如何使用 Jest 模拟拒绝承诺

Question

我在做什么？

我正在学习 Nestjs 课程，其中有一些单元测试。我编写了这个测试来检查存储库类中的signUp方法。问题是，为了触发异常，该行user.save()应该返回一个承诺拒绝（模拟写入数据库的一些问题）。我尝试了几种方法（见下文），但没有一个有效。

问题

结果是测试成功了，但是有一个unhandled Promise rejection. 这样，即使我断言它not.toThow()会以相同的方式成功unhandled Promise rejection

(node:10149) UnhandledPromiseRejectionWarning: Error: expect(received).rejects.toThrow()

Received promise resolved instead of rejected
Resolved to value: undefined
(Use `node --trace-warnings ...` to show where the warning was created)

我如何让它正确拒绝承诺？

代码

下面是我的测试代码和被测函数。

import { ConflictException } from '@nestjs/common';
import { Test } from '@nestjs/testing';
import { AuthCredentialsDto } from './dto/auth-credentials.dto';
import { UserRepository } from './user.repository';

describe('UserRepository', () => {
  let userRepository: UserRepository;

  let authCredentialsDto: AuthCredentialsDto = {
    username: 'usahh',
    password: 'passworD12!@',
  };

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      providers: [UserRepository],
    }).compile();

    userRepository = module.get<UserRepository>(UserRepository);
  });

  describe('signUp', () => {
    let save: any;
    beforeEach(() => {
      save = jest.fn();
      userRepository.create = jest.fn().mockReturnValue({ save });
    });

    it('throws a conflict exception if user already exist', () => {
      // My first try:
      // save.mockRejectedValue({
      //   code: '23505',
      // });

      // Then I tried this, with and without async await:
      save.mockImplementation(async () => {
        await Promise.reject({ code: '23505' });
      });
      expect(userRepository.signUp(authCredentialsDto)).rejects.toThrow(
        ConflictException,
      );
    });
  });
});

这里测试的函数是：

@EntityRepository(User)
export class UserRepository extends Repository<User> {
  async signUp(authCredentialsDto: AuthCredentialsDto): Promise<void> {
    const { username, password } = authCredentialsDto;
    const user = this.create();

    user.salt = await bcrypt.genSalt();
    user.username = username;
    user.password = await this.hashPassword(password, user.salt);

    try {
      await user.save();
    } catch (e) {
      if (e.code === '23505') {
        throw new ConflictException('Username already exists');
      } else {
        throw new InternalServerErrorException();
      }
    }
  }
}

Answer 1

这应该是异步测试，但它是同步的，即使有被拒绝的承诺，它也不会失败。

返回的承诺expect(...).rejects...需要被链接起来：

it('throws a conflict exception if user already exist', async () => {
  ...
  await expect(userRepository.signUp(authCredentialsDto)).rejects.toThrow(
    ConflictException,
  );
});

没有试错的余地mockImplementation。模拟应该返回被拒绝的承诺，并mockRejectedValue执行此操作。mockImplementation(async () => ...)写起来实在是太长了。