Tests: Fix spurious infinite timer issue

change/office-ui-fabric-react-2020-03-03-15-35-01-jg-fix-infinite-timers.json

@@ -0,0 +1,9 @@
+{
+  "type": "none",
+  "comment": "Add mock for Async to prevent spurious infinite timer issue.",
+  "packageName": "office-ui-fabric-react",
+  "email": "jagore@microsoft.com",
+  "commit": "5c78a6866573bcfa7096bc625c795c3205d69e89",
+  "dependentChangeType": "patch",
+  "date": "2020-03-03T23:35:01.016Z"
+}

packages/office-ui-fabric-react/__mocks__/@uifabric/utilities.ts

@@ -0,0 +1,33 @@
+export * from '@uifabric/utilities';
+import { Async } from '@uifabric/utilities';
+
+// Known issue with jest's runAllTimers and debounce implementations resulting in
+// "Ran 100000 timers, and there are still more! Assuming we've hit an infinite recursion and bailing out..."
+// https://github.com/facebook/jest/issues/3465
+// Mock impl inspired from issue.
+class MockAsync extends Async {
+  public debounce(callback: Function, timeout: number) {
+    let timeoutId: number | null = null;
+    const debounced = (...args: any[]) => {
+      if (timeoutId) {
+        clearTimeout(timeoutId);
+      }
+      // Callback functions throughout repo aren't binding properly, so we have to access
+      // Async's private _parent member and invoke callbacks the same way Async.debounce does.
+      const invokeFunction = () => callback.apply((this as any)._parent, args);
+      timeoutId = setTimeout(invokeFunction, timeout);
+    };
+
+    const cancel = () => {
+      if (timeoutId) {
+        clearTimeout(timeoutId);
+      }
+    };
+
+    (debounced as any).cancel = cancel;
+
+    return debounced as any;
+  }
+}
+
+export { MockAsync as Async };