使用Vitest作为测试运行器时NestJS Testing Module依赖注入失败

问题重现 #

使用NestJS Testing Module配合Vitest执行测试时，服务无法正确注入，调用方法出现Cannot read properties of undefined (reading 'foo')错误。手动实例化服务的测试可正常运行，但通过Testing Module注入的测试报错。

服务代码 #

RandomService

import { InjectedOneService } from './injected-one.service';
import { InjectedTwoService } from './intjected-two.service';

export class RandomService {
  constructor(
    private readonly injectedOneService: InjectedOneService,
    private readonly injectedTwoService: InjectedTwoService,
  ) { }
  getStr() {
    return `${this.injectedOneService.foo()} ${this.injectedTwoService.bar()}`;
  }
}

InjectedOneService

import { Injectable } from '@nestjs/common';

@Injectable()
export class InjectedOneService {
  foo() {
    return 'foo';
  }
}

InjectedTwoService

import { Injectable } from '@nestjs/common';

@Injectable()
export class InjectedTwoService {
  constructor() { }
  bar() {
    return 'bar';
  }
}

测试代码 #

import { Test, TestingModule } from '@nestjs/testing';
import { RandomService } from './service';
import { InjectedOneService } from './injected-one.service';
import { InjectedTwoService } from './intjected-two.service';
import { describe, it, expect, vi } from 'vitest';

describe('Service', () => {
  it('should work without testing module', () => {
    const injectedOneMockService = new InjectedOneService();
    const injectedTwoMockService = new InjectedTwoService();
    const result = new RandomService(
      injectedOneMockService,
      injectedTwoMockService,
    );
    injectedOneMockService.foo = vi.fn().mockImplementation(() => 'mock foo');
    expect(result).toBeDefined();
    expect(result.getStr()).toBe('mock foo bar');
  });
  it('should work with testing module', async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [RandomService, InjectedOneService, InjectedTwoService],
    }).compile();

    const injectedOneMockService =
      module.get<InjectedOneService>(InjectedOneService);

    const injectedTwoMockService =
      module.get<InjectedTwoService>(InjectedTwoService);

    const result = module.get<RandomService>(RandomService);

    injectedOneMockService.foo = vi.fn().mockImplementation(() => 'mock foo');
    injectedTwoMockService.bar = vi.fn().mockImplementation(() => 'mock bar');

    expect(result).toBeDefined();
    expect(result.getStr()).toBe('mock foo mock bar');
  });
});

报错信息 #

TypeError: Cannot read properties of undefined (reading 'foo')
 ❯ RandomService.getStr src/vitest-test/service.ts:10:39
      8|   ) { }
      9|   getStr() {
     10|     return `${this.injectedOneService.foo()} ${this.injectedTwoService.bar()}`;
       |                                       ^
     11|   }
     12| }

Vitest配置文件 #

import swc from 'unplugin-swc';
import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    globals: true,
    root: './',
  },
  plugins: [
    swc.vite({
      module: { type: 'es6' },
    }),
  ],
});

问题原因与解决方法 #

1. RandomService缺少@Injectable()装饰器 #

NestJS的依赖注入系统需要通过@Injectable()装饰器标记可注入的服务。你的RandomService未添加该装饰器，导致Testing Module无法识别并处理它的依赖注入逻辑，最终实例化时内部依赖为undefined。

修复代码：

import { Injectable } from '@nestjs/common'; // 导入装饰器
import { InjectedOneService } from './injected-one.service';
import { InjectedTwoService } from './injected-two.service';

@Injectable() // 添加装饰器
export class RandomService {
  constructor(
    private readonly injectedOneService: InjectedOneService,
    private readonly injectedTwoService: InjectedTwoService,
  ) { }
  getStr() {
    return `${this.injectedOneService.foo()} ${this.injectedTwoService.bar()}`;
  }
}

2. 导入路径拼写错误 #

检查RandomService和测试文件中对InjectedTwoService的导入路径：./intjected-two.service存在拼写错误（少了字母j），正确路径应为./injected-two.service。路径错误会导致实际注入的依赖为undefined，也是报错的潜在原因。

修复导入路径：

// RandomService中
import { InjectedTwoService } from './injected-two.service';

// 测试文件中
import { InjectedTwoService } from './injected-two.service';

验证修复 #

完成上述修改后重新运行测试，Testing Module即可正确注入依赖，测试应当正常通过。

内容的提问来源于stack exchange，提问作者Steeve