Fix util.promisify(setTimeout) on jest-fake-timers (#9180)

* Fix util.promisify(setTimeout) on jest-fake-timers

* Assert for promisify on instalation describe

* Fix typo in var

* Change custom timeout to not call the resolve function

* Expect promise to be resolved

* Add changelog

* Apply review changes

* Fix eslint

* Update CHANGELOG.md

* Run CI again

* Run CI again

* Run CI again

CHANGELOG.md

@@ -62,6 +62,7 @@
 - `[jest-types]` [**BREAKING**] Use less `null | undefined` in config types ([#9200](https://github.com/facebook/jest/pull/9200))
 - `[jest-utils]` Allow querying process.domain ([#9136](https://github.com/facebook/jest/pull/9136))
 - `[pretty-format]` Correctly detect memoized elements ([#9196](https://github.com/facebook/jest/pull/9196))
+- `[jest-fake-timers]` Support `util.promisify` on `setTimeout` ([#9180](https://github.com/facebook/jest/pull/9180))
 
 ### Chore & Maintenance
 

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

@@ -8,6 +8,7 @@
 import {runInNewContext} from 'vm';
 import mock = require('jest-mock');
 import FakeTimers from '../jestFakeTimers';
+import util = require('util');
 
 const timerConfig = {
   idToRef: (id: number) => id,
@@ -40,6 +41,20 @@ describe('FakeTimers', () => {
       expect(global.setTimeout).not.toBe(undefined);
     });
 
+    it('accepts to promisify setTimeout mock', async () => {
+      const global = ({process} as unknown) as NodeJS.Global;
+      const timers = new FakeTimers({
+        config,
+        global,
+        moduleMocker,
+        timerConfig,
+      });
+      timers.useFakeTimers();
+      const timeoutPromise = util.promisify(global.setTimeout)(0, 'resolved');
+      timers.runAllTimers();
+      await expect(timeoutPromise).resolves.toBe('resolved');
+    });
+
     it('installs clearTimeout mock', () => {
       const global = ({process} as unknown) as NodeJS.Global;
       const timers = new FakeTimers({

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

@@ -8,6 +8,7 @@
 import {ModuleMocker} from 'jest-mock';
 import {StackTraceConfig, formatStackTrace} from 'jest-message-util';
 import {setGlobal} from 'jest-util';
+import util = require('util');
 
 type Callback = (...args: Array<unknown>) => void;
 
@@ -368,6 +369,13 @@ export default class FakeTimers<TimerRef> {
       // @ts-ignore TODO: figure out better typings here
       this._moduleMocker.fn().mockImplementation(impl);
 
+    const promisifiableFakeSetTimeout = fn(this._fakeSetTimeout.bind(this));
+    promisifiableFakeSetTimeout[util.promisify.custom] = (
+      delay?: number,
+      arg?: any,
+    ) =>
+      new Promise(resolve => promisifiableFakeSetTimeout(resolve, delay, arg));
+
     // TODO: add better typings; these are mocks, but typed as regular timers
     this._fakeTimerAPIs = {
       clearImmediate: fn(this._fakeClearImmediate.bind(this)),
@@ -376,7 +384,7 @@ export default class FakeTimers<TimerRef> {
       nextTick: fn(this._fakeNextTick.bind(this)),
       setImmediate: fn(this._fakeSetImmediate.bind(this)),
       setInterval: fn(this._fakeSetInterval.bind(this)),
-      setTimeout: fn(this._fakeSetTimeout.bind(this)),
+      setTimeout: promisifiableFakeSetTimeout,
     };
   }