feat: use real Date.now to avoid using potentially mocked one (#12)

* (#12) use real Date.now to avoid using potentially mocked one

* (#12) make tsc happy with Date on Window interface and Date jest mock

src/index.ts

@@ -1,6 +1,17 @@
-// Used to avoid using Jest's fake timers.
-// See https://github.com/TheBrainFamily/wait-for-expect/issues/4 for more info
-const { setTimeout } = typeof window !== "undefined" ? window : global;
+// Augment Window interface with the Date declaratoin,
+// because typescript does not expose it for now.
+// Check https://github.com/Microsoft/TypeScript/issues/19816 for more info
+declare global {
+  /* eslint-disable-next-line no-undef */
+  interface Window {
+    Date: typeof Date;
+  }
+}
+// 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, Date: { now } } =
+  typeof window !== "undefined" ? window : global;
 
 /**
  * Waits for the expectation to pass and returns a Promise
@@ -15,10 +26,10 @@ const waitForExpect = function waitForExpect(
   timeout = 4500,
   interval = 50
 ) {
-  const startTime = Date.now();
+  const startTime = now();
   return new Promise((resolve, reject) => {
     const rejectOrRerun = (error: Error) => {
-      if (Date.now() - startTime >= timeout) {
+      if (now() - startTime >= timeout) {
         reject(error);
         return;
       }

src/withFakeTimers.spec.ts

@@ -2,10 +2,15 @@
 import "./toBeInRangeMatcher";
 import waitForExpect from "./index";
 
-// this is a copy of "it waits for expectation to pass" modified to use jestFakeTimers
-// This breakes when we remove the const { setTimeout } = typeof window !== "undefined" ? window : global;
+// this is a copy of "it waits for expectation to pass" modified to use jestFakeTimers and two ways of Date.now mocking
+// This breakes when we remove the const { setTimeout, Date: { now } } = typeof window !== "undefined" ? window : global;
 // line from the index.ts
 
+beforeEach(() => {
+  jest.restoreAllMocks();
+  jest.useRealTimers();
+});
+
 test("it works even if the timers are overwritten by jest", async () => {
   jest.useFakeTimers();
   let numberToChange = 10;
@@ -22,3 +27,55 @@ test("it works even if the timers are overwritten by jest", async () => {
     expect(numberToChange).toEqual(100);
   });
 });
+
+// Date.now might be mocked with two main ways:
+// via mocking whole Date, or by mocking just Date.now
+// hence two test cases covered both ways
+test("it works even if the Date was mocked", async () => {
+  /* eslint-disable no-global-assign */
+  // @ts-ignore: Cannot reassign to const Date
+  Date = jest.fn(() => ({
+    now() {
+      return 1482363367071;
+    }
+  }));
+  /* eslint-enable */
+  let numberToChange = 10;
+
+  setTimeout(() => {
+    numberToChange = 100;
+  }, 100);
+  let expectFailingMessage;
+  try {
+    await waitForExpect(() => {
+      expect(numberToChange).toEqual(101);
+    }, 1000);
+  } catch (e) {
+    expectFailingMessage = e.message;
+  }
+  expect(expectFailingMessage).toMatch("Expected value to equal:");
+  expect(expectFailingMessage).toMatch("101");
+  expect(expectFailingMessage).toMatch("Received:");
+  expect(expectFailingMessage).toMatch("100");
+});
+
+test("it works even if the Date.now was mocked", async () => {
+  Date.now = jest.fn(() => 1482363367071);
+  let numberToChange = 10;
+
+  setTimeout(() => {
+    numberToChange = 100;
+  }, 100);
+  let expectFailingMessage;
+  try {
+    await waitForExpect(() => {
+      expect(numberToChange).toEqual(101);
+    }, 1000);
+  } catch (e) {
+    expectFailingMessage = e.message;
+  }
+  expect(expectFailingMessage).toMatch("Expected value to equal:");
+  expect(expectFailingMessage).toMatch("101");
+  expect(expectFailingMessage).toMatch("Received:");
+  expect(expectFailingMessage).toMatch("100");
+});