如何在Jest中重置方法的mockImplementation并重新指定返回值

问题场景 #

已为TestService实现Mock，第一个测试用例因checkIfExistsInDB返回false执行成功，但第二个测试用例需要将该Mock方法改为返回true，尝试两种方法均无效：

初始Mock代码 #

jest.mock('../../lib/testService', () => ({
  TestService : jest.fn(() => ({
      callAPIProd: jest.fn().mockImplementation(() => Promise.resolve([{ DateTime: '2022-01-01'}])),
      getAuthToken: jest.fn().mockImplementation(() => Promise.resolve({ data: 'mock token' })),
      checkIfExistsInDB: jest.fn().mockImplementation(() => Promise.resolve(false)),
  })),
}));

测试用例代码 #

describe('getRates', () => {
    it('should insert rates into DB if not already available', async () => {
      const req = {} as Request;
      const res = { send: jest.fn() } as unknown as Response;

      await integration.getRates(req, res);

      expect(res.send).toHaveBeenCalledWith('New Rates Received');
    });

    it('should send response if rates are already available in DB', async () => {
      const req = {} as Request;
      const res = { send: jest.fn() } as unknown as Response;

      // 尝试过的两种无效方法
      // #1 重新调用jest.mock，无效
      // jest.mock(...)
      // #2 修改prototype，无效
      // TestService.prototype.checkIfExistsInDB = jest.fn().mockResolvedValue(true)

      await integration.getRates(req, res);

      expect(res.send).toHaveBeenCalledWith('Rates already available in DB');
    });
});

无效原因分析 #

1. 测试用例内调用jest.mock无效：jest.mock会被Jest自动提升到模块顶部执行，测试用例内部的jest.mock无法覆盖模块级的Mock配置。
2. 修改prototype无效：TestService是一个返回普通对象的工厂函数（jest.fn()），并非类，不存在prototype属性，修改它不会影响Mock实例的方法。

正确解决方案 #

步骤1：获取Mock实例的方法引用 #

在模块顶部导入TestService后，直接获取Mock工厂函数返回的实例引用：

import { TestService } from '../../lib/testService';
import integration from './your-integration-module'; // 替换为实际路径
import type { Request, Response } from 'express';

jest.mock('../../lib/testService', () => ({
  TestService : jest.fn(() => ({
      callAPIProd: jest.fn().mockResolvedValue([{ DateTime: '2022-01-01'}]),
      getAuthToken: jest.fn().mockResolvedValue({ data: 'mock token' }),
      checkIfExistsInDB: jest.fn().mockResolvedValue(false),
  })),
}));

// 获取TestService Mock返回的实例对象
const mockTestService = TestService.mock.results[0].value;

步骤2：在测试用例中重置并修改Mock返回值 #

在第二个测试用例中，直接重置目标Mock方法并设置新的返回值：

describe('getRates', () => {
    // 可选：每次测试前重置所有Mock，避免用例间污染
    beforeEach(() => {
        jest.clearAllMocks();
        // 重置为默认返回值
        mockTestService.checkIfExistsInDB.mockResolvedValue(false);
    });

    it('should insert rates into DB if not already available', async () => {
      const req = {} as Request;
      const res = { send: jest.fn() } as unknown as Response;

      await integration.getRates(req, res);

      expect(res.send).toHaveBeenCalledWith('New Rates Received');
    });

    it('should send response if rates are already available in DB', async () => {
      const req = {} as Request;
      const res = { send: jest.fn() } as unknown as Response;

      // 修改checkIfExistsInDB的返回值为true
      mockTestService.checkIfExistsInDB.mockResolvedValue(true);

      await integration.getRates(req, res);

      expect(res.send).toHaveBeenCalledWith('Rates already available in DB');
    });
});

替代写法：使用mockReturnValueOnce #

如果只需要当前测试用例临时改变返回值，也可以用mockReturnValueOnce：

mockTestService.checkIfExistsInDB.mockReturnValueOnce(Promise.resolve(true));

内容的提问来源于stack exchange，提问作者MUK