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

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 +369,12 @@ export default class FakeTimers<TimerRef> {
       // @ts-ignore TODO: figure out better typings here
       this._moduleMocker.fn().mockImplementation(impl);
 
+    const promisifiedFakeSetTimeout = fn(this._fakeSetTimeout.bind(this));
+    promisifiedFakeSetTimeout[util.promisify.custom] = (
+      delay?: number,
+      arg?: any,
+    ) => new Promise(resolve => promisifiedFakeSetTimeout(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 +383,7 @@ 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: promisifiedFakeSetTimeout,
     };
   }