在使用 Jest 测试 Angular 应用程序时，如何避免常见的错误？

在前端开发过程中，测试是一个极其重要的环节，而 Jest 是一个非常好用的 JavaScript 测试框架。在使用 Jest 测试 Angular 应用程序时，经常会遇到一些常见的错误。本篇文章将详细介绍这些错误，并指导你如何避免它们。

常见的错误

TypeError: Cannot read property 'x' of undefined

在测试 Angular 组件时，我们通常需要引入 ComponentFixture 和 TestBed。如果在编写测试用例时，我们忘记在 beforeEach 函数中声明这些变量，就会出现 Cannot read property 'x' of undefined 的错误。

// javascriptcn.com code example
describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ],
      imports: [ ... ],
      providers: [ ... ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance; // 这里出现了错误
    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });
});

为避免这个错误，我们应该在 beforeEach 函数中正确声明变量。

// javascriptcn.com code example
describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ],
      imports: [ ... ],
      providers: [ ... ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });
});

Failed: Template parse errors

在测试 Angular 组件时，我们经常需要测试它们的模板。如果模板中存在语法错误，就会导致测试失败，并提示 Failed: Template parse errors 错误。

<app-my-component></app-my-component>

// javascriptcn.com code example
describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ],
      imports: [ ... ],
      providers: [ ... ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });

  it('should render title', () => {
    const element: HTMLElement = fixture.nativeElement;
    const title = element.querySelector('h1');
    expect(title.textContent).toContain('Hello World'); // 这里出现了错误
  });
});

为避免这个错误，我们应该在测试之前检查模板的语法，确保没有语法错误。

<app-my-component>
  <h1>Hello World</h1>
</app-my-component>

// javascriptcn.com code example
describe('MyComponent', () => {
  let component: MyComponent;
  let fixture: ComponentFixture<MyComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ],
      imports: [ ... ],
      providers: [ ... ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(MyComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });

  it('should render title', () => {
    const element: HTMLElement = fixture.nativeElement;
    const title = element.querySelector('h1');
    expect(title.textContent).toContain('Hello World');
  });
});

TypeError: Cannot read property 'subscribe' of undefined

在测试 Angular 服务时，我们通常会使用 HttpClient。如果在编写测试用例时，我们忘记在 beforeEach 函数中声明这个服务，就会出现 Cannot read property 'subscribe' of undefined 的错误。

// javascriptcn.com code example
describe('MyService', () => {
  let service: MyService;

  beforeEach(() => {
    TestBed.configureTestingModule({});
    service = TestBed.inject(MyService); // 这里出现了错误
  });

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

  it('should get data', () => {
    service.getData().subscribe(result => {
      expect(result).toBe('Hello World');
    }); // 这里还会出现错误
  });
});

为避免这个错误，我们应该在 beforeEach 函数中正确声明服务，并使用 HttpClientTestingModule 来模拟 HttpClient。

// javascriptcn.com code example
describe('MyService', () => {
  let service: MyService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [ HttpClientTestingModule ],
      providers: [ MyService ]
    });
    service = TestBed.inject(MyService);
    httpMock = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    httpMock.verify();
  });

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

  it('should get data', () => {
    const data = 'Hello World';
    const url = 'https://example.com/api/data';
    service.getData().subscribe(result => {
      expect(result).toBe(data);
    });
    const req = httpMock.expectOne(url);
    expect(req.request.method).toBe('GET');
    req.flush(data);
  });
});

结论

在使用 Jest 测试 Angular 应用程序时，我们应该避免常见的错误，确保测试用例能够正确运行。要注意在 beforeEach 函数中正确声明变量和服务，并检查模板的语法，确保没有语法错误。在测试服务时，应该使用 HttpClientTestingModule 来模拟 HttpClient。希望这篇文章能对你有所帮助！

来源：JavaScript中文网 ，转载请注明来源 https://www.javascriptcn.com/post/673473f30bc820c58249306b