[TestUtils.act] warn when using TestUtils.act in node (#14768)

* warn when using TestUtils.act in node

* s/warns/throws

* s/throw/warn

* consistent ellipses

Author: Sunil Pai

Diff for: packages/react-dom/src/test-utils/ReactTestUtils.js

@@ -151,8 +151,8 @@ function validateClassInstance(inst, methodName) {
   );
 }
 
-// stub element used by act() when flushing effects
-let actContainerElement = document.createElement('div');
+// a stub element, lazily initialized, used by act() when flushing effects
+let actContainerElement = null;
 
 /**
  * Utilities for making it easy to test React components.
@@ -391,9 +391,25 @@ const ReactTestUtils = {
   SimulateNative: {},
 
   act(callback: () => void): Thenable {
+    if (actContainerElement === null) {
+      // warn if we can't actually create the stub element
+      if (__DEV__) {
+        warningWithoutStack(
+          typeof document !== 'undefined' &&
+            document !== null &&
+            typeof document.createElement === 'function',
+          'It looks like you called TestUtils.act(...) in a non-browser environment. ' +
+            "If you're using TestRenderer for your tests, you should call " +
+            'TestRenderer.act(...) instead of TestUtils.act(...).',
+        );
+      }
+      // then make it
+      actContainerElement = document.createElement('div');
+    }
+
+    const result = ReactDOM.unstable_batchedUpdates(callback);
     // note: keep these warning messages in sync with
     // createReactNoop.js and ReactTestRenderer.js
-    const result = ReactDOM.unstable_batchedUpdates(callback);
     if (__DEV__) {
       if (result !== undefined) {
         let addendum;

Diff for: packages/react-test-renderer/src/__tests__/ReactTestRenderer-test.internal.js

@@ -1023,7 +1023,7 @@ describe('ReactTestRenderer', () => {
   });
 
   describe('act', () => {
-    it('works', () => {
+    it('can use .act() to batch updates and effects', () => {
       function App(props) {
         React.useEffect(() => {
           props.callback();
@@ -1043,5 +1043,19 @@ describe('ReactTestRenderer', () => {
 
       expect(called).toBe(true);
     });
+    it('warns and throws if you use TestUtils.act instead of TestRenderer.act in node', () => {
+      // we warn when you try to load 2 renderers in the same 'scope'
+      // so as suggested, we call resetModules() to carry on with the test
+      jest.resetModules();
+      const {act} = require('react-dom/test-utils');
+      expect(() => {
+        expect(() => act(() => {})).toThrow('document is not defined');
+      }).toWarnDev(
+        [
+          'It looks like you called TestUtils.act(...) in a non-browser environment',
+        ],
+        {withoutStack: 1},
+      );
+    });
   });
 });