Support writing to this.refs from userspace

Previously, the `refs` property of a class component instance was
read-only by user code — only React could write to it, and until/unless
a string ref was used, it pointed to a shared empty object that was
frozen in dev to prevent userspace mutations.

Because string refs are deprecated, we want users to be able to codemod
all their string refs to callback refs. The safest way to do this is to
output a callback ref that assigns to `this.refs`.

So to support this, we need to make `this.refs` writable by userspace.

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -34,7 +34,6 @@ import {
   createFiberFromText,
   createFiberFromPortal,
 } from './ReactFiber.new';
-import {emptyRefsObject} from './ReactFiberClassComponent.new';
 import {isCompatibleFamilyForHotReloading} from './ReactFiberHotReloading.new';
 import {StrictLegacyMode} from './ReactTypeOfMode';
 import {getIsHydrating} from './ReactFiberHydrationContext.new';
@@ -199,11 +198,7 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = resolvedInst.refs;
-        if (refs === emptyRefsObject) {
-          // This is a lazy pooled frozen object, so we need to initialize.
-          refs = resolvedInst.refs = {};
-        }
+        const refs = resolvedInst.refs;
         if (value === null) {
           delete refs[stringRef];
         } else {

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -34,7 +34,6 @@ import {
   createFiberFromText,
   createFiberFromPortal,
 } from './ReactFiber.old';
-import {emptyRefsObject} from './ReactFiberClassComponent.old';
 import {isCompatibleFamilyForHotReloading} from './ReactFiberHotReloading.old';
 import {StrictLegacyMode} from './ReactTypeOfMode';
 import {getIsHydrating} from './ReactFiberHydrationContext.old';
@@ -199,11 +198,7 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = resolvedInst.refs;
-        if (refs === emptyRefsObject) {
-          // This is a lazy pooled frozen object, so we need to initialize.
-          refs = resolvedInst.refs = {};
-        }
+        const refs = resolvedInst.refs;
         if (value === null) {
           delete refs[stringRef];
         } else {

packages/react-reconciler/src/ReactFiberClassComponent.new.js

@@ -12,7 +12,6 @@ import type {Lanes} from './ReactFiberLane.new';
 import type {UpdateQueue} from './ReactFiberClassUpdateQueue.new';
 import type {Flags} from './ReactFiberFlags';
 
-import * as React from 'react';
 import {
   LayoutStatic,
   MountLayoutDev,
@@ -82,10 +81,6 @@ import {
 
 const fakeInternalInstance = {};
 
-// React.Component uses a shared frozen object by default.
-// We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject = new React.Component().refs;
-
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
@@ -871,7 +866,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);
 

packages/react-reconciler/src/ReactFiberClassComponent.old.js

@@ -12,7 +12,6 @@ import type {Lanes} from './ReactFiberLane.old';
 import type {UpdateQueue} from './ReactFiberClassUpdateQueue.old';
 import type {Flags} from './ReactFiberFlags';
 
-import * as React from 'react';
 import {
   LayoutStatic,
   MountLayoutDev,
@@ -82,10 +81,6 @@ import {
 
 const fakeInternalInstance = {};
 
-// React.Component uses a shared frozen object by default.
-// We'll use it to determine whether we need to initialize legacy refs.
-export const emptyRefsObject = new React.Component().refs;
-
 let didWarnAboutStateAssignmentForComponent;
 let didWarnAboutUninitializedState;
 let didWarnAboutGetSnapshotBeforeUpdateWithoutDidUpdate;
@@ -871,7 +866,7 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
+  instance.refs = {};
 
   initializeUpdateQueue(workInProgress);
 

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

@@ -0,0 +1,74 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+let React;
+let ReactNoop;
+let act;
+
+describe('ReactFiberRefs', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+    act = require('jest-react').act;
+  });
+
+  test('strings refs can be codemodded to callback refs', async () => {
+    let app;
+    class App extends React.Component {
+      render() {
+        app = this;
+        return (
+          <div
+            prop="Hello!"
+            ref={el => {
+              // `refs` used to be a shared frozen object unless/until a string
+              // ref attached by the reconciler, but it's not anymore so that we
+              // can codemod string refs to userspace callback refs.
+              this.refs.div = el;
+            }}
+          />
+        );
+      }
+    }
+
+    const root = ReactNoop.createRoot();
+    await act(async () => 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__);
+  });
+});