diff --git a/protobuf/CHANGELOG.md b/protobuf/CHANGELOG.md
index a987ef8..209c21b 100644
--- a/protobuf/CHANGELOG.md
+++ b/protobuf/CHANGELOG.md
@@ -8,8 +8,12 @@
 * Some of the private `PbFieldType` members are made public, to allow using
   them in internal libraries. This type is for internal use only. ([#1027])
 
+* Improve performance of `GeneratedMessage` members: `writeToJsonMap`,
+  `writeToJson`, `mergeFromJson`, `mergeFromJsonMap`. ([#1028])
+
 [#1026]: https://github.com/google/protobuf.dart/pull/1026
 [#1027]: https://github.com/google/protobuf.dart/pull/1027
+[#1028]: https://github.com/google/protobuf.dart/pull/1028
 
 ## 4.1.1
 
diff --git a/protobuf/lib/src/protobuf/generated_message.dart b/protobuf/lib/src/protobuf/generated_message.dart
index 35caa69..9b5c866 100644
--- a/protobuf/lib/src/protobuf/generated_message.dart
+++ b/protobuf/lib/src/protobuf/generated_message.dart
@@ -227,7 +227,7 @@
   /// Unknown field data, data for which there is no metadata for the associated
   /// field, will only be included if this message was deserialized from the
   /// same wire format.
-  Map<String, dynamic> writeToJsonMap() => _writeToJsonMap(_fieldSet);
+  Map<String, dynamic> writeToJsonMap() => json_lib.writeToJsonMap(_fieldSet);
 
   /// Returns a JSON string that encodes this message.
   ///
@@ -246,7 +246,7 @@
   /// Unknown field data, data for which there is no metadata for the associated
   /// field, will only be included if this message was deserialized from the
   /// same wire format.
-  String writeToJson() => jsonEncode(writeToJsonMap());
+  String writeToJson() => json_lib.writeToJsonString(_fieldSet);
 
   /// Returns an Object representing Proto3 JSON serialization of `this`.
   ///
@@ -318,19 +318,9 @@
     String data, [
     ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY,
   ]) {
-    /// Disable lazy creation of Dart objects for a dart2js speedup.
-    /// This is a slight regression on the Dart VM.
-    /// TODO(skybrian) we could skip the reviver if we're running
-    /// on the Dart VM for a slight speedup.
-    final Map<String, dynamic> jsonMap = jsonDecode(
-      data,
-      reviver: _emptyReviver,
-    );
-    _mergeFromJsonMap(_fieldSet, jsonMap, extensionRegistry);
+    json_lib.mergeFromJsonString(_fieldSet, data, extensionRegistry);
   }
 
-  static Object? _emptyReviver(Object? k, Object? v) => v;
-
   /// Merges field values from a JSON object represented as a Dart map.
   ///
   /// The encoding is described in [GeneratedMessage.writeToJson].
@@ -338,7 +328,7 @@
     Map<String, dynamic> json, [
     ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY,
   ]) {
-    _mergeFromJsonMap(_fieldSet, json, extensionRegistry);
+    json_lib.mergeFromJsonMap(_fieldSet, json, extensionRegistry);
   }
 
   /// Adds an extension field value to a repeated field.
diff --git a/protobuf/lib/src/protobuf/internal.dart b/protobuf/lib/src/protobuf/internal.dart
index 176084d..e3a29d0 100644
--- a/protobuf/lib/src/protobuf/internal.dart
+++ b/protobuf/lib/src/protobuf/internal.dart
@@ -8,14 +8,7 @@
 library;
 
 import 'dart:collection' show ListBase, MapBase;
-import 'dart:convert'
-    show
-        Utf8Decoder,
-        Utf8Encoder,
-        base64Decode,
-        base64Encode,
-        jsonDecode,
-        jsonEncode;
+import 'dart:convert' show Utf8Decoder, Utf8Encoder, base64Decode, base64Encode;
 import 'dart:math' as math;
 import 'dart:typed_data' show ByteData, Endian, Uint8List;
 
@@ -23,6 +16,7 @@
 import 'package:meta/meta.dart' show UseResult;
 
 import 'consts.dart';
+import 'json/json.dart' as json_lib;
 import 'json_parsing_context.dart';
 import 'permissive_compare.dart';
 import 'type_registry.dart';
@@ -45,7 +39,6 @@
 part 'field_type.dart';
 part 'generated_message.dart';
 part 'generated_service.dart';
-part 'json.dart';
 part 'message_set.dart';
 part 'pb_list.dart';
 part 'pb_map.dart';
diff --git a/protobuf/lib/src/protobuf/json.dart b/protobuf/lib/src/protobuf/json/json.dart
similarity index 84%
rename from protobuf/lib/src/protobuf/json.dart
rename to protobuf/lib/src/protobuf/json/json.dart
index 5021fc0..05d1ac1 100644
--- a/protobuf/lib/src/protobuf/json.dart
+++ b/protobuf/lib/src/protobuf/json/json.dart
@@ -2,9 +2,20 @@
 // 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.
 
-part of 'internal.dart';
+import 'dart:convert' show base64Decode, base64Encode;
 
-Map<String, dynamic> _writeToJsonMap(FieldSet fs) {
+import 'package:fixnum/fixnum.dart' show Int64;
+
+import '../consts.dart';
+import '../internal.dart';
+import '../utils.dart';
+
+// Use json_vm.dart with VM and dart2wasm, json_web.dart with dart2js.
+// json_web.dart uses JS interop for parsing, and JS interop is too slow on
+// Wasm. VM's patch performs better in Wasm.
+export 'json_vm.dart' if (dart.library.html) 'json_web.dart';
+
+Map<String, dynamic> writeToJsonMap(FieldSet fs) {
   dynamic convertToMap(dynamic fieldValue, int fieldType) {
     final baseType = PbFieldType.baseType(fieldType);
 
@@ -61,15 +72,15 @@
   List writeMap(PbMap fieldValue, MapFieldInfo fi) => List.from(
     fieldValue.entries.map(
       (MapEntry e) => {
-        '${PbMap._keyFieldNumber}': convertToMap(e.key, fi.keyFieldType),
-        '${PbMap._valueFieldNumber}': convertToMap(e.value, fi.valueFieldType),
+        '$mapKeyFieldNumber': convertToMap(e.key, fi.keyFieldType),
+        '$mapValueFieldNumber': convertToMap(e.value, fi.valueFieldType),
       },
     ),
   );
 
   final result = <String, dynamic>{};
-  for (final fi in fs._infosSortedByTag) {
-    final value = fs._values[fi.index!];
+  for (final fi in fs.infosSortedByTag) {
+    final value = fs.values[fi.index!];
     if (value == null || (value is List && value.isEmpty)) {
       continue; // It's missing, repeated, or an empty byte array.
     }
@@ -82,18 +93,18 @@
     }
     result['${fi.tagNumber}'] = convertToMap(value, fi.type);
   }
-  final extensions = fs._extensions;
+  final extensions = fs.extensions;
   if (extensions != null) {
-    for (final tagNumber in sorted(extensions._tagNumbers)) {
-      final value = extensions._values[tagNumber];
+    for (final tagNumber in sorted(extensions.tagNumbers)) {
+      final value = extensions.values[tagNumber];
       if (value is List && value.isEmpty) {
         continue; // It's repeated or an empty byte array.
       }
-      final fi = extensions._getInfoOrNull(tagNumber)!;
+      final fi = extensions.getInfoOrNull(tagNumber)!;
       result['$tagNumber'] = convertToMap(value, fi.type);
     }
   }
-  final unknownJsonData = fs._unknownJsonData;
+  final unknownJsonData = fs.unknownJsonData;
   if (unknownJsonData != null) {
     unknownJsonData.forEach((key, value) {
       result[key] = value;
@@ -104,20 +115,20 @@
 
 // Merge fields from a previously decoded JSON object.
 // (Called recursively on nested messages.)
-void _mergeFromJsonMap(
+void mergeFromJsonMap(
   FieldSet fs,
   Map<String, dynamic> json,
   ExtensionRegistry? registry,
 ) {
-  fs._ensureWritable();
+  fs.ensureWritable();
   final keys = json.keys;
-  final meta = fs._meta;
+  final meta = fs.meta;
   for (final key in keys) {
     var fi = meta.byTagAsString[key];
     if (fi == null) {
-      fi = registry?.getExtension(fs._messageName, int.parse(key));
+      fi = registry?.getExtension(fs.messageName, int.parse(key));
       if (fi == null) {
-        (fs._unknownJsonData ??= {})[key] = json[key];
+        (fs.unknownJsonData ??= {})[key] = json[key];
         continue;
       }
     }
@@ -144,7 +155,7 @@
   FieldInfo fi,
   ExtensionRegistry? registry,
 ) {
-  final repeated = fi._ensureRepeatedField(meta, fs);
+  final repeated = fi.ensureRepeatedField(meta, fs);
   // Micro optimization. Using "for in" generates the following and iterator
   // alloc:
   //   for (t1 = J.get$iterator$ax(json), t2 = fi.tagNumber, t3 = fi.type,
@@ -175,23 +186,23 @@
   ExtensionRegistry? registry,
 ) {
   final entryMeta = fi.mapEntryBuilderInfo;
-  final map = fi._ensureMapField(meta, fs);
+  final map = fi.ensureMapField(meta, fs);
   for (final jsonEntryDynamic in jsonList) {
     final jsonEntry = jsonEntryDynamic as Map<String, dynamic>;
     final entryFieldSet = FieldSet(null, entryMeta);
     final convertedKey = _convertJsonValue(
       entryMeta,
       entryFieldSet,
-      jsonEntry['${PbMap._keyFieldNumber}'],
-      PbMap._keyFieldNumber,
+      jsonEntry['$mapKeyFieldNumber'],
+      mapKeyFieldNumber,
       fi.keyFieldType,
       registry,
     );
     var convertedValue = _convertJsonValue(
       entryMeta,
       entryFieldSet,
-      jsonEntry['${PbMap._valueFieldNumber}'],
-      PbMap._valueFieldNumber,
+      jsonEntry['$mapValueFieldNumber'],
+      mapValueFieldNumber,
       fi.valueFieldType,
       registry,
     );
@@ -223,10 +234,10 @@
   // Therefore we run _validateField for debug builds only to validate
   // correctness of conversion.
   assert(() {
-    fs._validateField(fi, value);
+    fs.validateField(fi, value);
     return true;
   }());
-  fs._setFieldUnchecked(meta, fi, value);
+  fs.setFieldUnchecked(meta, fi, value);
 }
 
 /// Converts [value] from the JSON format to the Dart data type suitable for
@@ -298,7 +309,7 @@
         // The following call will return null if the enum value is unknown.
         // In that case, we want the caller to ignore this value, so we return
         // null from this method as well.
-        return meta._decodeEnum(tagNumber, registry, value);
+        return meta.decodeEnum(tagNumber, registry, value);
       }
       expectedType = 'int or stringified int';
       break;
@@ -333,8 +344,8 @@
     case PbFieldType.MESSAGE_BIT:
       if (value is Map) {
         final messageValue = value as Map<String, dynamic>;
-        final subMessage = meta._makeEmptyMessage(tagNumber, registry);
-        _mergeFromJsonMap(subMessage._fieldSet, messageValue, registry);
+        final subMessage = meta.makeEmptyMessage(tagNumber, registry);
+        mergeFromJsonMap(subMessage.fieldSet, messageValue, registry);
         return subMessage;
       }
       expectedType = 'nested message or group';
diff --git a/protobuf/lib/src/protobuf/json/json_vm.dart b/protobuf/lib/src/protobuf/json/json_vm.dart
new file mode 100644
index 0000000..831756a
--- /dev/null
+++ b/protobuf/lib/src/protobuf/json/json_vm.dart
@@ -0,0 +1,23 @@
+// Copyright (c) 2025, 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:convert' show jsonDecode, jsonEncode;
+
+import '../internal.dart';
+import 'json.dart';
+
+String writeToJsonString(FieldSet fs) => jsonEncode(writeToJsonMap(fs));
+
+/// Merge fields from a [json] string.
+void mergeFromJsonString(
+  FieldSet fs,
+  String json,
+  ExtensionRegistry? registry,
+) {
+  final jsonMap = jsonDecode(json);
+  if (jsonMap is! Map<String, dynamic>) {
+    throw ArgumentError.value(json, 'json', 'Does not parse to a JSON object.');
+  }
+  mergeFromJsonMap(fs, jsonMap, registry);
+}
diff --git a/protobuf/lib/src/protobuf/json/json_web.dart b/protobuf/lib/src/protobuf/json/json_web.dart
new file mode 100644
index 0000000..b0a8e2e
--- /dev/null
+++ b/protobuf/lib/src/protobuf/json/json_web.dart
@@ -0,0 +1,471 @@
+// Copyright (c) 2025, 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:convert' show base64Decode, base64Encode;
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import 'package:fixnum/fixnum.dart' show Int64;
+
+import '../consts.dart';
+import '../internal.dart';
+import '../utils.dart';
+
+@JS('JSON')
+extension type _JSON._(JSObject _) implements JSObject {
+  @JS('JSON.stringify')
+  external static JSString _stringify(JSObject value);
+
+  @JS('JSON.parse')
+  external static JSAny? _parse(JSString text);
+}
+
+@JS('Number')
+extension type _Number._(JSObject _) implements JSObject {
+  @JS('Number.isInteger')
+  external static bool _isInteger(JSAny value);
+}
+
+@JS('Object.keys')
+external JSArray<JSString> _objectKeys(JSObject obj);
+
+@JS('Object.prototype')
+external JSObject get _objectPrototype;
+
+@JS('Object.getPrototypeOf')
+external JSObject _getPrototypeOf(JSAny obj);
+
+extension on JSAny {
+  /// Returns this typed as [T] while omitting the `as` cast. For use after an
+  /// `isA` check.
+  @pragma('dart2js:as:trust')
+  @pragma('dart2js:prefer-inline')
+  T _as<T extends JSAny>() => this as T;
+}
+
+String writeToJsonString(FieldSet fs) {
+  final rawJs = _writeToRawJs(fs);
+  return _JSON._stringify(rawJs).toDart;
+}
+
+JSObject _writeToRawJs(FieldSet fs) {
+  JSAny convertToRawJs(dynamic fieldValue, int fieldType) {
+    final baseType = PbFieldType.baseType(fieldType);
+
+    if (PbFieldType.isRepeated(fieldType)) {
+      final PbList list = fieldValue;
+      final length = list.length;
+      final jsArray = JSArray.withLength(length);
+      for (var i = 0; i < length; i++) {
+        final entry = list[i];
+        jsArray[i] = convertToRawJs(entry, baseType);
+      }
+      return jsArray;
+    }
+
+    switch (baseType) {
+      case PbFieldType.INT32_BIT:
+      case PbFieldType.SINT32_BIT:
+      case PbFieldType.UINT32_BIT:
+      case PbFieldType.FIXED32_BIT:
+      case PbFieldType.SFIXED32_BIT:
+        final int value = fieldValue;
+        return value.toJS;
+
+      case PbFieldType.BOOL_BIT:
+        final bool value = fieldValue;
+        return value.toJS;
+
+      case PbFieldType.STRING_BIT:
+        final String value = fieldValue;
+        return value.toJS;
+
+      case PbFieldType.FLOAT_BIT:
+      case PbFieldType.DOUBLE_BIT:
+        final double value = fieldValue;
+        if (value.isNaN) {
+          return nan.toJS;
+        }
+        if (value.isInfinite) {
+          return value.isNegative ? negativeInfinity.toJS : infinity.toJS;
+        }
+        if (value.toInt() == value) {
+          return value.toInt().toJS;
+        }
+        return value.toJS;
+
+      case PbFieldType.BYTES_BIT:
+        // Encode 'bytes' as a base64-encoded string.
+        final List<int> value = fieldValue;
+        return base64Encode(value).toJS;
+
+      case PbFieldType.ENUM_BIT:
+        final ProtobufEnum enum_ = fieldValue;
+        return enum_.value.toJS; // assume |value| < 2^52
+
+      case PbFieldType.INT64_BIT:
+      case PbFieldType.SINT64_BIT:
+      case PbFieldType.SFIXED64_BIT:
+        final Int64 int_ = fieldValue;
+        return int_.toString().toJS;
+
+      case PbFieldType.UINT64_BIT:
+      case PbFieldType.FIXED64_BIT:
+        final Int64 int_ = fieldValue;
+        return int_.toStringUnsigned().toJS;
+
+      case PbFieldType.GROUP_BIT:
+      case PbFieldType.MESSAGE_BIT:
+        final GeneratedMessage msg = fieldValue;
+        return _writeToRawJs(msg.fieldSet);
+
+      default:
+        throw UnsupportedError('Unknown type $fieldType');
+    }
+  }
+
+  JSArray writeMap(PbMap fieldValue, MapFieldInfo fi) {
+    final length = fieldValue.entries.length;
+    final jsArray = JSArray.withLength(length);
+    var index = 0;
+    for (final entry in fieldValue.entries) {
+      final entryJsObj = JSObject();
+      entryJsObj.setProperty(
+        mapKeyFieldNumber.toJS,
+        convertToRawJs(entry.key, fi.keyFieldType),
+      );
+      entryJsObj.setProperty(
+        mapValueFieldNumber.toJS,
+        convertToRawJs(entry.value, fi.valueFieldType),
+      );
+      jsArray[index] = entryJsObj;
+      index++;
+    }
+    return jsArray;
+  }
+
+  final result = JSObject();
+  for (final fi in fs.infosSortedByTag) {
+    final value = fs.values[fi.index!];
+    if (value == null || (value is List && value.isEmpty)) {
+      continue; // It's missing, repeated, or an empty byte array.
+    }
+    if (PbFieldType.isMapField(fi.type)) {
+      result.setProperty(
+        fi.tagNumber.toJS,
+        writeMap(value, fi as MapFieldInfo<dynamic, dynamic>),
+      );
+      continue;
+    }
+    result.setProperty(fi.tagNumber.toJS, convertToRawJs(value, fi.type));
+  }
+  final extensions = fs.extensions;
+  if (extensions != null) {
+    for (final tagNumber in sorted(extensions.tagNumbers)) {
+      final value = extensions.values[tagNumber];
+      if (value is List && value.isEmpty) {
+        continue; // It's repeated or an empty byte array.
+      }
+      final fi = extensions.getInfoOrNull(tagNumber)!;
+      result.setProperty(tagNumber.toJS, convertToRawJs(value, fi.type));
+    }
+  }
+  final unknownJsonData = fs.unknownJsonData;
+  if (unknownJsonData != null) {
+    unknownJsonData.forEach((key, value) {
+      result.setProperty(key.toJS, value);
+    });
+  }
+  return result;
+}
+
+/// Merge fields from a [json] string.
+void mergeFromJsonString(
+  FieldSet fs,
+  String json,
+  ExtensionRegistry? registry,
+) {
+  final JSAny? parsed;
+  try {
+    parsed = _JSON._parse(json.toJS);
+  } catch (e) {
+    throw FormatException(e.toString());
+  }
+  if (parsed == null || !parsed.isA<JSObject>()) {
+    throw ArgumentError.value(json, 'json', 'Does not parse to a JSON object.');
+  }
+  _mergeFromRawJsMap(fs, parsed._as<JSObject>(), registry);
+}
+
+void _mergeFromRawJsMap(
+  FieldSet fs,
+  JSObject json,
+  ExtensionRegistry? registry,
+) {
+  fs.ensureWritable();
+
+  final meta = fs.meta;
+  final keys = _objectKeys(json);
+  final length = keys.length;
+  for (var i = 0; i < length; i++) {
+    final jsKey = keys[i];
+    final key = jsKey.toDart;
+    var fi = meta.byTagAsString[key];
+    if (fi == null) {
+      fi = registry?.getExtension(fs.messageName, int.parse(key));
+      if (fi == null) {
+        (fs.unknownJsonData ??= {})[key] = json.getProperty<JSAny>(jsKey);
+        continue;
+      }
+    }
+    if (fi.isMapField) {
+      _appendRawJsMap(
+        meta,
+        fs,
+        json.getProperty<JSArray<JSObject>>(jsKey),
+        fi as MapFieldInfo<dynamic, dynamic>,
+        registry,
+      );
+    } else if (fi.isRepeated) {
+      _appendRawJsList(
+        meta,
+        fs,
+        json.getProperty<JSArray<JSAny>>(jsKey),
+        fi,
+        registry,
+      );
+    } else {
+      _setRawJsField(meta, fs, json.getProperty<JSAny>(jsKey), fi, registry);
+    }
+  }
+}
+
+void _appendRawJsList(
+  BuilderInfo meta,
+  FieldSet fs,
+  JSArray<JSAny> jsonList,
+  FieldInfo fi,
+  ExtensionRegistry? registry,
+) {
+  final repeated = fi.ensureRepeatedField(meta, fs);
+  // Micro optimization. Using "for in" generates the following and iterator
+  // alloc:
+  //   for (t1 = J.get$iterator$ax(json), t2 = fi.tagNumber, t3 = fi.type,
+  //       t4 = J.getInterceptor$ax(repeated); t1.moveNext$0();)
+  final length = jsonList.length;
+  for (var i = 0; i < length; i++) {
+    final value = jsonList[i];
+    var convertedValue = _convertRawJsValue(
+      meta,
+      fs,
+      value,
+      fi.tagNumber,
+      fi.type,
+      registry,
+    );
+    // In the case of an unknown enum value, the converted value may return
+    // null. The default enum value should be used in these cases, which is
+    // stored in the FieldInfo.
+    convertedValue ??= fi.defaultEnumValue;
+    repeated.add(convertedValue);
+  }
+}
+
+void _appendRawJsMap(
+  BuilderInfo meta,
+  FieldSet fs,
+  JSArray<JSObject> jsonList,
+  MapFieldInfo fi,
+  ExtensionRegistry? registry,
+) {
+  final entryMeta = fi.mapEntryBuilderInfo;
+  final map = fi.ensureMapField(meta, fs);
+  final length = jsonList.length;
+
+  for (var i = 0; i < length; i++) {
+    final value = jsonList[i];
+    final entryFieldSet = FieldSet(null, entryMeta);
+
+    final convertedKey = _convertRawJsValue(
+      entryMeta,
+      entryFieldSet,
+      value.getProperty<JSAny>(mapKeyFieldNumber.toJS),
+      mapKeyFieldNumber,
+      fi.keyFieldType,
+      registry,
+    );
+    var convertedValue = _convertRawJsValue(
+      entryMeta,
+      entryFieldSet,
+      value.getProperty<JSAny>(mapValueFieldNumber.toJS),
+      mapValueFieldNumber,
+      fi.valueFieldType,
+      registry,
+    );
+    // In the case of an unknown enum value, the converted value may return
+    // null. The default enum value should be used in these cases, which is
+    // stored in the FieldInfo.
+    convertedValue ??= fi.defaultEnumValue;
+    map[convertedKey] = convertedValue;
+  }
+}
+
+void _setRawJsField(
+  BuilderInfo meta,
+  FieldSet fs,
+  JSAny json,
+  FieldInfo fi,
+  ExtensionRegistry? registry,
+) {
+  final value = _convertRawJsValue(
+    meta,
+    fs,
+    json,
+    fi.tagNumber,
+    fi.type,
+    registry,
+  );
+  if (value == null) return;
+  // _convertRawJsValue throws exception when it fails to do conversion.
+  // Therefore we run _validateField for debug builds only to validate
+  // correctness of conversion.
+  assert(() {
+    fs.validateField(fi, value);
+    return true;
+  }());
+  fs.setFieldUnchecked(meta, fi, value);
+}
+
+/// Converts [value] from the JSON format to the Dart data type suitable for
+/// inserting into the corresponding [GeneratedMessage] field.
+///
+/// Returns the converted value. Returns `null` if it is an unknown enum value,
+/// in which case the caller should figure out the default enum value to return
+/// instead.
+///
+/// Throws [ArgumentError] if it cannot convert the value.
+Object? _convertRawJsValue(
+  BuilderInfo meta,
+  FieldSet fs,
+  JSAny value,
+  int tagNumber,
+  int fieldType,
+  ExtensionRegistry? registry,
+) {
+  String expectedType; // for exception message
+  switch (PbFieldType.baseType(fieldType)) {
+    case PbFieldType.BOOL_BIT:
+      if (value.isA<JSBoolean>()) {
+        return value._as<JSBoolean>().toDart;
+      } else if (value.isA<JSString>()) {
+        final dartStr = value._as<JSString>().toDart;
+        if (dartStr == 'true') {
+          return true;
+        } else if (dartStr == 'false') {
+          return false;
+        }
+      } else if (value.isA<JSNumber>()) {
+        final dartNum = value._as<JSNumber>().toDartDouble;
+        if (dartNum == 1) {
+          return true;
+        } else if (dartNum == 0) {
+          return false;
+        }
+      }
+      expectedType = 'bool (true, false, "true", "false", 1, 0)';
+    case PbFieldType.BYTES_BIT:
+      if (value.isA<JSString>()) {
+        return base64Decode(value._as<JSString>().toDart);
+      }
+      expectedType = 'Base64 String';
+    case PbFieldType.STRING_BIT:
+      if (value.isA<JSString>()) {
+        return value._as<JSString>().toDart;
+      }
+      expectedType = 'String';
+    case PbFieldType.FLOAT_BIT:
+    case PbFieldType.DOUBLE_BIT:
+      // Allow quoted values, although we don't emit them.
+      if (value.isA<JSNumber>()) {
+        final jsNum = value._as<JSNumber>();
+        return _Number._isInteger(jsNum) ? jsNum.toDartInt : jsNum.toDartDouble;
+      } else if (value.isA<JSString>()) {
+        return double.parse(value._as<JSString>().toDart);
+      }
+      expectedType = 'num or stringified num';
+    case PbFieldType.ENUM_BIT:
+      // Allow quoted values, although we don't emit them.
+      if (value.isA<JSString>()) {
+        value = int.parse(value._as<JSString>().toDart).toJS;
+      }
+      if (_Number._isInteger(value)) {
+        // The following call will return null if the enum value is unknown.
+        // In that case, we want the caller to ignore this value, so we return
+        // null from this method as well.
+        return meta.decodeEnum(
+          tagNumber,
+          registry,
+          value._as<JSNumber>().toDartInt,
+        );
+      }
+      expectedType = 'int or stringified int';
+    case PbFieldType.INT32_BIT:
+    case PbFieldType.SINT32_BIT:
+    case PbFieldType.SFIXED32_BIT:
+      if (_Number._isInteger(value)) {
+        return value._as<JSNumber>().toDartInt;
+      }
+      if (value.isA<JSString>()) {
+        return int.parse(value._as<JSString>().toDart);
+      }
+      expectedType = 'int or stringified int';
+    case PbFieldType.UINT32_BIT:
+    case PbFieldType.FIXED32_BIT:
+      int? validatedValue;
+      if (_Number._isInteger(value)) {
+        validatedValue = value._as<JSNumber>().toDartInt;
+      }
+      if (value.isA<JSString>()) {
+        validatedValue = int.parse(value._as<JSString>().toDart);
+      }
+      if (validatedValue != null && validatedValue < 0) {
+        validatedValue += 2 * (1 << 31);
+      }
+      if (validatedValue != null) return validatedValue;
+      expectedType = 'int or stringified int';
+    case PbFieldType.INT64_BIT:
+    case PbFieldType.SINT64_BIT:
+    case PbFieldType.UINT64_BIT:
+    case PbFieldType.FIXED64_BIT:
+    case PbFieldType.SFIXED64_BIT:
+      if (_Number._isInteger(value)) {
+        return Int64(value._as<JSNumber>().toDartInt);
+      }
+      if (value.isA<JSString>()) {
+        return Int64.parseInt(value._as<JSString>().toDart);
+      }
+      expectedType = 'int or stringified int';
+    case PbFieldType.GROUP_BIT:
+    case PbFieldType.MESSAGE_BIT:
+      if (_getPrototypeOf(value).strictEquals(_objectPrototype).toDart) {
+        final subMessage = meta.makeEmptyMessage(tagNumber, registry);
+        _mergeFromRawJsMap(
+          subMessage.fieldSet,
+          value._as<JSObject>(),
+          registry,
+        );
+        return subMessage;
+      }
+      expectedType = 'nested message or group';
+    default:
+      throw ArgumentError(
+        'Unknown type $fieldType when decoding a '
+        '${meta.qualifiedMessageName} message field with tag $tagNumber.',
+      );
+  }
+  throw ArgumentError(
+    'Expected type $expectedType, got $value when decoding a '
+    '${meta.qualifiedMessageName} message field with tag $tagNumber.',
+  );
+}