feat: Add runWithRealTimers & getSetTimeoutFn functions

* Add runWithRealTimers & getSetTimeoutFn functions

BREAKING CHANGE: If you are using fake jest timers by default, but upgraded to v2.0 here, you might have to upgrade your tests. Sorry! :(


Co-authored-by: Brais Piñeiro <brapifra@gmail.com>

Author: lgandecki

src/helpers.ts

@@ -0,0 +1,30 @@
+/* eslint-disable import/prefer-default-export */
+/* eslint-env jest */
+// Used to avoid using Jest's fake timers and Date.now mocks
+// See https://github.com/TheBrainFamily/wait-for-expect/issues/4 and
+// https://github.com/TheBrainFamily/wait-for-expect/issues/12 for more info
+const globalObj = typeof window === "undefined" ? global : window;
+
+// Currently this fn only supports jest timers, but it could support other test runners in the future.
+function runWithRealTimers(callback: () => any) {
+  const usingJestFakeTimers =
+    // eslint-disable-next-line no-underscore-dangle
+    (globalObj.setTimeout as any)._isMockFunction &&
+    typeof jest !== "undefined";
+
+  if (usingJestFakeTimers) {
+    jest.useRealTimers();
+  }
+
+  const callbackReturnValue = callback();
+
+  if (usingJestFakeTimers) {
+    jest.useFakeTimers();
+  }
+
+  return callbackReturnValue;
+}
+
+export function getSetTimeoutFn() {
+  return runWithRealTimers(() => globalObj.setTimeout);
+}

src/index.ts

@@ -1,3 +1,5 @@
+import { getSetTimeoutFn } from "./helpers";
+
 const defaults = {
   timeout: 4500,
   interval: 50
@@ -16,10 +18,7 @@ const waitForExpect = function waitForExpect(
   timeout = defaults.timeout,
   interval = defaults.interval
 ) {
-  // Used to avoid using Jest's fake timers and Date.now mocks
-  // See https://github.com/TheBrainFamily/wait-for-expect/issues/4 and
-  // https://github.com/TheBrainFamily/wait-for-expect/issues/12 for more info
-  const { setTimeout } = typeof window !== "undefined" ? window : global;
+  const setTimeout = getSetTimeoutFn();
 
   // eslint-disable-next-line no-param-reassign
   if (interval < 1) interval = 1;

src/withFakeTimers.spec.ts

@@ -12,7 +12,7 @@ beforeEach(() => {
   jest.useRealTimers();
 });
 
-test("it works with real timers even if they were set to fake before importing the module", async () => {
+test("it always uses real timers even if they were set to fake before importing the module", async () => {
   jest.useFakeTimers();
   /* eslint-disable global-require */
   const importedWaitForExpect = require("./index");
@@ -27,6 +27,8 @@ test("it works with real timers even if they were set to fake before importing t
     numberToChange = 100;
   }, randomTimeout);
 
+  jest.useFakeTimers();
+
   await importedWaitForExpect(() => {
     expect(numberToChange).toEqual(100);
   });