使用mockFn.mockImplementation (fn)。

import { funcToMock } from './somewhere';
jest.mock('./somewhere');

beforeEach(() => {
  funcToMock.mockImplementation(() => { /* default implementation */ });
  // (funcToMock as jest.Mock)... in TS
});

test('case that needs a different implementation of funcToMock', () => {
  funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
  // (funcToMock as jest.Mock)... in TS

  // ...
});

使用mockFn.mockImplementation (fn)。

import { funcToMock } from './somewhere';
jest.mock('./somewhere');

beforeEach(() => {
  funcToMock.mockImplementation(() => { /* default implementation */ });
  // (funcToMock as jest.Mock)... in TS
});

test('case that needs a different implementation of funcToMock', () => {
  funcToMock.mockImplementation(() => { /* implementation specific to this test */ });
  // (funcToMock as jest.Mock)... in TS

  // ...
});

当模拟单个方法时(当它需要保持类/模块实现的其余部分完整时)，我发现以下方法有助于从单个测试重置任何实现调整。

I found this approach to be the concisest one, with no need to jest.mock something at the beginning of the file etc. You need just the code you see below to mock MyClass.methodName. Another advantage is that by default spyOn keeps the original method implementation but also saves all the stats (# of calls, arguments, results etc.) to test against, and keeping the default implementation is a must in some cases. So you have the flexibility to keep the default implementation or to change it with a simple addition of .mockImplementation as mentioned in the code below.

代码是Typescript的，注释突出了JS的区别(准确地说，区别在一行中)。用Jest 26.6测试。

describe('test set', () => {
    let mockedFn: jest.SpyInstance<void>; // void is the return value of the mocked function, change as necessary
    // For plain JS use just: let mockedFn;

    beforeEach(() => {
        mockedFn = jest.spyOn(MyClass.prototype, 'methodName');
        // Use the following instead if you need not to just spy but also to replace the default method implementation:
        // mockedFn = jest.spyOn(MyClass.prototype, 'methodName').mockImplementation(() => {/*custom implementation*/});
    });

    afterEach(() => {
        // Reset to the original method implementation (non-mocked) and clear all the mock data
        mockedFn.mockRestore();
    });

    it('does first thing', () => {
        /* Test with the default mock implementation */
    });

    it('does second thing', () => {
        mockedFn.mockImplementation(() => {/*custom implementation just for this test*/});
        /* Test utilising this custom mock implementation. It is reset after the test. */
    });

    it('does third thing', () => {
        /* Another test with the default mock implementation */
    });
});

编写测试的一个很好的模式是创建一个设置工厂函数，该函数返回测试当前模块所需的数据。

下面是第二个示例之后的一些示例代码，尽管它允许以可重用的方式提供默认值和覆盖值。

const spyReturns = returnValue => jest.fn(() => returnValue);

describe("scenario", () => {
  beforeEach(() => {
    jest.resetModules();
  });

  const setup = (mockOverrides) => {
    const mockedFunctions =  {
      a: spyReturns(true),
      b: spyReturns(true),
      ...mockOverrides
    }
    jest.doMock('../myModule', () => mockedFunctions)
    return {
      mockedModule: require('../myModule')
    }
  }

  it("should return true for module a", () => {
    const { mockedModule } = setup();
    expect(mockedModule.a()).toEqual(true)
  });

  it("should return override for module a", () => {
    const EXPECTED_VALUE = "override"
    const { mockedModule } = setup({ a: spyReturns(EXPECTED_VALUE)});
    expect(mockedModule.a()).toEqual(EXPECTED_VALUE)
  });
});

必须使用jest.resetModules()重置缓存的模块，这一点很重要。这可以在beforeEach或类似的拆卸函数中完成。

更多信息请参见笑话对象文档:https://jestjs.io/docs/jest-object。

有点晚了，但如果有人对此有意见。

我们使用TypeScript、ES6和babel进行react-native开发。

我们通常在根目录__mocks__中模拟外部NPM模块。

我想为特定的测试重写aws-amplify的Auth类中某个模块的特定函数。

    import { Auth } from 'aws-amplify';
    import GetJwtToken from './GetJwtToken';
    ...
    it('When idToken should return "123"', async () => {
      const spy = jest.spyOn(Auth, 'currentSession').mockImplementation(() => ({
        getIdToken: () => ({
          getJwtToken: () => '123',
        }),
      }));

      const result = await GetJwtToken();
      expect(result).toBe('123');
      spy.mockRestore();
    });

要点: https://gist.github.com/thomashagstrom/e5bffe6c3e3acec592201b6892226af2

教程: https://medium.com/p/b4ac52a005d#19c5

这是我在这个博客https://mikeborozdin.com/post/changing-jest-mocks-between-tests/上发现的一个很酷的方法

import { sayHello } from './say-hello';
import * as config from './config';

jest.mock('./config', () => ({
  __esModule: true,
  CAPITALIZE: null
}));

describe('say-hello', () => {
  test('Capitalizes name if config requires that', () => {
    config.CAPITALIZE = true;

    expect(sayHello('john')).toBe('Hi, John');
  });

  test('does not capitalize name if config does not require that', () => {
    config.CAPITALIZE = false;

    expect(sayHello('john')).toBe('Hi, john');
  });
});