Nullable dataset ext

web/CHANGELOG.md

@@ -11,6 +11,9 @@
 - Added `childNodesAsList` to `Node` and `childrenAsList` to `Element` via 
   extensions to support mutable operations on node lists.
 - Added `asList` to `NodeList` via extension.
+- Added `data` to `HTMLElement`, `SVGElement` and `MathMLElement` via
+  extension. It provides nullable interface for `dataset` allowing to check if
+  data attribute is set.
 
 ## 1.1.1
 

web/lib/src/helpers.dart

@@ -22,6 +22,7 @@
 library;
 
 export 'helpers/cross_origin.dart' show CrossOriginLocation, CrossOriginWindow;
+export 'helpers/dataset.dart';
 export 'helpers/enums.dart';
 export 'helpers/events/events.dart';
 export 'helpers/events/providers.dart';

web/lib/src/helpers/dataset.dart

@@ -0,0 +1,62 @@
+// Copyright (c) 2026, the Dart 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.
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import '../dom/html.dart';
+import '../dom/mathml_core.dart';
+import '../dom/svg.dart';
+
+/// Provides nullable api on `DOMStringMap`.
+///
+/// Native `DOMStringMap` returns non nullable `DOMString` from getter and
+/// returns `undefined` when given key does not exist. As in Dart there is no
+/// `undefined`, this wrapper will return null in such case.
+/// This extension also allows removing `dataset` elements.
+extension type NullableDOMStringMap._(JSObject _) implements JSObject {
+  /// Retrieves `dataset` element.
+  ///
+  /// When it is not set (`data-*` attribute is missing) returns `null`.
+  external String? operator [](String name);
+
+  /// Sets `dataset` element (and corresponding `data-*` attribute).
+  ///
+  /// Removes attribute if `value` is `null`.
+  void operator []=(
+    String name,
+    String? value,
+  ) {
+    if (value != null) {
+      setProperty(name.toJS, value.toJS);
+    } else {
+      remove(name);
+    }
+  }
+
+  /// Removes `dataset` element (and corresponding `data-*` attribute)
+  String? remove(String name) {
+    final ret = this[name];
+    delete(name.toJS);
+    return ret;
+  }
+}
+
+extension HTMLElementDatasetExtension on HTMLElement {
+  /// Wrapper for nullable [dataset]. See [NullableDOMStringMap] for details.
+  @JS('dataset')
+  external NullableDOMStringMap get data;
+}
+
+extension SVGElementDatasetExtension on SVGElement {
+  /// Wrapper for nullable [dataset]. See [NullableDOMStringMap] for details.
+  @JS('dataset')
+  external NullableDOMStringMap get data;
+}
+
+extension MathMLElementDatasetExtension on MathMLElement {
+  /// Wrapper for nullable [dataset]. See [NullableDOMStringMap] for details.
+  @JS('dataset')
+  external NullableDOMStringMap get data;
+}

web/test/helpers_test.dart

@@ -289,4 +289,33 @@ void main() {
   test('Uri.toJS throws an ArgumentError for a relative URL', () {
     expect(() => Uri.parse('/path').toJS, throwsArgumentError);
   });
+
+  test('nullable dataset extension', () {
+    final elements = [SVGSVGElement(), HTMLDivElement(), MathMLElement.mi()];
+
+    for (var element in elements) {
+      element.setAttribute('data-foo', 'bar');
+      element.setAttribute('data-foo-camel', 'bar');
+      final data = element.isA<SVGElement>()
+          ? (element as SVGElement).data
+          : element.isA<MathMLElement>()
+              ? (element as MathMLElement).data
+              : (element as HTMLElement).data;
+      //read existing and not existing data
+      expect(data['foo'], equals('bar'));
+      expect(data['fooCamel'], equals('bar'));
+      expect(data['nonexisting'], isNull);
+
+      //update data
+      data['foo'] = data['fooCamel'] = 'bar2';
+      expect(data['foo'], equals('bar2'));
+      expect(data['fooCamel'], equals('bar2'));
+
+      //unset data
+      data['foo'] = null;
+      expect(data.remove('fooCamel'), equals('bar2'));
+      expect(data['foo'], isNull);
+      expect(element.getAttribute('data-foo-camel'), isNull);
+    }
+  });
 }