Always recreate mocks in FakeTimers.useFakeTimers()

If the flag resetMocks or jest.resetAllMocks is used in the test, all the timer
tests following that will fail with no way of recovering.

jest.resetAllMocks resets everything, including the one created by fakeTimers.
There is also no public methods that we can use to reinstantiate a new
FakeTimer object to mitigate that.

Given both mock and fake timers are part of the jest, they should be working
together out-of-the box without extra code gluing them. We change the
useFakeTimers method to always create new mocks so it would not be reverted.

integration_tests/__tests__/timer_after_resetAllMocks-test.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @emails oncall+jsinfra
+ */
+'use strict';
+
+const runJest = require('../runJest');
+const skipOnWindows = require('skipOnWindows');
+
+skipOnWindows.suite();
+
+test('run timers after resetAllMocks test', () => {
+  const result = runJest('timer_after_resetAllMocks');
+  expect(result.status).toBe(0);
+});

integration_tests/timer_after_resetAllMocks/index.js

@@ -0,0 +1,11 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+'use strict';
+
+module.exports = () => {};

integration_tests/timer_after_resetAllMocks/package.json

@@ -0,0 +1,6 @@
+{
+  "jest": {
+    "testEnvironment": "node",
+    "resetMocks": false
+  }
+}

integration_tests/timer_after_resetAllMocks/timer_and_mock.test.js

@@ -0,0 +1,18 @@
+describe('timers', () => {
+  it('should work before calling resetAllMocks', () => {
+    jest.useFakeTimers();
+    const f = jest.fn();
+    setImmediate(() => f());
+    jest.runAllImmediates();
+    expect(f.mock.calls.length).toBe(1);
+  });
+
+  it('should not break after calling resetAllMocks', () => {
+    jest.resetAllMocks();
+    jest.useFakeTimers();
+    const f = jest.fn();
+    setImmediate(() => f());
+    jest.runAllImmediates();
+    expect(f.mock.calls.length).toBe(1);
+  });
+});

packages/jest-util/src/FakeTimers.js

@@ -53,6 +53,7 @@ class FakeTimers {
   _global: Global;
   _immediates: Array<Tick>;
   _maxLoops: number;
+  _moduleMocker: ModuleMocker;
   _now: number;
   _ticks: Array<Tick>;
   _timerAPIs: TimerAPI;
@@ -70,8 +71,7 @@ class FakeTimers {
     this._config = config;
     this._maxLoops = maxLoops || 100000;
     this._uuidCounter = 1;
-
-    this.reset();
+    this._moduleMocker = moduleMocker;
 
     // Store original timer APIs for future reference
     this._timerAPIs = {
@@ -83,25 +83,16 @@ class FakeTimers {
       setTimeout: global.setTimeout,
     };
 
-    const fn = impl => moduleMocker.getMockFn().mockImpl(impl);
-
-    this._fakeTimerAPIs = {
-      clearImmediate: fn(this._fakeClearImmediate.bind(this)),
-      clearInterval: fn(this._fakeClearTimer.bind(this)),
-      clearTimeout: fn(this._fakeClearTimer.bind(this)),
-      setImmediate: fn(this._fakeSetImmediate.bind(this)),
-      setInterval: fn(this._fakeSetInterval.bind(this)),
-      setTimeout: fn(this._fakeSetTimeout.bind(this)),
-    };
-
     // If there's a process.nextTick on the global, mock it out
     // (only applicable to node/node-emulating environments)
     if (typeof global.process === 'object'
       && typeof global.process.nextTick === 'function') {
       this._timerAPIs.nextTick = global.process.nextTick;
-      this._fakeTimerAPIs.nextTick = fn(this._fakeNextTick.bind(this));
     }
 
+    this.reset();
+    this._createMocks();
+
     // These globally-accessible function are now deprecated!
     // They will go away very soon, so do not use them!
     // Instead, use the versions available on the `jest` object
@@ -330,6 +321,8 @@ class FakeTimers {
       typeof this._global.process === 'object'
       && typeof this._global.process.nextTick === 'function';
 
+    this._createMocks();
+
     this._global.clearImmediate = this._fakeTimerAPIs.clearImmediate;
     this._global.clearInterval = this._fakeTimerAPIs.clearInterval;
     this._global.clearTimeout = this._fakeTimerAPIs.clearTimeout;
@@ -355,6 +348,25 @@ class FakeTimers {
     }
   }
 
+  _createMocks() {
+    const fn = impl => this._moduleMocker.getMockFn().mockImpl(impl);
+
+    this._fakeTimerAPIs = {
+      clearImmediate: fn(this._fakeClearImmediate.bind(this)),
+      clearInterval: fn(this._fakeClearTimer.bind(this)),
+      clearTimeout: fn(this._fakeClearTimer.bind(this)),
+      setImmediate: fn(this._fakeSetImmediate.bind(this)),
+      setInterval: fn(this._fakeSetInterval.bind(this)),
+      setTimeout: fn(this._fakeSetTimeout.bind(this)),
+    };
+
+    // If there's a process.nextTick on the global, mock it out
+    // (only applicable to node/node-emulating environments)
+    if (this._timerAPIs.nextTick) {
+      this._fakeTimerAPIs.nextTick = fn(this._fakeNextTick.bind(this));
+    }
+  }
+
   _fakeClearTimer(uuid: TimerID) {
     if (this._timers.hasOwnProperty(uuid)) {
       delete this._timers[uuid];