Minimal example showing issue with runWithRealTimers and setimmediate shim

jest.config.js

@@ -1,6 +1,7 @@
 module.exports = {
   preset: 'react-native',
   setupFilesAfterEnv: ['./jest-setup.ts'],
+  testEnvironment: "jsdom", // This causes setImmediate to be undefined.
   testPathIgnorePatterns: ['build/', 'examples/', 'experiments-app/', 'timer-utils'],
   testTimeout: 60000,
   transformIgnorePatterns: ['/node_modules/(?!(@react-native|react-native)/).*/'],

package.json

@@ -86,6 +86,7 @@
     "eslint-plugin-simple-import-sort": "^12.1.1",
     "flow-bin": "~0.170.0",
     "jest": "^29.7.0",
+    "jest-environment-jsdom": "^29.7.0",
     "prettier": "^2.8.8",
     "react": "19.0.0",
     "react-native": "0.79.1",

src/__tests__/runWithRealTimers.test.ts

@@ -0,0 +1,13 @@
+import { setImmediate, setTimeout } from '../helpers/timers';
+
+describe('runWithRealTimers', () => {
+  test('real setImmediate is called when timers are fake', async () => {
+    jest.useFakeTimers({ legacyFakeTimers: false });
+    const timeout = new Promise((_, reject) => setTimeout(reject, 1000))
+    const immediate = new Promise(resolve => setImmediate(resolve))
+    const result = await Promise.race([timeout, immediate])
+      .then(() => 'resolve')
+      .catch(() => 'reject')
+    expect(result).toEqual('resolve')
+  });
+});