Go back to shared refs instance object

It turns out we already made refs writable in facebook#25696, which has been
in canary for over a year. The approach in that PR also has the benefit
of being slightly more perf sensitive because it still uses a shared
object until the fiber is mounted. So let's just go back to that.

packages/react-reconciler/src/ReactFiberClassComponent.js

@@ -19,13 +19,12 @@ import {
 } from './ReactFiberFlags';
 import {
   debugRenderPhaseSideEffectsForStrictMode,
-  disableDefaultPropsExceptForClasses,
   disableLegacyContext,
-  disableStringRefs,
   enableDebugTracing,
+  enableSchedulingProfiler,
   enableLazyContextPropagation,
   enableRefAsProp,
-  enableSchedulingProfiler,
+  disableDefaultPropsExceptForClasses,
 } from 'shared/ReactFeatureFlags';
 import ReactStrictModeWarnings from './ReactStrictModeWarnings';
 import {isMounted} from './ReactFiberTreeReflection';
@@ -820,12 +819,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  if (!disableStringRefs) {
-    // When string refs are used in create-react-class legacy components,
-    // we need to make refs writable unless we patch all such copies of the
-    // class code that sets to a frozen emptyObject.
-    instance.refs = {};
-  }
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);
 

packages/react-reconciler/src/__tests__/ReactFiberRefs-test.js

@@ -162,4 +162,31 @@ describe('ReactFiberRefs', () => {
     await act(() => root.render(<App />));
     expect(app.refs.div.prop).toBe('Hello!');
   });
+
+  test('class refs are initialized to a frozen shared object', async () => {
+    const refsCollection = new Set();
+    class Component extends React.Component {
+      constructor(props) {
+        super(props);
+        refsCollection.add(this.refs);
+      }
+      render() {
+        return <div />;
+      }
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(() =>
+      root.render(
+        <>
+          <Component />
+          <Component />
+        </>,
+      ),
+    );
+
+    expect(refsCollection.size).toBe(1);
+    const refsInstance = Array.from(refsCollection)[0];
+    expect(Object.isFrozen(refsInstance)).toBe(__DEV__);
+  });
 });

packages/react/src/ReactBaseClasses.js

@@ -8,13 +8,19 @@
 import ReactNoopUpdateQueue from './ReactNoopUpdateQueue';
 import assign from 'shared/assign';
 
+const emptyObject = {};
+if (__DEV__) {
+  Object.freeze(emptyObject);
+}
+
 /**
  * Base class helpers for the updating state of a component.
  */
 function Component(props, context, updater) {
   this.props = props;
   this.context = context;
-  this.refs = {};
+  // If a component has string refs, we will assign a different object later.
+  this.refs = emptyObject;
   // We initialize the default updater but the real one gets injected by the
   // renderer.
   this.updater = updater || ReactNoopUpdateQueue;
@@ -127,7 +133,7 @@ function PureComponent(props, context, updater) {
   this.props = props;
   this.context = context;
   // If a component has string refs, we will assign a different object later.
-  this.refs = {};
+  this.refs = emptyObject;
   this.updater = updater || ReactNoopUpdateQueue;
 }