Introduce type guard isMessage

packages/protobuf/src/index.ts

@@ -22,6 +22,7 @@ export { codegenInfo } from "./codegen-info.js";
 
 export { Message } from "./message.js";
 export type { AnyMessage, PartialMessage, PlainMessage } from "./message.js";
+export { isMessage } from "./is-message.js";
 
 export type { FieldInfo } from "./field.js";
 export type { FieldList } from "./field-list.js";

packages/protobuf/src/is-message.ts

@@ -0,0 +1,56 @@
+// Copyright 2021-2024 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import type { MessageType } from "./message-type.js";
+import type { AnyMessage } from "./message.js";
+import { Message } from "./message.js";
+
+/**
+ * Check whether the given object is an instance of the given message type.
+ *
+ * This function is equivalent to the `instanceof` operator. For example,
+ * `isMessage(foo, MyMessage)` is the same as `foo instanceof MyMessage`, and
+ * `isMessage(foo)` is the same as `foo instanceof Message`.
+ *
+ * Just like `instanceof`, `isMessage` narrows the type. The advantage of
+ * `isMessage` is that it compares identity by the message type name, not by
+ * class identity. This makes it robust against the dual package hazard and
+ * similar situations, where the same message is duplicated.
+ */
+export function isMessage<T extends Message<T> = AnyMessage>(
+  arg: unknown,
+  type?: MessageType<T>,
+): arg is T {
+  if (arg === null || typeof arg != "object") {
+    return false;
+  }
+  if (
+    !Object.getOwnPropertyNames(Message.prototype).every(
+      (m) =>
+        m in arg && typeof (arg as Record<string, unknown>)[m] == "function",
+    )
+  ) {
+    return false;
+  }
+  const actualType = (arg as { getType(): unknown }).getType();
+  if (
+    actualType === null ||
+    typeof actualType != "object" ||
+    !("typeName" in actualType) ||
+    typeof actualType.typeName != "string"
+  ) {
+    return false;
+  }
+  return type === undefined ? true : actualType.typeName == type.typeName;
+}