Minimal example showing the issue with setimmediate shim

Author: edkimmel

jest-setup.ts

@@ -1,3 +1,4 @@
+import './src/__tests__/setImmediateShim'
 import { resetToDefaults, configure } from './src/pure';
 
 beforeEach(() => {

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')
+  });
+});

src/__tests__/setImmediateShim.ts

@@ -0,0 +1,5 @@
+const globalObj = typeof window === 'undefined' ? global : window;
+// @ts-expect-error
+globalObj.setImmediate = function(fn: () => void) {
+  return globalObj.setTimeout(fn, 0);
+}