feat: Allow mocking property value in tests

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Features
 
+- `[@jest/globals, jest-mock]` Add `jest.replaceProperty()` that replaces property value ([#13496](https://github.com/facebook/jest/pull/13496))
 - `[jest-haste-map]` ignore Sapling vcs directories (`.sl/`) ([#13674](https://github.com/facebook/jest/pull/13674))
 - `[jest-resolve]` Support subpath imports ([#13705](https://github.com/facebook/jest/pull/13705))
 - `[jest-runtime]` Add `jest.isolateModulesAsync` for scoped module initialization of asynchronous functions ([#13680](https://github.com/facebook/jest/pull/13680))

docs/JestObjectAPI.md

@@ -608,13 +608,62 @@ See [Mock Functions](MockFunctionAPI.md#jestfnimplementation) page for details o
 
 Determines if the given function is a mocked function.
 
+### `jest.replaceProperty(object, propertyKey, value)`
+
+Replace `object[propertyKey]` with a `value`. The property must already exist on the object. The same property might be replaced multiple times. Returns a Jest [replaced property](MockFunctionAPI.md#replaced-properties).
+
+:::note
+
+To mock properties that are defined as getters or setters, use [`jest.spyOn(object, methodName, accessType)`](#jestspyonobject-methodname-accesstype) instead. To mock functions, use [`jest.spyOn(object, methodName)`](#jestspyonobject-methodname) instead.
+
+:::
+
+:::tip
+
+All properties replaced with `jest.replaceProperty` could be restored to the original value by calling [jest.restoreAllMocks](#jestrestoreallmocks) on [afterEach](GlobalAPI.md#aftereachfn-timeout) method.
+
+:::
+
+Example:
+
+```js
+const utils = {
+  isLocalhost() {
+    return process.env.HOSTNAME === 'localhost';
+  },
+};
+
+module.exports = utils;
+```
+
+Example test:
+
+```js
+const utils = require('./utils');
+
+afterEach(() => {
+  // restore replaced property
+  jest.restoreAllMocks();
+});
+
+test('isLocalhost returns true when HOSTNAME is localhost', () => {
+  jest.replaceProperty(process, 'env', {HOSTNAME: 'localhost'});
+  expect(utils.isLocalhost()).toBe(true);
+});
+
+test('isLocalhost returns false when HOSTNAME is not localhost', () => {
+  jest.replaceProperty(process, 'env', {HOSTNAME: 'not-localhost'});
+  expect(utils.isLocalhost()).toBe(false);
+});
+```
+
 ### `jest.spyOn(object, methodName)`
 
 Creates a mock function similar to `jest.fn` but also tracks calls to `object[methodName]`. Returns a Jest [mock function](MockFunctionAPI.md).
 
 :::note
 
-By default, `jest.spyOn` also calls the **spied** method. This is different behavior from most other test libraries. If you want to overwrite the original function, you can use `jest.spyOn(object, methodName).mockImplementation(() => customImplementation)` or `object[methodName] = jest.fn(() => customImplementation);`
+By default, `jest.spyOn` also calls the **spied** method. This is different behavior from most other test libraries. If you want to overwrite the original function, you can use `jest.spyOn(object, methodName).mockImplementation(() => customImplementation)` or `jest.replaceProperty(object, methodName, jest.fn(() => customImplementation));`
 
 :::
 
@@ -713,6 +762,10 @@ test('plays audio', () => {
 });
 ```
 
+### `jest.Replaced<Source>`
+
+See [TypeScript Usage](MockFunctionAPI.md#replacedpropertyreplacevaluevalue) chapter of Mock Functions page for documentation.
+
 ### `jest.Spied<Source>`
 
 See [TypeScript Usage](MockFunctionAPI.md#jestspiedsource) chapter of Mock Functions page for documentation.
@@ -731,7 +784,7 @@ Returns the `jest` object for chaining.
 
 ### `jest.restoreAllMocks()`
 
-Restores all mocks back to their original value. Equivalent to calling [`.mockRestore()`](MockFunctionAPI.md#mockfnmockrestore) on every mocked function. Beware that `jest.restoreAllMocks()` only works when the mock was created with `jest.spyOn`; other mocks will require you to manually restore them.
+Restores all mocks and replaced properties back to their original value. Equivalent to calling [`.mockRestore()`](MockFunctionAPI.md#mockfnmockrestore) on every mocked function and [`.restore()`](MockFunctionAPI.md#replacedpropertyrestore) on every replaced property. Beware that `jest.restoreAllMocks()` only works for mocks created with [`jest.spyOn()`](#jestspyonobject-methodname) and properties replaced with [`jest.replaceProperty()`](#jestreplacepropertyobject-propertykey-value); other mocks will require you to manually restore them.
 
 ## Fake Timers
 

docs/MockFunctionAPI.md

@@ -515,6 +515,20 @@ test('async test', async () => {
 });
 ```
 
+## Replaced Properties
+
+### `replacedProperty.replaceValue(value)`
+
+Changes the value of already replaced property. This is useful when you want to replace property and then adjust the value in specific tests. As an alternative, you can call [`jest.replaceProperty()`](JestObjectAPI.md#jestreplacepropertyobject-propertykey-value) multiple times on same property.
+
+### `replacedProperty.restore()`
+
+Restores object's property to the original value.
+
+Beware that `replacedProperty.restore()` only works when the property value was replaced with [`jest.replaceProperty()`](JestObjectAPI.md#jestreplacepropertyobject-propertykey-value).
+
+The [`restoreMocks`](configuration#restoremocks-boolean) configuration option is available to restore replaced properties automatically before each test.
+
 ## TypeScript Usage
 
 <TypeScriptExamplesNote />
@@ -594,6 +608,39 @@ test('returns correct data', () => {
 
 Types of classes, functions or objects can be passed as type argument to `jest.Mocked<Source>`. If you prefer to constrain the input type, use: `jest.MockedClass<Source>`, `jest.MockedFunction<Source>` or `jest.MockedObject<Source>`.
 
+### `jest.Replaced<Source>`
+
+The `jest.Replaced<Source>` utility type returns the `Source` type wrapped with type definitions of Jest [replaced property](#replaced-properties).
+
+```ts title="src/utils.ts"
+export function isLocalhost(): boolean {
+  return process.env['HOSTNAME'] === 'localhost';
+}
+```
+
+```ts
+import {afterEach, expect, it, jest} from '@jest/globals';
+import {isLocalhost} from '../utils';
+
+let replacedEnv: jest.Replaced<typeof process.env> | undefined = undefined;
+
+afterEach(() => {
+  replacedEnv?.restore();
+});
+
+it('isLocalhost should detect localhost environment', () => {
+  replacedEnv = jest.replaceProperty(process, 'env', {HOSTNAME: 'localhost'});
+
+  expect(isLocalhost()).toBe(true);
+});
+
+it('isLocalhost should detect non-localhost environment', () => {
+  replacedEnv = jest.replaceProperty(process, 'env', {HOSTNAME: 'example.com'});
+
+  expect(isLocalhost()).toBe(false);
+});
+```
+
 ### `jest.mocked(source, options?)`
 
 The `mocked()` helper method wraps types of the `source` object and its deep nested members with type definitions of Jest mock function. You can pass `{shallow: true}` as the `options` argument to disable the deeply mocked behavior.

examples/manual-mocks/__tests__/utils.test.js

@@ -0,0 +1,17 @@
+import {isLocalhost} from '../utils';
+
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
+it('isLocalhost should detect localhost environment', () => {
+  jest.replaceProperty(process, 'env', {HOSTNAME: 'localhost'});
+
+  expect(isLocalhost()).toBe(true);
+});
+
+it('isLocalhost should detect non-localhost environment', () => {
+  jest.replaceProperty(process, 'env', {HOSTNAME: 'example.com'});
+
+  expect(isLocalhost()).toBe(false);
+});

examples/manual-mocks/utils.js

@@ -0,0 +1,3 @@
+export function isLocalhost() {
+  return process.env.HOSTNAME === 'localhost';
+}

examples/typescript/__tests__/utils.test.ts

@@ -0,0 +1,24 @@
+// Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+
+import {afterEach, beforeEach, expect, it, jest} from '@jest/globals';
+import {isLocalhost} from '../utils';
+
+let replacedEnv: jest.Replaced<typeof process.env> | undefined = undefined;
+
+beforeEach(() => {
+  replacedEnv = jest.replaceProperty(process, 'env', {});
+});
+
+afterEach(() => {
+  replacedEnv?.restore();
+});
+
+it('isLocalhost should detect localhost environment', () => {
+  replacedEnv.replaceValue({HOSTNAME: 'localhost'});
+
+  expect(isLocalhost()).toBe(true);
+});
+
+it('isLocalhost should detect non-localhost environment', () => {
+  expect(isLocalhost()).toBe(false);
+});

examples/typescript/utils.ts

@@ -0,0 +1,5 @@
+// Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+
+export function isLocalhost() {
+  return process.env.HOSTNAME === 'localhost';
+}

packages/jest-environment/src/index.ts

@@ -223,6 +223,13 @@ export interface Jest {
    * mocked behavior.
    */
   mocked: ModuleMocker['mocked'];
+  /**
+   * Replaces property on an object with another value.
+   *
+   * @remarks
+   * For mocking functions or 'get' or 'set' accessors, use `jest.spyOn()` instead.
+   */
+  replaceProperty: ModuleMocker['replaceProperty'];
   /**
    * Returns a mock module instead of the actual module, bypassing all checks
    * on whether the module should be required normally or not.
@@ -239,8 +246,9 @@ export interface Jest {
    */
   resetModules(): Jest;
   /**
-   * Restores all mocks back to their original value. Equivalent to calling
-   * `.mockRestore()` on every mocked function.
+   * Restores all mocks and replaced properties back to their original value.
+   * Equivalent to calling `.mockRestore()` on every mocked function
+   * and `.restore()` on every replaced property.
    *
    * Beware that `jest.restoreAllMocks()` only works when the mock was created
    * with `jest.spyOn()`; other mocks will require you to manually restore them.

packages/jest-globals/src/index.ts

@@ -16,6 +16,7 @@ import type {
   MockedClass as JestMockedClass,
   MockedFunction as JestMockedFunction,
   MockedObject as JestMockedObject,
+  Replaced as JestReplaced,
   Spied as JestSpied,
   SpiedClass as JestSpiedClass,
   SpiedFunction as JestSpiedFunction,
@@ -63,6 +64,10 @@ declare namespace jest {
    * Wraps an object type with Jest mock type definitions.
    */
   export type MockedObject<T extends object> = JestMockedObject<T>;
+  /**
+   * Constructs the type of a replaced property.
+   */
+  export type Replaced<T> = JestReplaced<T>;
   /**
    * Constructs the type of a spied class or function.
    */

packages/jest-mock/__typetests__/mock-functions.test.ts

@@ -15,11 +15,13 @@ import {
 } from 'tsd-lite';
 import {
   Mock,
+  Replaced,
   SpiedClass,
   SpiedFunction,
   SpiedGetter,
   SpiedSetter,
   fn,
+  replaceProperty,
   spyOn,
 } from 'jest-mock';
 
@@ -492,3 +494,65 @@ expectError(
     (key: string, value: number) => {},
   ),
 );
+
+// replaceProperty + Replaced
+
+const obj = {
+  fn: () => {},
+
+  property: 1,
+};
+
+expectType<Replaced<number>>(replaceProperty(obj, 'property', 1));
+expectType<void>(replaceProperty(obj, 'property', 1).replaceValue(1).restore());
+
+expectError(replaceProperty(obj, 'invalid', 1));
+expectError(replaceProperty(obj, 'property', 'not a number'));
+expectError(replaceProperty(obj, 'fn', () => {}));
+
+expectError(replaceProperty(obj, 'property', 1).replaceValue('not a number'));
+
+interface ComplexObject {
+  numberOrUndefined: number | undefined;
+  optionalString?: string;
+  [key: `dynamic prop ${number}`]: boolean;
+  multipleTypes: number | string | {foo: number} | null;
+}
+declare const complexObject: ComplexObject;
+
+// Resulting type should retain the original property type
+expectType<Replaced<number | undefined>>(
+  replaceProperty(complexObject, 'numberOrUndefined', undefined),
+);
+expectType<Replaced<number | undefined>>(
+  replaceProperty(complexObject, 'numberOrUndefined', 1),
+);
+
+expectError(
+  replaceProperty(
+    complexObject,
+    'numberOrUndefined',
+    'string is not valid TypeScript type',
+  ),
+);
+
+expectType<Replaced<string | undefined>>(
+  replaceProperty(complexObject, 'optionalString', 'foo'),
+);
+expectType<Replaced<string | undefined>>(
+  replaceProperty(complexObject, 'optionalString', undefined),
+);
+
+expectType<Replaced<boolean>>(
+  replaceProperty(complexObject, 'dynamic prop 1', true),
+);
+expectError(replaceProperty(complexObject, 'dynamic prop 1', undefined));
+
+expectError(replaceProperty(complexObject, 'not a property', undefined));
+
+expectType<Replaced<ComplexObject['multipleTypes']>>(
+  replaceProperty(complexObject, 'multipleTypes', 1)
+    .replaceValue('foo')
+    .replaceValue({foo: 1})
+    .replaceValue(null),
+);