在类中模拟实例答案

【问题标题】：Mocking an instance within a class在类中模拟实例
【发布时间】：2021-11-19 20:38:19
【问题描述】：

晚上好，

我正在尝试为以下打字稿类创建一些测试。我正在尝试创建一个测试，以便我可以验证内部 BroadcastChannel 是否已调用其 postMessage 方法，但我无法创建相关的间谍来执行此操作。我猜这是因为我没有将间谍附加到类中的实际实例，但我不确定。

export class BroadcastChannelService<T> {
  private readonly broadcastChannel: BroadcastChannel;

  constructor(name: CHANNEL_NAMES) {
    this.broadcastChannel = new BroadcastChannel(name);
  }

  postMessage = (msg: T) => {
    this.broadcastChannel.postMessage(msg);
  }
}

以下是我目前的测试

import { BroadcastChannel } from 'broadcast-channel';

import { BroadcastChannelService } from '../../services';

jest.mock('broadcast-channel');
const mockedBroadcastChannel = BroadcastChannel as jest.Mocked<typeof BroadcastChannel>;

describe('BroadcastChannelService', () => {
  let subject: BroadcastChannelService<string>;

  describe('constructor', () => {
    afterAll(() => {
      jest.resetAllMocks();
    });

    test('is successful', () => {
      // eslint-disable-next-line no-unused-vars
      subject = new BroadcastChannelService<string>('GOOGLE_AUTH');
      expect(mockedBroadcastChannel).toBeCalledWith('GOOGLE_AUTH');
      expect(mockedBroadcastChannel).toBeCalledTimes(1);
    });
  });

  describe('postMessage', () => {
    beforeAll(() => {
      subject = new BroadcastChannelService('GOOGLE_AUTH');
      subject.postMessage('Hello World');
    });

    afterAll(() => {
      jest.resetAllMocks();
    });

    test('is successful', () => {
    });
  });
});

【问题讨论】：

标签： typescript jestjs mocking

【解决方案1】：

所以诀窍是使用spyOn 和mockedBroadcastChannel.prototype

import { BroadcastChannel } from 'broadcast-channel';
import { BroadcastChannelService } from '../../services';

jest.mock('broadcast-channel');
const mockedBroadcastChannel = BroadcastChannel as jest.Mocked<typeof BroadcastChannel>;

describe('BroadcastChannelService', () => {
  let subject: BroadcastChannelService<string>;

  describe('constructor', () => {
    test('is successful', () => {
      // eslint-disable-next-line no-unused-vars
      subject = new BroadcastChannelService<string>('GOOGLE_AUTH');
      expect(mockedBroadcastChannel).toBeCalledWith('GOOGLE_AUTH');
      expect(mockedBroadcastChannel).toBeCalledTimes(1);
    });
  });

  describe('postMessage', () => {
    test('is successful', () => {
      const postMessageSpy = jest.spyOn(mockedBroadcastChannel.prototype, 'postMessage');
      subject = new BroadcastChannelService<string>('GOOGLE_AUTH');
      subject.postMessage('Hello World');
      expect(postMessageSpy).toBeCalledTimes(1);
      expect(postMessageSpy).toBeCalledWith('Hello World');
    });
  });
});

【讨论】：