fix: React next fails on build (#726) (#732)

* fix: Fix build on react@next

* Working on ^16.9 and lower than 16.8

* Solution works in 16.8 also

* Fixes from code review

* Import only the function we need

package.json

@@ -44,7 +44,8 @@
   "license": "MIT",
   "dependencies": {
     "@babel/runtime": "^7.10.3",
-    "@testing-library/dom": "^7.17.1"
+    "@testing-library/dom": "^7.17.1",
+    "semver": "^7.3.2"
   },
   "devDependencies": {
     "@reach/router": "^1.3.3",

src/flush-microtasks.js

@@ -1,3 +1,6 @@
+import React from 'react'
+import satisfies from 'semver/functions/satisfies'
+
 /* istanbul ignore file */
 // the part of this file that we need tested is definitely being run
 // and the part that is not cannot easily have useful tests written
@@ -15,8 +18,15 @@ function getIsUsingFakeTimers() {
   )
 }
 
+const globalObj = typeof window === 'undefined' ? global : window
+let Scheduler = globalObj.Scheduler
+const isModernScheduleCallbackSupported = satisfies(React.version, '>16.8.6', {
+  includePrerelease: true,
+})
+
 let didWarnAboutMessageChannel = false
 let enqueueTask
+
 try {
   // read require off the module object to get around the bundlers.
   // we don't want them to detect a require and bundle a Node polyfill.
@@ -25,6 +35,8 @@ try {
   // assuming we're in node, let's try to get node's
   // version of setImmediate, bypassing fake timers if any.
   enqueueTask = nodeRequire.call(module, 'timers').setImmediate
+  // import React's scheduler so we'll be able to schedule our tasks later on.
+  Scheduler = nodeRequire.call(module, 'scheduler')
 } catch (_err) {
   // we're in a browser
   // we can't use regular timers because they may still be faked
@@ -49,6 +61,20 @@ try {
   }
 }
 
+function scheduleCallback(cb) {
+  const NormalPriority = Scheduler
+    ? Scheduler.NormalPriority || Scheduler.unstable_NormalPriority
+    : null
+
+  const scheduleFn = Scheduler
+    ? Scheduler.scheduleCallback || Scheduler.unstable_scheduleCallback
+    : callback => callback()
+
+  return isModernScheduleCallbackSupported
+    ? scheduleFn(NormalPriority, cb)
+    : scheduleFn(cb)
+}
+
 export default function flushMicroTasks() {
   return {
     then(resolve) {
@@ -59,7 +85,11 @@ export default function flushMicroTasks() {
         jest.advanceTimersByTime(0)
         resolve()
       } else {
-        enqueueTask(resolve)
+        scheduleCallback(() => {
+          enqueueTask(() => {
+            resolve()
+          })
+        })
       }
     },
   }