Angular+Jest测试报错：TypeError: this.helperService.add不是函数

错误原因 #

• 你在TestBed的providers数组中重复注册了HelperService，后一个{provide: HelperService, useValue: subSpy}会覆盖前一个配置，最终注入到TestService的是subSpy——它只是subtract方法的spy函数，并非完整的HelperService实例，自然不存在add方法。
• 直接将jest.spyOn返回的单个方法spy作为useValue注入是错误逻辑，TestService依赖的是HelperService的完整实例，而非单个孤立的方法spy。

解决方案 #

方法一：创建包含spy方法的模拟服务实例

在beforeEach中构建一个模拟的HelperService对象，为其add和subtract方法分别设置spy，再将这个模拟对象作为依赖注入：

import { TestBed } from '@angular/core/testing';
import { HelperService } from './helper.service';
import { TestService } from './test.service';

describe('Test Service', () => {
    let testService: TestService;
    let mockHelperService: jest.Mocked<HelperService>;

    beforeEach(() => {
        // 构建模拟服务，包含两个方法的spy
        mockHelperService = {
            add: jest.fn().mockReturnValue(50),
            subtract: jest.fn()
        } as jest.Mocked<HelperService>;

        TestBed.configureTestingModule({
            providers: [
                TestService,
                { provide: HelperService, useValue: mockHelperService }
            ],
        });

        testService = TestBed.inject(TestService);
    });

    it('should be created', () => {
        expect(testService).toBeTruthy();
    });

    it('should add two numbers', () => {
        const result = testService.add(25, 25);
        expect(result).toBe(50);
        // 额外验证spy是否被正确调用
        expect(mockHelperService.add).toHaveBeenCalledWith(25, 25);
    });
});

方法二：注入真实服务后对方法打spy

如果需要保留真实HelperService的部分逻辑（本案例中虽无必要，但方法具备通用性），可以先注入真实服务实例，再对目标方法打spy：

import { TestBed } from '@angular/core/testing';
import { HelperService } from './helper.service';
import { TestService } from './test.service';

describe('Test Service', () => {
    let testService: TestService;
    let helperService: HelperService;
    let addSpy: jest.SpyInstance;
    let subSpy: jest.SpyInstance;

    beforeEach(() => {
        TestBed.configureTestingModule({
            providers: [TestService, HelperService]
        });

        testService = TestBed.inject(TestService);
        helperService = TestBed.inject(HelperService);

        // 对真实服务实例的方法打spy
        addSpy = jest.spyOn(helperService, 'add').mockReturnValue(50);
        subSpy = jest.spyOn(helperService, 'subtract');
    });

    // 测试后恢复原始方法，避免影响其他测试用例
    afterEach(() => {
        addSpy.mockRestore();
        subSpy.mockRestore();
    });

    it('should be created', () => {
        expect(testService).toBeTruthy();
    });

    it('should add two numbers', () => {
        const result = testService.add(25, 25);
        expect(result).toBe(50);
        expect(addSpy).toHaveBeenCalledWith(25, 25);
    });
});

内容的提问来源于stack exchange，提问作者Anis83