Implement tagName property in ReadOnlyElement (#39278)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39278

Implements tagName as the name of the component prefixed with `RN:`.

Changelog: [internal]

Reviewed By: NickGerleman

Differential Revision: D48951824

fbshipit-source-id: 4a8387adff8ed504423d7ead7b95943bfd77ae8c

packages/react-native/Libraries/DOM/Nodes/ReadOnlyElement.js

@@ -137,7 +137,13 @@ export default class ReadOnlyElement extends ReadOnlyNode {
   }
 
   get tagName(): string {
-    throw new TypeError('Unimplemented');
+    const node = getShadowNode(this);
+
+    if (node != null) {
+      return nullthrows(getFabricUIManager()).getTagName(node);
+    }
+
+    return '';
   }
 
   get textContent(): string | null {

packages/react-native/Libraries/ReactNative/FabricUIManager.js

@@ -91,6 +91,7 @@ export interface Spec {
   +getScrollPosition: (
     node: Node,
   ) => ?[/* scrollLeft: */ number, /* scrollTop: */ number];
+  +getTagName: (node: Node) => string;
 
   /**
    * Support methods for the Pointer Capture APIs.
@@ -131,6 +132,10 @@ const CACHED_PROPERTIES = [
   'getBoundingClientRect',
   'getOffset',
   'getScrollPosition',
+  'getTagName',
+  'hasPointerCapture',
+  'setPointerCapture',
+  'releasePointerCapture',
 ];
 
 // This is exposed as a getter because apps using the legacy renderer AND

packages/react-native/Libraries/ReactNative/__mocks__/FabricUIManager.js

@@ -497,6 +497,11 @@ const FabricUIManagerMock: IFabricUIManagerMock = {
     },
   ),
 
+  getTagName: jest.fn((node: Node): string => {
+    ensureHostNode(node);
+    return 'RN:' + fromNode(node).viewName;
+  }),
+
   __getInstanceHandleFromNode(node: Node): InternalInstanceHandle {
     return fromNode(node).instanceHandle;
   },

packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -1207,6 +1207,43 @@ jsi::Value UIManagerBinding::get(
         });
   }
 
+  if (methodName == "getTagName") {
+    // This is a method to access the normalized tag name of a shadow node, to
+    // implement `Element.prototype.tagName` (see
+    // https://developer.mozilla.org/en-US/docs/Web/API/Element/tagName).
+
+    // getTagName(shadowNode: ShadowNode): string
+    auto paramCount = 1;
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        paramCount,
+        [methodName, paramCount](
+            jsi::Runtime& runtime,
+            const jsi::Value& /*thisValue*/,
+            const jsi::Value* arguments,
+            size_t count) -> jsi::Value {
+          validateArgumentCount(runtime, methodName, paramCount, count);
+
+          auto shadowNode = shadowNodeFromValue(runtime, arguments[0]);
+
+          std::string canonicalComponentName = shadowNode->getComponentName();
+
+          // FIXME(T162807327): Remove Android-specific prefixes and unify
+          // shadow node implementations
+          if (canonicalComponentName == "AndroidTextInput") {
+            canonicalComponentName = "TextInput";
+          } else if (canonicalComponentName == "AndroidSwitch") {
+            canonicalComponentName = "Switch";
+          }
+
+          // Prefix with RN:
+          canonicalComponentName.insert(0, "RN:");
+
+          return jsi::String::createFromUtf8(runtime, canonicalComponentName);
+        });
+  }
+
   /**
    * Pointer Capture APIs
    */