `?? null` as a workaround to `undefined` coming through and causing issues: https://github.com/dart-lang/sdk/issues/36116
+ // ignore: unnecessary_null_in_if_null_operators
+ return DartValueWrapper.unwrapIfNeeded(js_util.getProperty(jsObject, nonNullableJsPropertyName(key)) ?? null);
  }
 
  @override

test/react_client/js_backed_map_test.dart

@@ -5,6 +5,7 @@ library react.js_backed_map_test.dart;
 import 'package:js/js.dart';
 import 'package:js/js_util.dart';
 import 'package:react/react_client/js_backed_map.dart';
+import 'package:react/react_client/react_interop.dart';
 import 'package:test/test.dart';
 
 import '../shared_type_tester.dart';
@@ -49,6 +50,40 @@ main() {
  });
  });
 
+ // Tests related to our workaround for https://github.com/dart-lang/sdk/issues/36116.
+ group(
+ 'map lookup returns null instead of undefined, so that the value does not interfere with Dart function calls:',
+ () {
+ test('test setup check: getNullishValueType works as expected', () {
+ expect(getNullishValueType(jsUndefined), 'undefined');
+ expect(getNullishValueType(jsNull), 'null');
+ });
+
+ test('returns null and not undefined', () {
+ final jsBackedMap = JsBackedMap();
+ final value = jsBackedMap['missingKey'];
+ // We can't use `identical(value, jsUndefined)` (or the `same` matcher which uses `identical`)
+ // since it's special-cased to return true when passed two nullish (null or undefined) values.
+ // Instead, we'll use a custom JS interop function to classify the value
+ expect(getNullishValueType(value), 'null');
+ });
+
+ test('test setup check: triggersDdcOptionalArgumentBug works as expected to reproduce and detect the bug', () {
+ expect(triggersDdcOptionalArgumentBug(jsUndefined), isTrue,
+ reason: 'if this expectation fails in newer Dart SDKs, then this bug may have been fixed.');
+ expect(triggersDdcOptionalArgumentBug(null), isFalse);
+ // Since this bug only affects DDC, this test will fail in dart2js.
+ }, tags: 'no-dart2js');
+
+ // We only need to run this test in DDC since the bug does not effect dart2js, but it doesn't hurt to also
+ // validate behavior in dart2js, and could catch new, similar dart2js bugs.
+ test('regression test: returned value does not trigger argument issues', () {
+ final jsBackedMap = JsBackedMap();
+ final value = jsBackedMap['missingKey'];
+ expect(triggersDdcOptionalArgumentBug(value), isFalse);
+ });
+ });
+
  group('constructor', () {
  test('(default) creates an instance backed by a new JS object', () {
  final jsBackedMap = JsBackedMap();
@@ -210,5 +245,23 @@ main() {
 
 class TestObject {}
 
+@JS()
+external String getNullishValueType(dynamic value);
+
+class _Default {
+ const _Default();
+}
+
+const _default = _Default();
+
+bool _defaultArgumentSpecifiedUsed([x = _default]) => x == _default;
+
+bool triggersDdcOptionalArgumentBug(dynamic value) {
+ // We'd expect this call to always return false,
+ // but it returns true in DDC if `value` is `undefined`.
+ // https://github.com/dart-lang/sdk/issues/36116
+ return _defaultArgumentSpecifiedUsed(value);
+}
+
 @JS('Object.freeze')
 external void _objectFreeze(JsMap object);

test/react_client/js_backed_map_test.html

@@ -8,6 +8,16 @@
 
  <script>
  function JsType() {}
+
+ function getNullishValueType(value) {
+ if (typeof value === 'undefined') {
+ return 'undefined';
+ }
+ if (value === null) {
+ return 'null';
+ }
+ throw new Error('value was neither null nor undefined');
+ }
  </script>
 
  <link rel="x-dart-test" href="js_backed_map_test.dart">

test/shared_type_tester.dart

@@ -22,6 +22,10 @@ void sharedTypeTests(
  bool skipJsAnonInteropTypes = false,
  bool skipJsInteropTypes = true,
 }) {
+ test('null', () {
+ testTypeValue(null);
+ });
+
  if (!skipNormalDartObjects) {
  test('normal Dart objects', () {
  final object = Foo('f');