add jest.advanceTimersToNextTimer method

CHANGELOG.md

@@ -14,6 +14,7 @@
 - `[jest-validate]` Allow `maxWorkers` as part of the `jest.config.js` ([#8565](https://github.com/facebook/jest/pull/8565))
 - `[jest-runtime]` Allow passing configuration objects to transformers ([#7288](https://github.com/facebook/jest/pull/7288))
 - `[@jest/core, @jest/test-sequencer]` Support async sort in custom `testSequencer` ([#8642](https://github.com/facebook/jest/pull/8642))
+- `[jest-runtime, @jest/fake-timers]` Add `jest.advanceTimersToNextTimer` ([#8713](https://github.com/facebook/jest/pull/8713))
 
 ### Fixes
 

docs/JestObjectAPI.md

@@ -623,6 +623,12 @@ Executes only the macro-tasks that are currently pending (i.e., only the tasks t
 
 This is useful for scenarios such as one where the module being tested schedules a `setTimeout()` whose callback schedules another `setTimeout()` recursively (meaning the scheduling never stops). In these scenarios, it's useful to be able to run forward in time by a single step at a time.
 
+### `jest.advanceTimersToNextTimer(steps)`
+
+Advances all timers by the needed milliseconds so that only the next timeouts/intervals will run.
+
+Optionally, you can provide `steps`, so it will run `steps` amount of next timeouts/intervals.
+
 ### `jest.clearAllTimers()`
 
 Removes any pending timers from the timer system.

packages/jest-environment/src/index.ts

@@ -65,6 +65,11 @@ export interface Jest {
   /**
    * Disables automatic mocking in the module loader.
    */
+  advanceTimersToNextTimer(steps?: number): void;
+  /**
+   * Advances all timers by the needed milliseconds so that only the next timeouts/intervals will run.
+   * Optionally, you can provide steps, so it will run steps amount of next timeouts/intervals.
+   */
   autoMockOff(): Jest;
   /**
    * Enables automatic mocking in the module loader.

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

@@ -651,6 +651,100 @@ describe('FakeTimers', () => {
     });
   });
 
+  describe('advanceTimersToNextTimer', () => {
+    it('runs timers in order', () => {
+      const global = ({process} as unknown) as NodeJS.Global;
+      const timers = new FakeTimers({
+        config,
+        global,
+        moduleMocker,
+        timerConfig,
+      });
+      timers.useFakeTimers();
+
+      const runOrder: Array<string> = [];
+      const mock1 = jest.fn(() => runOrder.push('mock1'));
+      const mock2 = jest.fn(() => runOrder.push('mock2'));
+      const mock3 = jest.fn(() => runOrder.push('mock3'));
+      const mock4 = jest.fn(() => runOrder.push('mock4'));
+
+      global.setTimeout(mock1, 100);
+      global.setTimeout(mock2, 0);
+      global.setTimeout(mock3, 0);
+      global.setInterval(() => {
+        mock4();
+      }, 200);
+
+      timers.advanceTimersToNextTimer();
+      // Move forward to t=0
+      expect(runOrder).toEqual(['mock2', 'mock3']);
+
+      timers.advanceTimersToNextTimer();
+      // Move forward to t=100
+      expect(runOrder).toEqual(['mock2', 'mock3', 'mock1']);
+
+      timers.advanceTimersToNextTimer();
+      // Move forward to t=200
+      expect(runOrder).toEqual(['mock2', 'mock3', 'mock1', 'mock4']);
+
+      timers.advanceTimersToNextTimer();
+      // Move forward to t=400
+      expect(runOrder).toEqual(['mock2', 'mock3', 'mock1', 'mock4', 'mock4']);
+    });
+
+    it('run correct amount of steps', () => {
+      const global = ({process} as unknown) as NodeJS.Global;
+      const timers = new FakeTimers({
+        config,
+        global,
+        moduleMocker,
+        timerConfig,
+      });
+      timers.useFakeTimers();
+
+      const runOrder: Array<string> = [];
+      const mock1 = jest.fn(() => runOrder.push('mock1'));
+      const mock2 = jest.fn(() => runOrder.push('mock2'));
+      const mock3 = jest.fn(() => runOrder.push('mock3'));
+      const mock4 = jest.fn(() => runOrder.push('mock4'));
+
+      global.setTimeout(mock1, 100);
+      global.setTimeout(mock2, 0);
+      global.setTimeout(mock3, 0);
+      global.setInterval(() => {
+        mock4();
+      }, 200);
+
+      // Move forward to t=200
+      timers.advanceTimersToNextTimer(2);
+      expect(runOrder).toEqual(['mock2', 'mock3', 'mock1']);
+
+      // Move forward to t=600
+      timers.advanceTimersToNextTimer(3);
+      expect(runOrder).toEqual([
+        'mock2',
+        'mock3',
+        'mock1',
+        'mock4',
+        'mock4',
+        'mock4',
+      ]);
+    });
+
+    it('does nothing when no timers have been scheduled', () => {
+      const global = ({process} as unknown) as NodeJS.Global;
+      const timers = new FakeTimers({
+        config,
+        global,
+        moduleMocker,
+        timerConfig,
+      });
+      timers.useFakeTimers();
+
+      timers.advanceTimersToNextTimer();
+    });
+  });
+
   describe('reset', () => {
     it('resets all pending setTimeouts', () => {
       const global = ({process} as unknown) as NodeJS.Global;

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

@@ -237,6 +237,20 @@ export default class FakeTimers<TimerRef> {
       .forEach(([timerHandle]) => this._runTimerHandle(timerHandle));
   }
 
+  advanceTimersToNextTimer(steps: number = 1): void {
+    const nextExpiry = Array.from(this._timers.values()).reduce(
+      (minExpiry: number | null, timer: Timer): number => {
+        if (minExpiry === null || timer.expiry < minExpiry) return timer.expiry;
+        return minExpiry;
+      },
+      null,
+    );
+    if (nextExpiry !== null && steps > 0) {
+      this.advanceTimersByTime(nextExpiry - this._now);
+      this.advanceTimersToNextTimer(steps - 1);
+    }
+  }
+
   advanceTimersByTime(msToRun: number) {
     this._checkFakeTimers();
     // Only run a generous number of timers and then bail.

packages/jest-runtime/src/index.ts

@@ -1037,6 +1037,8 @@ class Runtime {
         this._environment.global.jasmine.addMatchers(matchers),
       advanceTimersByTime: (msToRun: number) =>
         _getFakeTimers().advanceTimersByTime(msToRun),
+      advanceTimersToNextTimer: () =>
+        _getFakeTimers().advanceTimersToNextTimer(),
       autoMockOff: disableAutomock,
       autoMockOn: enableAutomock,
       clearAllMocks,