fix: setImmediate fake timer issue with jsdom (#1928)

Author: mdjastrzebski

src/helpers/__tests__/timers.test.ts

@@ -5,4 +5,39 @@ describe('timers', () => {
     jest.useFakeTimers();
     expect(jestFakeTimersAreEnabled()).toEqual(false);
   });
+
+  it('setImmediate polyfill keeps using real timers when fake timers are enabled', async () => {
+    // Regression test for https://github.com/callstack/react-native-testing-library/issues/1767.
+    // The module captures its timer functions once at import time. When the environment
+    // has no native `setImmediate` (e.g. jsdom/browser) it falls back to a polyfill built
+    // on `setTimeout`; that `setTimeout` must be the real one, so the polyfill keeps
+    // working after fake timers are re-applied (as happens during auto-cleanup).
+    delete process.env.RNTL_SKIP_AUTO_DETECT_FAKE_TIMERS;
+    jest.useRealTimers();
+
+    // Simulate a missing native `setImmediate` before the module captures its functions.
+    const originalSetImmediate = globalThis.setImmediate;
+    // @ts-expect-error force the polyfill fallback
+    delete globalThis.setImmediate;
+
+    let setImmediate!: (fn: () => void) => unknown;
+    try {
+      jest.isolateModules(() => {
+        // eslint-disable-next-line @typescript-eslint/no-require-imports
+        setImmediate = require('../timers').setImmediate;
+      });
+    } finally {
+      globalThis.setImmediate = originalSetImmediate;
+    }
+
+    jest.useFakeTimers({ legacyFakeTimers: false });
+    try {
+      // Before the fix this never resolved (the polyfill read the fake `setTimeout`),
+      // so the test would hang until the Jest timeout.
+      const resolved = await new Promise<boolean>((resolve) => setImmediate(() => resolve(true)));
+      expect(resolved).toBe(true);
+    } finally {
+      jest.useRealTimers();
+    }
+  }, 10_000);
 });

src/helpers/timers.ts

@@ -63,22 +63,28 @@ function getFakeTimersConfigFromType(type: FakeTimersTypes) {
 
 const jestFakeTimersAreEnabled = (): boolean => Boolean(getJestFakeTimersType());
 
-// we only run our tests in node, and setImmediate is supported in node.
-function setImmediatePolyfill(fn: () => void) {
-  return globalObj.setTimeout(fn, 0);
-}
-
 type BindTimeFunctions = {
   clearTimeoutFn: typeof clearTimeout;
   setImmediateFn: typeof setImmediate;
   setTimeoutFn: typeof setTimeout;
 };
 
 function bindTimeFunctions(): BindTimeFunctions {
+  // Capture the current (real) `setTimeout` so the `setImmediate` polyfill keeps
+  // using real timers even after fake timers are re-applied. Reading
+  // `globalObj.setTimeout` lazily inside the polyfill would pick up fake timers.
+  const realSetTimeout = globalObj.setTimeout;
+
+  // `setImmediate` exists in Node, but not in environments such as jsdom, so we
+  // fall back to a `setTimeout`-based polyfill there (see #1767).
+  function setImmediatePolyfill(fn: () => void) {
+    return realSetTimeout(fn, 0);
+  }
+
   return {
     clearTimeoutFn: globalObj.clearTimeout,
     setImmediateFn: globalObj.setImmediate || setImmediatePolyfill,
-    setTimeoutFn: globalObj.setTimeout,
+    setTimeoutFn: realSetTimeout,
   };
 }