Implement node.isConnected

Summary:
This implements the `isConnected` method as defined in https://github.com/react-native-community/discussions-and-proposals/pull/607.

This requires a new method in Fabric because we can't derive that from the existing methods.

Changelog: [internal]

bypass-github-export-checks

Reviewed By: sammy-SC

Differential Revision: D44346661

fbshipit-source-id: 1eefa589f2937d1dac2461350ddc8df006f0e873

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

@@ -47,7 +47,13 @@ export default class ReadOnlyNode {
   }
 
   get isConnected(): boolean {
-    throw new TypeError('Unimplemented');
+    const shadowNode = getShadowNode(this);
+
+    if (shadowNode == null) {
+      return false;
+    }
+
+    return nullthrows(getFabricUIManager()).isConnected(shadowNode);
   }
 
   get lastChild(): ReadOnlyNode | null {

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

@@ -72,6 +72,7 @@ export type Spec = {|
   ) => void,
   +getParentNode: (node: Node) => ?InternalInstanceHandle,
   +getChildNodes: (node: Node) => $ReadOnlyArray<InternalInstanceHandle>,
+  +isConnected: (node: Node) => boolean,
 |};
 
 // This is exposed as a getter because apps using the legacy renderer AND

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

@@ -285,6 +285,9 @@ const FabricUIManagerMock: FabricUIManager = {
       );
     },
   ),
+  isConnected: jest.fn((node: Node): boolean => {
+    return getNodeInCurrentTree(node) != null;
+  }),
 };
 
 global.nativeFabricUIManager = FabricUIManagerMock;

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

@@ -836,6 +836,32 @@ jsi::Value UIManagerBinding::get(
         });
   }
 
+  if (methodName == "isConnected") {
+    // This is a React Native implementation of `Node.prototype.isConnected`
+    // (see https://developer.mozilla.org/en-US/docs/Web/API/Node/isConnected).
+
+    // Indicates whether a version of the given shadow node is present in
+    // the current revision of an active shadow tree.
+
+    // isConnected(shadowNode: ShadowNode): boolean
+    return jsi::Function::createFromHostFunction(
+        runtime,
+        name,
+        1,
+        [uiManager](
+            jsi::Runtime &runtime,
+            jsi::Value const & /*thisValue*/,
+            jsi::Value const *arguments,
+            size_t /*count*/) noexcept -> jsi::Value {
+          auto shadowNode = shadowNodeFromValue(runtime, arguments[0]);
+
+          auto newestCloneOfShadowNode =
+              uiManager->getNewestCloneOfShadowNode(*shadowNode);
+
+          return jsi::Value(newestCloneOfShadowNode != nullptr);
+        });
+  }
+
   return jsi::Value::undefined();
 }