feat: upgrade to react 18 (#593)

src/__snapshots__/index.test.jsx.snap

@@ -1,39 +1,43 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`app registry subscribe: APP_INIT_ERROR.  snapshot: displays an ErrorPage to root element 1`] = `
-<ErrorPage
-  message="test-error-message"
-/>
+<UNDEFINED>
+  <ErrorPage
+    message="test-error-message"
+  />
+</UNDEFINED>
 `;
 
 exports[`app registry subscribe: APP_READY.  links App to root element 1`] = `
-<AppProvider
-  store={
-    {
-      "redux": "store",
+<UNDEFINED>
+  <AppProvider
+    store={
+      {
+        "redux": "store",
+      }
     }
-  }
->
-  <NoticesWrapper>
-    <Routes>
-      <Route
-        element={
-          <PageWrap>
-            <App />
-          </PageWrap>
-        }
-        path="/"
-      />
-      <Route
-        element={
-          <Navigate
-            replace={true}
-            to="/"
-          />
-        }
-        path="*"
-      />
-    </Routes>
-  </NoticesWrapper>
-</AppProvider>
+  >
+    <NoticesWrapper>
+      <Routes>
+        <Route
+          element={
+            <PageWrap>
+              <App />
+            </PageWrap>
+          }
+          path="/"
+        />
+        <Route
+          element={
+            <Navigate
+              replace={true}
+              to="/"
+            />
+          }
+          path="*"
+        />
+      </Routes>
+    </NoticesWrapper>
+  </AppProvider>
+</UNDEFINED>
 `;

src/index.jsx

@@ -2,8 +2,8 @@
 import 'core-js/stable';
 import 'regenerator-runtime/runtime';
 
-import React from 'react';
-import ReactDOM from 'react-dom';
+import React, { StrictMode } from 'react';
+import { createRoot } from 'react-dom/client';
 import {
   Route, Navigate, Routes,
 } from 'react-router-dom';
@@ -30,23 +30,29 @@ import App from './App';
 import NoticesWrapper from './components/NoticesWrapper';
 
 subscribe(APP_READY, () => {
-  ReactDOM.render(
-    <AppProvider store={store}>
-      <NoticesWrapper>
-        <Routes>
-          <Route path="/" element={<PageWrap><App /></PageWrap>} />
-          <Route path="*" element={<Navigate to="/" replace />} />
-        </Routes>
-      </NoticesWrapper>
-    </AppProvider>,
-    document.getElementById('root'),
+  const root = createRoot(document.getElementById('root'));
+
+  root.render(
+    <StrictMode>
+      <AppProvider store={store}>
+        <NoticesWrapper>
+          <Routes>
+            <Route path="/" element={<PageWrap><App /></PageWrap>} />
+            <Route path="*" element={<Navigate to="/" replace />} />
+          </Routes>
+        </NoticesWrapper>
+      </AppProvider>
+    </StrictMode>,
   );
 });
 
 subscribe(APP_INIT_ERROR, (error) => {
-  ReactDOM.render(
-    <ErrorPage message={error.message} />,
-    document.getElementById('root'),
+  const root = createRoot(document.getElementById('root'));
+
+  root.render(
+    <StrictMode>
+      <ErrorPage message={error.message} />
+    </StrictMode>,
   );
 });
 

src/index.test.jsx

@@ -1,5 +1,3 @@
-import { render } from 'react-dom';
-
 import {
   APP_INIT_ERROR,
   APP_READY,
@@ -11,9 +9,20 @@ import {
 import { configuration } from './config';
 import * as app from '.';
 
-jest.mock('react-dom', () => ({
-  render: jest.fn(),
-}));
+// These need to be var not let so they get hoisted
+// and can be used by jest.mock (which is also hoisted)
+var mockRender; // eslint-disable-line no-var
+var mockCreateRoot; // eslint-disable-line no-var
+jest.mock('react-dom/client', () => {
+  mockRender = jest.fn();
+  mockCreateRoot = jest.fn(() => ({
+    render: mockRender,
+  }));
+
+  return ({
+    createRoot: mockCreateRoot,
+  });
+});
 
 jest.mock('@edx/frontend-platform', () => ({
   mergeConfig: jest.fn(),
@@ -32,7 +41,9 @@ describe('app registry', () => {
   let getElement;
 
   beforeEach(() => {
-    render.mockClear();
+    mockCreateRoot.mockClear();
+    mockRender.mockClear();
+
     getElement = window.document.getElementById;
     window.document.getElementById = jest.fn(id => ({ id }));
   });
@@ -44,18 +55,16 @@ describe('app registry', () => {
     const callArgs = subscribe.mock.calls[0];
     expect(callArgs[0]).toEqual(APP_READY);
     callArgs[1]();
-    const [rendered, target] = render.mock.calls[0];
+    const [rendered] = mockRender.mock.calls[0];
     expect(rendered).toMatchSnapshot();
-    expect(target).toEqual(document.getElementById('root'));
   });
   test('subscribe: APP_INIT_ERROR.  snapshot: displays an ErrorPage to root element', () => {
     const callArgs = subscribe.mock.calls[1];
     expect(callArgs[0]).toEqual(APP_INIT_ERROR);
     const error = { message: 'test-error-message' };
     callArgs[1](error);
-    const [rendered, target] = render.mock.calls[0];
+    const [rendered] = mockRender.mock.calls[0];
     expect(rendered).toMatchSnapshot();
-    expect(target).toEqual(document.getElementById('root'));
   });
   test('initialize is called with requireAuthenticatedUser', () => {
     expect(initialize).toHaveBeenCalledTimes(1);

src/setupTest.jsx

@@ -1,6 +1,5 @@
 /* eslint-disable import/no-extraneous-dependencies */
 import '@testing-library/jest-dom';
-import '@testing-library/jest-dom/extend-expect';
 
 jest.mock('react', () => ({
   ...jest.requireActual('react'),