getTimerCount will not include cancelled immediates

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Fixes
 
+- `[jest-fake-timers]` `getTimerCount` will not include cancelled immediates ([#8764](https://github.com/facebook/jest/pull/8764))
+
 ### Chore & Maintenance
 
 ### Performance

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

@@ -1303,5 +1303,22 @@ describe('FakeTimers', () => {
 
       expect(timers.getTimerCount()).toEqual(3);
     });
+
+    it('not includes cancelled immediates', () => {
+      const timers = new FakeTimers({
+        config,
+        global,
+        moduleMocker,
+        timerConfig,
+      });
+
+      timers.useFakeTimers();
+
+      global.setImmediate(() => {});
+      expect(timers.getTimerCount()).toEqual(1);
+      timers.clearAllTimers();
+
+      expect(timers.getTimerCount()).toEqual(0);
+    });
   });
 });

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

@@ -53,7 +53,6 @@ const setGlobal = (
 };
 
 export default class FakeTimers<TimerRef> {
-  private _cancelledImmediates!: Record<string, boolean>;
   private _cancelledTicks!: Record<string, boolean>;
   private _config: StackTraceConfig;
   private _disposed?: boolean;
@@ -105,9 +104,7 @@ export default class FakeTimers<TimerRef> {
   }
 
   clearAllTimers() {
-    this._immediates.forEach(immediate =>
-      this._fakeClearImmediate(immediate.uuid),
-    );
+    this._immediates = [];
     this._timers.clear();
   }
 
@@ -118,7 +115,6 @@ export default class FakeTimers<TimerRef> {
 
   reset() {
     this._cancelledTicks = {};
-    this._cancelledImmediates = {};
     this._now = 0;
     this._ticks = [];
     this._immediates = [];
@@ -177,10 +173,10 @@ export default class FakeTimers<TimerRef> {
   }
 
   private _runImmediate(immediate: Tick) {
-    if (!this._cancelledImmediates.hasOwnProperty(immediate.uuid)) {
-      // Callback may throw, so update the map prior calling.
-      this._cancelledImmediates[immediate.uuid] = true;
+    try {
       immediate.callback();
+    } finally {
+      this._fakeClearImmediate(immediate.uuid);
     }
   }
 
@@ -402,7 +398,9 @@ export default class FakeTimers<TimerRef> {
   }
 
   private _fakeClearImmediate(uuid: TimerID) {
-    this._cancelledImmediates[uuid] = true;
+    this._immediates = this._immediates.filter(
+      immediate => immediate.uuid !== uuid,
+    );
   }
 
   private _fakeNextTick(callback: Callback, ...args: Array<any>) {
@@ -432,19 +430,20 @@ export default class FakeTimers<TimerRef> {
       return null;
     }
 
-    const uuid = this._uuidCounter++;
+    const uuid = String(this._uuidCounter++);
 
     this._immediates.push({
       callback: () => callback.apply(null, args),
-      uuid: String(uuid),
+      uuid,
     });
 
-    const cancelledImmediates = this._cancelledImmediates;
     this._timerAPIs.setImmediate(() => {
-      if (!cancelledImmediates.hasOwnProperty(uuid)) {
-        // Callback may throw, so update the map prior calling.
-        cancelledImmediates[String(uuid)] = true;
-        callback.apply(null, args);
+      if (this._immediates.find(x => x.uuid === uuid)) {
+        try {
+          callback.apply(null, args);
+        } finally {
+          this._fakeClearImmediate(uuid);
+        }
       }
     });