jest [nfc]: Take Jest's new defaulting of modern fake-timer implementation

Jest 27 flipped the default for fake timers to the "modern"
implementation, from the "legacy" implementation:
  https://jestjs.io/blog/2021/05/25/jest-27

Now, in the config, we can just say "fake" when we mean "modern
fake". Since the goal is to eventually forget about the "legacy"
implementation, apply that simpler name across our test code.

Author: chrisbobbe

jest.config.js

@@ -79,7 +79,7 @@ const projectForPlatform = platform => {
     globals: {
       __TEST__: true,
     },
-    timers: 'modern',
+    timers: 'fake',
     globalSetup: './jest/globalSetup.js',
     setupFiles: [
       './jest/globalFetch.js',

jest/jestSetup.js

@@ -6,7 +6,7 @@ import { URL, URLSearchParams } from 'react-native-url-polyfill';
 // $FlowIgnore[untyped-import] - this is not anywhere near critical
 import mockAsyncStorage from '@react-native-async-storage/async-storage/jest/async-storage-mock';
 
-import { assertUsingModernFakeTimers } from '../src/__tests__/lib/fakeTimers';
+import { assertUsingFakeTimers } from '../src/__tests__/lib/fakeTimers';
 
 // Use the same `URL` polyfill we do in the app.
 //
@@ -21,8 +21,8 @@ import { assertUsingModernFakeTimers } from '../src/__tests__/lib/fakeTimers';
 polyfillGlobal('URL', () => URL);
 polyfillGlobal('URLSearchParams', () => URLSearchParams);
 
-// Default should be set with `timers: 'modern'` in Jest config.
-assertUsingModernFakeTimers();
+// Default should be set with `timers: 'fake'` in Jest config.
+assertUsingFakeTimers();
 
 // Mock `react-native` ourselves, following upstream advice [1] [2].
 //

src/__tests__/lib/fakeTimers.js

@@ -2,17 +2,21 @@
 import { sleep } from '../../utils/async';
 
 /**
- * Ensure the "modern" fake-timer implementation is used.
+ * Ensure the fake-timer implementation is used.
  *
- * By setting `timers: 'modern'` in our Jest config, the modern
+ * By setting `timers: 'fake'` in our Jest config, the fake-timer
  * implementation is the default. May be used to double-check that
  * this default is in fact set, in our Jest setup file.
  *
  * Also, in one or two files, we switch over to using real timers,
  * with `jest.useRealTimers()`. May be used in those files to make
  * sure this setting doesn't linger where we don't want it to.
+ *
+ * Note: As of Jest 27, there are "modern" and "legacy" fake-timer
+ * implementations. This checks for the "modern" one, which is the only one
+ * we should use.
  */
-export const assertUsingModernFakeTimers = () => {
+export const assertUsingFakeTimers = () => {
   // "Note: This function is only available when using modern fake
   // timers implementation"
   //
@@ -25,11 +29,9 @@ export const assertUsingModernFakeTimers = () => {
  */
 export const fakeSleep = async (ms: number): Promise<void> => {
   try {
-    assertUsingModernFakeTimers();
+    assertUsingFakeTimers();
   } catch {
-    throw new Error(
-      'Tried to call `fakeSleep` without "modern" fake-timer implementation enabled.',
-    );
+    throw new Error('Tried to call `fakeSleep` without fake-timer implementation enabled.');
   }
 
   const sleepPromise = sleep(ms);

src/__tests__/metatests.js

@@ -1,7 +1,7 @@
 /** @jest-environment jest-environment-jsdom-global */
 // @flow strict-local
 
-import { assertUsingModernFakeTimers } from './lib/fakeTimers';
+import { assertUsingFakeTimers } from './lib/fakeTimers';
 
 /**
  * This file should not test any part of the application. It exists to test that
@@ -75,8 +75,8 @@ describe('jsdom-global', () => {
   });
 });
 
-describe('Jest modern fake timers', () => {
-  assertUsingModernFakeTimers();
+describe('Jest fake timers', () => {
+  assertUsingFakeTimers();
 
   afterEach(() => {
     // clear any unset timers

src/utils/__tests__/async-test.js

@@ -1,10 +1,10 @@
 /* @flow strict-local */
 import { sleep, BackoffMachine } from '../async';
-import { assertUsingModernFakeTimers } from '../../__tests__/lib/fakeTimers';
+import { assertUsingFakeTimers } from '../../__tests__/lib/fakeTimers';
 
 describe('BackoffMachine', () => {
   beforeAll(() => {
-    assertUsingModernFakeTimers();
+    assertUsingFakeTimers();
   });
 
   afterEach(() => {
@@ -60,7 +60,7 @@ const sleepMeasure = async (expectedMs: number) => {
 
 describe('sleep (ideal)', () => {
   beforeAll(() => {
-    assertUsingModernFakeTimers();
+    assertUsingFakeTimers();
   });
 
   afterEach(() => {