(jest-fake-timers): Add now() API to get the fake clock time (#13244)

Author: robhogan

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Features
 
+- `[@jest/fake-timers]` Add `jest.now()` to return the current fake clock time ([#13244](https://github.com/facebook/jest/pull/13244))
+
 ### Fixes
 
 ### Chore & Maintenance

docs/JestObjectAPI.md

@@ -799,6 +799,10 @@ This means, if any timers have been scheduled (but have not yet executed), they
 
 Returns the number of fake timers still left to run.
 
+### `jest.now()`
+
+Returns the time in ms of the current fake clock. This is equivalent to `Date.now()` if `Date` has been mocked.
+
 ### `jest.setSystemTime(now?: number | Date)`
 
 Set the current system time used by fake timers. Simulates a user changing the system clock while your program is running. It affects the current time but it does not in itself cause e.g. timers to fire; they will fire exactly as they would have done without the call to `jest.setSystemTime()`.

packages/jest-environment/src/index.ts

@@ -152,6 +152,10 @@ export interface Jest {
    * Returns the number of fake timers still left to run.
    */
   getTimerCount(): number;
+  /**
+   * Returns the current time in ms of the fake timer clock.
+   */
+  now(): number;
   /**
    * Determines if the given function is a mocked function.
    */

packages/jest-fake-timers/src/__tests__/legacyFakeTimers.test.ts

@@ -1589,4 +1589,45 @@ describe('FakeTimers', () => {
       expect(timers.getTimerCount()).toEqual(0);
     });
   });
+
+  describe('now', () => {
+    it('returns the current clock', () => {
+      const timers = new FakeTimers({
+        config,
+        global: globalThis,
+        moduleMocker,
+        timerConfig,
+      });
+
+      timers.useFakeTimers();
+      globalThis.setTimeout(() => {}, 2);
+      globalThis.setTimeout(() => {}, 100);
+
+      expect(timers.now()).toEqual(0);
+
+      // This should run the 2ms timer, and then advance _now by 3ms
+      timers.advanceTimersByTime(5);
+      expect(timers.now()).toEqual(5);
+
+      // Advance _now even though there are no timers to run
+      timers.advanceTimersByTime(5);
+      expect(timers.now()).toEqual(10);
+
+      // Run up to the 100ms timer
+      timers.runAllTimers();
+      expect(timers.now()).toEqual(100);
+
+      // Verify that runOnlyPendingTimers advances now only up to the first
+      // recursive timer
+      globalThis.setTimeout(function infinitelyRecursingCallback() {
+        globalThis.setTimeout(infinitelyRecursingCallback, 20);
+      }, 10);
+      timers.runOnlyPendingTimers();
+      expect(timers.now()).toEqual(110);
+
+      // Reset should set now back to 0
+      timers.reset();
+      expect(timers.now()).toEqual(0);
+    });
+  });
 });

packages/jest-fake-timers/src/__tests__/modernFakeTimers.test.ts

@@ -948,4 +948,44 @@ describe('FakeTimers', () => {
       expect(timers.getTimerCount()).toEqual(0);
     });
   });
+
+  describe('now', () => {
+    it('returns the current clock', () => {
+      const timers = new FakeTimers({
+        config: makeProjectConfig(),
+        global: globalThis,
+      });
+
+      timers.useFakeTimers();
+      timers.setSystemTime(0);
+      globalThis.setTimeout(() => {}, 2);
+      globalThis.setTimeout(() => {}, 100);
+
+      expect(timers.now()).toEqual(0);
+
+      // This should run the 2ms timer, and then advance _now by 3ms
+      timers.advanceTimersByTime(5);
+      expect(timers.now()).toEqual(5);
+
+      // Advance _now even though there are no timers to run
+      timers.advanceTimersByTime(5);
+      expect(timers.now()).toEqual(10);
+
+      // Run up to the 100ms timer
+      timers.runAllTimers();
+      expect(timers.now()).toEqual(100);
+
+      // Verify that runOnlyPendingTimers advances now only up to the first
+      // recursive timer
+      globalThis.setTimeout(function infinitelyRecursingCallback() {
+        globalThis.setTimeout(infinitelyRecursingCallback, 20);
+      }, 10);
+      timers.runOnlyPendingTimers();
+      expect(timers.now()).toEqual(110);
+
+      // For modern timers, reset() explicitly preserves the clock time
+      timers.reset();
+      expect(timers.now()).toEqual(110);
+    });
+  });
 });

packages/jest-fake-timers/src/legacyFakeTimers.ts

@@ -134,6 +134,10 @@ export default class FakeTimers<TimerRef = unknown> {
     this._timers = new Map();
   }
 
+  now(): number {
+    return this._now;
+  }
+
   runAllTicks(): void {
     this._checkFakeTimers();
     // Only run a generous number of ticks and then bail.
@@ -200,13 +204,15 @@ export default class FakeTimers<TimerRef = unknown> {
     // This is just to help avoid recursive loops
     let i;
     for (i = 0; i < this._maxLoops; i++) {
-      const nextTimerHandle = this._getNextTimerHandle();
+      const nextTimerHandleAndExpiry = this._getNextTimerHandleAndExpiry();
 
       // If there are no more timer handles, stop!
-      if (nextTimerHandle === null) {
+      if (nextTimerHandleAndExpiry === null) {
         break;
       }
 
+      const [nextTimerHandle, expiry] = nextTimerHandleAndExpiry;
+      this._now = expiry;
       this._runTimerHandle(nextTimerHandle);
 
       // Some of the immediate calls could be enqueued
@@ -239,7 +245,10 @@ export default class FakeTimers<TimerRef = unknown> {
 
     timerEntries
       .sort(([, left], [, right]) => left.expiry - right.expiry)
-      .forEach(([timerHandle]) => this._runTimerHandle(timerHandle));
+      .forEach(([timerHandle, timer]) => {
+        this._now = timer.expiry;
+        this._runTimerHandle(timerHandle);
+      });
   }
 
   advanceTimersToNextTimer(steps = 1): void {
@@ -265,21 +274,16 @@ export default class FakeTimers<TimerRef = unknown> {
     // This is just to help avoid recursive loops
     let i;
     for (i = 0; i < this._maxLoops; i++) {
-      const timerHandle = this._getNextTimerHandle();
+      const timerHandleAndExpiry = this._getNextTimerHandleAndExpiry();
 
       // If there are no more timer handles, stop!
-      if (timerHandle === null) {
-        break;
-      }
-      const timerValue = this._timers.get(timerHandle);
-      if (timerValue === undefined) {
+      if (timerHandleAndExpiry === null) {
         break;
       }
-      const nextTimerExpiry = timerValue.expiry;
+      const [timerHandle, nextTimerExpiry] = timerHandleAndExpiry;
+
       if (this._now + msToRun < nextTimerExpiry) {
-        // There are no timers between now and the target we're running to, so
-        // adjust our time cursor and quit
-        this._now += msToRun;
+        // There are no timers between now and the target we're running to
         break;
       } else {
         msToRun -= nextTimerExpiry - this._now;
@@ -288,6 +292,9 @@ export default class FakeTimers<TimerRef = unknown> {
       }
     }
 
+    // Advance the clock by whatever time we still have left to run
+    this._now += msToRun;
+
     if (i === this._maxLoops) {
       throw new Error(
         `Ran ${this._maxLoops} timers, and there are still more! ` +
@@ -557,7 +564,7 @@ export default class FakeTimers<TimerRef = unknown> {
     return this._timerConfig.idToRef(uuid);
   }
 
-  private _getNextTimerHandle() {
+  private _getNextTimerHandleAndExpiry(): [string, number] | null {
     let nextTimerHandle = null;
     let soonestTime = MS_IN_A_YEAR;
 
@@ -568,13 +575,19 @@ export default class FakeTimers<TimerRef = unknown> {
       }
     });
 
-    return nextTimerHandle;
+    if (nextTimerHandle === null) {
+      return null;
+    }
+
+    return [nextTimerHandle, soonestTime];
   }
 
   private _runTimerHandle(timerHandle: TimerID) {
     const timer = this._timers.get(timerHandle);
 
     if (!timer) {
+      // Timer has been cleared - we'll hit this when a timer is cleared within
+      // another timer in runOnlyPendingTimers
       return;
     }
 

packages/jest-fake-timers/src/modernFakeTimers.ts

@@ -122,6 +122,10 @@ export default class FakeTimers {
     return Date.now();
   }
 
+  now(): number {
+    return this._clock.now;
+  }
+
   getTimerCount(): number {
     if (this._checkFakeTimers()) {
       return this._clock.countTimers();

packages/jest-runtime/src/index.ts

@@ -2128,6 +2128,7 @@ export default class Runtime {
       isolateModules,
       mock,
       mocked,
+      now: () => _getFakeTimers().now(),
       requireActual: this.requireActual.bind(this, from),
       requireMock: this.requireMock.bind(this, from),
       resetAllMocks,

packages/jest-types/__typetests__/jest.test.ts

@@ -413,6 +413,9 @@ expectError(jest.clearAllTimers(false));
 expectType<number>(jest.getTimerCount());
 expectError(jest.getTimerCount(true));
 
+expectType<number>(jest.now());
+expectError(jest.now('1995-12-17T03:24:00'));
+
 expectType<number>(jest.getRealSystemTime());
 expectError(jest.getRealSystemTime(true));