Support writing to this.refs from userspace (#28867)

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.

Author: acdlite

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

@@ -82,10 +82,6 @@ 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 +867,6 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
 
   initializeUpdateQueue(workInProgress);
 

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

@@ -82,10 +82,6 @@ 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 +867,6 @@ function mountClassInstance(
   const instance = workInProgress.stateNode;
   instance.props = newProps;
   instance.state = workInProgress.memoizedState;
-  instance.refs = emptyRefsObject;
 
   initializeUpdateQueue(workInProgress);
 

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

@@ -0,0 +1,47 @@
+/**
+ * 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!');
+  });
+});

packages/react/src/ReactBaseClasses.js

@@ -8,19 +8,13 @@
 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;
-  // If a component has string refs, we will assign a different object later.
-  this.refs = emptyObject;
+  this.refs = {};
   // We initialize the default updater but the real one gets injected by the
   // renderer.
   this.updater = updater || ReactNoopUpdateQueue;
@@ -132,8 +126,7 @@ ComponentDummy.prototype = Component.prototype;
 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 = emptyObject;
+  this.refs = {};
   this.updater = updater || ReactNoopUpdateQueue;
 }