Warn when Using DefaultProps on Function Components (#16210)

As part of the process to deprecate defaultProps on function components (as per a larger proposal outlined in (https://github.com/reactjs/rfcs/blob/createlement-rfc/text/0000-create-element-changes.md)), add a warning whenever someone does this.

packages/react-dom/src/__tests__/ReactDeprecationWarnings-test.internal.js

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) Facebook, Inc. and its 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 ReactTestUtils;
+let ReactFeatureFlags;
+
+describe('ReactDeprecationWarnings', () => {
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactFeatureFlags = require('shared/ReactFeatureFlags');
+    ReactTestUtils = require('react-dom/test-utils');
+    ReactFeatureFlags.warnAboutDefaultPropsOnFunctionComponents = true;
+  });
+
+  afterEach(() => {
+    ReactFeatureFlags.warnAboutDefaultPropsOnFunctionComponents = false;
+  });
+
+  it('should warn when given defaultProps', () => {
+    function FunctionalComponent(props) {
+      return null;
+    }
+
+    FunctionalComponent.defaultProps = {
+      testProp: true,
+    };
+
+    expect(() =>
+      ReactTestUtils.renderIntoDocument(<FunctionalComponent />),
+    ).toWarnDev(
+      'Warning: FunctionalComponent: Support for defaultProps ' +
+        'will be removed from function components in a future major ' +
+        'release. Use JavaScript default parameters instead.',
+      {withoutStack: true},
+    );
+  });
+});

packages/react-dom/src/__tests__/ReactFunctionComponent-test.js

@@ -365,6 +365,8 @@ describe('ReactFunctionComponent', () => {
     );
   });
 
+  // TODO: change this test after we deprecate default props support
+  // for function components
   it('should support default props and prop types', () => {
     function Child(props) {
       return <div>{props.test}</div>;

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -62,6 +62,7 @@ import {
   enableSuspenseServerRenderer,
   enableFlareAPI,
   enableFundamentalAPI,
+  warnAboutDefaultPropsOnFunctionComponents,
 } from 'shared/ReactFeatureFlags';
 import invariant from 'shared/invariant';
 import shallowEqual from 'shared/shallowEqual';
@@ -187,6 +188,7 @@ export let didWarnAboutReassigningProps;
 let didWarnAboutMaxDuration;
 let didWarnAboutRevealOrder;
 let didWarnAboutTailOptions;
+let didWarnAboutDefaultPropsOnFunctionComponent;
 
 if (__DEV__) {
   didWarnAboutBadClass = {};
@@ -198,6 +200,7 @@ if (__DEV__) {
   didWarnAboutMaxDuration = false;
   didWarnAboutRevealOrder = {};
   didWarnAboutTailOptions = {};
+  didWarnAboutDefaultPropsOnFunctionComponent = {};
 }
 
 export function reconcileChildren(
@@ -1424,6 +1427,23 @@ function validateFunctionComponentInDev(workInProgress: Fiber, Component: any) {
     }
   }
 
+  if (
+    warnAboutDefaultPropsOnFunctionComponents &&
+    Component.defaultProps !== undefined
+  ) {
+    const componentName = getComponentName(Component) || 'Unknown';
+
+    if (!didWarnAboutDefaultPropsOnFunctionComponent[componentName]) {
+      warningWithoutStack(
+        false,
+        '%s: Support for defaultProps will be removed from function components ' +
+          'in a future major release. Use JavaScript default parameters instead.',
+        componentName,
+      );
+      didWarnAboutDefaultPropsOnFunctionComponent[componentName] = true;
+    }
+  }
+
   if (typeof Component.getDerivedStateFromProps === 'function') {
     const componentName = getComponentName(Component) || 'Unknown';
 

packages/shared/ReactFeatureFlags.js

@@ -83,3 +83,8 @@ export const enableUserBlockingEvents = false;
 // in the update queue. This allows reporting and tracing of what is causing
 // the user to see a loading state.
 export const enableSuspenseCallback = false;
+
+// Part of the simplification of React.createElement so we can eventually move
+// from React.createElement to React.jsx
+// https://github.com/reactjs/rfcs/blob/createlement-rfc/text/0000-create-element-changes.md
+export const warnAboutDefaultPropsOnFunctionComponents = false;

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -38,6 +38,7 @@ export const warnAboutMissingMockScheduler = true;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.native-oss.js

@@ -33,6 +33,7 @@ export const warnAboutMissingMockScheduler = false;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.persistent.js

@@ -33,6 +33,7 @@ export const warnAboutMissingMockScheduler = true;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.test-renderer.js

@@ -33,6 +33,7 @@ export const warnAboutMissingMockScheduler = false;
 export const revertPassiveEffectsChange = false;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = false;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.test-renderer.www.js

@@ -33,6 +33,7 @@ export const enableJSXTransformAPI = true;
 export const warnAboutMissingMockScheduler = true;
 export const enableUserBlockingEvents = false;
 export const enableSuspenseCallback = true;
+export const warnAboutDefaultPropsOnFunctionComponents = false;
 
 // Only used in www builds.
 export function addUserTimingListener() {

packages/shared/forks/ReactFeatureFlags.www.js

@@ -78,6 +78,8 @@ export const warnAboutMissingMockScheduler = true;
 
 export const enableSuspenseCallback = true;
 
+export const warnAboutDefaultPropsOnFunctionComponents = false;
+
 // Flow magic to verify the exports of this file match the original version.
 // eslint-disable-next-line no-unused-vars
 type Check<_X, Y: _X, X: Y = _X> = null;