fix: make cleanup use async act (#427)

- awaits an `act(async () => {})` inside `cleanup-after-each`

Some possible Q&A:
- why not do the same in sync cleanup()?: if peeps are using react-testing-library already, it's suuuper unlikely they'll have hanging sync effects/updates. Decided not to add code without a good reason.

(bonus: fixes a lint violation in act-compat.js)

Author: threepointone

cleanup-after-each.js

@@ -1 +1,3 @@
-afterEach(require('./dist').cleanup)
+afterEach(() => {
+  return require('./dist/cleanup-async')()
+})

src/__tests__/cleanup-after-each.js

@@ -0,0 +1,32 @@
+import React from 'react'
+import {render} from '../index'
+import cleanupAsync from '../cleanup-async'
+
+afterEach(() => {
+  return cleanupAsync()
+})
+
+const log = []
+let ctr = 0
+
+function App() {
+  async function somethingAsync() {
+    await null
+    log.push(ctr++)
+  }
+  React.useEffect(() => {
+    somethingAsync()
+  }, [])
+  return 123
+}
+
+it('cleanup-after-each does not leave any hanging microtasks: part 1', () => {
+  render(<App />)
+  expect(document.body.textContent).toBe('123')
+  expect(log).toEqual([])
+})
+
+it('cleanup-after-each does not leave any hanging microtasks: part 2', () => {
+  expect(log).toEqual([0])
+  expect(document.body.innerHTML).toBe('')
+})

src/act-compat.js

@@ -1,7 +1,7 @@
 import React from 'react'
 import ReactDOM from 'react-dom'
-import {reactDomSixteenPointNineIsReleased} from './react-dom-16.9.0-is-released'
 import * as testUtils from 'react-dom/test-utils'
+import {reactDomSixteenPointNineIsReleased} from './react-dom-16.9.0-is-released'
 
 const reactAct = testUtils.act
 const actSupported = reactAct !== undefined

src/cleanup-async.js

@@ -0,0 +1,10 @@
+// This file is for use by the top-level export
+// @testing-library/react/cleanup-after-each
+// It is not meant to be used directly
+
+module.exports = async function cleanupAsync() {
+  const {asyncAct} = require('./act-compat')
+  const {cleanup} = require('./index')
+  await asyncAct(async () => {})
+  cleanup()
+}