feat: Use concurrent React when available (#937)

BREAKING CHANGE: If you have React 18 installed, we'll use the new [`createRoot` API](reactwg/react-18#5) by default which comes with a set of [changes while also enabling support for concurrent features](reactwg/react-18#4).
To can opt-out of this change by using `render(ui, { legacyRoot: true } )`. But be aware that the legacy root API is deprecated in React 18 and its usage will trigger console warnings.

.github/workflows/validate.yml

@@ -16,6 +16,7 @@ jobs:
     # ignore all-contributors PRs
     if: ${{ !contains(github.head_ref, 'all-contributors') }}
     strategy:
+      fail-fast: false
       matrix:
         # TODO: relax `'16.9.1'` to `16` once GitHub has 16.9.1 cached. 16.9.0 is broken due to https://github.com/nodejs/node/issues/40030
         node: [12, 14, '16.9.1']
@@ -52,6 +53,8 @@ jobs:
 
       - name: ⬆️ Upload coverage report
         uses: codecov/codecov-action@v1
+        with:
+          flags: ${{ matrix.react }}
 
   release:
     needs: main

jest.config.js

@@ -0,0 +1,15 @@
+const {jest: jestConfig} = require('kcd-scripts/config')
+
+module.exports = Object.assign(jestConfig, {
+  coverageThreshold: {
+    ...jestConfig.coverageThreshold,
+    // full coverage across the build matrix (React 17, 18) but not in a single job
+    './src/pure': {
+      // minimum coverage of jobs using React 17 and 18
+      branches: 80,
+      functions: 78,
+      lines: 84,
+      statements: 84,
+    },
+  },
+})

package.json

@@ -45,7 +45,7 @@
   "license": "MIT",
   "dependencies": {
     "@babel/runtime": "^7.12.5",
-    "@testing-library/dom": "^8.0.0"
+    "@testing-library/dom": "^8.5.0"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^5.11.6",

src/__tests__/cleanup.js

@@ -83,10 +83,7 @@ describe('fake timers and missing act warnings', () => {
     expect(microTaskSpy).toHaveBeenCalledTimes(0)
     // console.error is mocked
     // eslint-disable-next-line no-console
-    expect(console.error).toHaveBeenCalledTimes(
-      // ReactDOM.render is deprecated in React 18
-      React.version.startsWith('18') ? 1 : 0,
-    )
+    expect(console.error).toHaveBeenCalledTimes(0)
   })
 
   test('cleanup does not swallow missing act warnings', () => {
@@ -118,16 +115,10 @@ describe('fake timers and missing act warnings', () => {
     expect(deferredStateUpdateSpy).toHaveBeenCalledTimes(1)
     // console.error is mocked
     // eslint-disable-next-line no-console
-    expect(console.error).toHaveBeenCalledTimes(
-      // ReactDOM.render is deprecated in React 18
-      React.version.startsWith('18') ? 2 : 1,
-    )
+    expect(console.error).toHaveBeenCalledTimes(1)
     // eslint-disable-next-line no-console
-    expect(
-      console.error.mock.calls[
-        // ReactDOM.render is deprecated in React 18
-        React.version.startsWith('18') ? 1 : 0
-      ][0],
-    ).toMatch('a test was not wrapped in act(...)')
+    expect(console.error.mock.calls[0][0]).toMatch(
+      'a test was not wrapped in act(...)',
+    )
   })
 })

src/__tests__/end-to-end.js

@@ -17,6 +17,8 @@ function ComponentWithLoader() {
     let cancelled = false
     fetchAMessage().then(data => {
       if (!cancelled) {
+        // Will trigger "missing act" warnings in React 18 with real timers
+        // Need to wait for an action on https://github.com/reactwg/react-18/discussions/23#discussioncomment-1087897
         setState({data, loading: false})
       }
     })

src/__tests__/new-act.js

@@ -1,4 +1,4 @@
-let asyncAct, consoleErrorMock
+let asyncAct
 
 jest.mock('react-dom/test-utils', () => ({
   act: cb => {
@@ -9,11 +9,11 @@ jest.mock('react-dom/test-utils', () => ({
 beforeEach(() => {
   jest.resetModules()
   asyncAct = require('../act-compat').asyncAct
-  consoleErrorMock = jest.spyOn(console, 'error').mockImplementation(() => {})
+  jest.spyOn(console, 'error').mockImplementation(() => {})
 })
 
 afterEach(() => {
-  consoleErrorMock.mockRestore()
+  console.error.mockRestore()
 })
 
 test('async act works when it does not exist (older versions of react)', async () => {

src/__tests__/no-act.js

@@ -12,7 +12,15 @@ afterEach(() => {
   consoleErrorMock.mockRestore()
 })
 
+// no react-dom/test-utils also means no isomorphic act since isomorphic act got released after test-utils act
 jest.mock('react-dom/test-utils', () => ({}))
+jest.mock('react', () => {
+  const ReactActual = jest.requireActual('react')
+
+  delete ReactActual.unstable_act
+
+  return ReactActual
+})
 
 test('act works even when there is no act from test utils', () => {
   const callback = jest.fn()

src/__tests__/old-act.js

@@ -1,13 +1,13 @@
-let asyncAct, consoleErrorMock
+let asyncAct
 
 beforeEach(() => {
   jest.resetModules()
   asyncAct = require('../act-compat').asyncAct
-  consoleErrorMock = jest.spyOn(console, 'error').mockImplementation(() => {})
+  jest.spyOn(console, 'error').mockImplementation(() => {})
 })
 
 afterEach(() => {
-  consoleErrorMock.mockRestore()
+  console.error.mockRestore()
 })
 
 jest.mock('react-dom/test-utils', () => ({

src/__tests__/render.js

@@ -101,3 +101,36 @@ test('flushes useEffect cleanup functions sync on unmount()', () => {
 
   expect(spy).toHaveBeenCalledTimes(1)
 })
+
+test('throws if `legacyRoot: false` is used with an incomaptible version', () => {
+  const isConcurrentReact = typeof ReactDOM.createRoot === 'function'
+
+  const performConcurrentRender = () => render(<div />, {legacyRoot: false})
+
+  // eslint-disable-next-line jest/no-if -- jest doesn't support conditional tests
+  if (isConcurrentReact) {
+    // eslint-disable-next-line jest/no-conditional-expect -- yes, jest still doesn't support conditional tests
+    expect(performConcurrentRender).not.toThrow()
+  } else {
+    // eslint-disable-next-line jest/no-conditional-expect -- yes, jest still doesn't support conditional tests
+    expect(performConcurrentRender).toThrowError(
+      `Attempted to use concurrent React with \`react-dom@${ReactDOM.version}\`. Be sure to use the \`next\` or \`experimental\` release channel (https://reactjs.org/docs/release-channels.html).`,
+    )
+  }
+})
+
+test('can be called multiple times on the same container', () => {
+  const container = document.createElement('div')
+
+  const {unmount} = render(<strong />, {container})
+
+  expect(container).toContainHTML('<strong></strong>')
+
+  render(<em />, {container})
+
+  expect(container).toContainHTML('<em></em>')
+
+  unmount()
+
+  expect(container).toBeEmptyDOMElement()
+})

src/__tests__/stopwatch.js

@@ -53,8 +53,5 @@ test('unmounts a component', async () => {
   // and get an error.
   await sleep(5)
   // eslint-disable-next-line no-console
-  expect(console.error).toHaveBeenCalledTimes(
-    // ReactDOM.render is deprecated in React 18
-    React.version.startsWith('18') ? 1 : 0,
-  )
+  expect(console.error).not.toHaveBeenCalled()
 })

src/act-compat.js

@@ -2,8 +2,9 @@ import * as React from 'react'
 import ReactDOM from 'react-dom'
 import * as testUtils from 'react-dom/test-utils'
 
-const reactAct = testUtils.act
-const actSupported = reactAct !== undefined
+const isomorphicAct = React.unstable_act
+const domAct = testUtils.act
+const actSupported = domAct !== undefined
 
 // act is supported react-dom@16.8.0
 // so for versions that don't have act from test utils
@@ -14,7 +15,7 @@ function actPolyfill(cb) {
   ReactDOM.render(<div />, document.createElement('div'))
 }
 
-const act = reactAct || actPolyfill
+const act = isomorphicAct || domAct || actPolyfill
 
 let youHaveBeenWarned = false
 let isAsyncActSupported = null
@@ -50,7 +51,7 @@ function asyncAct(cb) {
         }
         let cbReturn, result
         try {
-          result = reactAct(() => {
+          result = domAct(() => {
             cbReturn = cb()
             return cbReturn
           })