Angular 5+ testing made easy with shallow rendering and easy mocking.
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karma-test-shim.ts feat: Basic mocking of services Feb 3, 2018


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Angular 5+ testing made easy with shallow rendering and easy mocking.

The Docs

The problem

Testing in Angular is HARD. TestBed is powerful but its use in component specs ends with lots of duplication.

Here's a standard TestBed spec for a component that uses a few other components, a directive and a pipe and handles click events:

describe('MyComponent', () => {
  beforeEach(async => {
    return TestBed.configureTestModule({
      imports: [
      declarations: [
        MyComponent, // <-- All I want to do is test this!!
        // We either must list all our dependencies here
        // -- OR --
        // Use NO_ERRORS_SCHEMA which allows any HTML to be used
        // even if it is invalid!
      providers: [
    }).compileComponents().then(() => {
      let myService = TestBed.get(MyService); // Not type safe
      spyOn(myService, 'foo').and.returnValue('mocked foo');

  it('renders a link with the provided label text', () => {
    const fixture = TestBed.createComponent(TestHostComponent);
    fixture.componentInstance.labelText = 'my text';
    const link = fixture.debugElement.query(By.css('a'));

    expect(a.nativeElement.innerText).toBe('my text');
  it('sends "foo" to bound click events', () => {
    const fixture = TestBed.createComponent(TestHostComponent);
    spyOn(fixture.componentInstance, 'handleClick');
    const myComponentElement = fixture.debugElement.query(By.directive(MyComponent));;

  template: `
    <my-component [linkText]="linkText" (click)="handleClick($event)">
class TestHostComponent {
  linkLabel: string;
  handleClick() {}

Whew!!! That was a lot of boilerplate. Here's just some of the issues:

  • Our TestBed module looks very similar if not identical to the NgModule I've probably already added MyComponent to. Total module duplication.
  • Since I've duplicated my module in my spec, I'm not actually sure the real module was setup correctly.
  • I've used REAL components and services in my spec which means I have not isolated the component I'm interested in testing.
    • This also means I have to follow, and provide all the dependencies of those real components to the TestBed module.
  • I had to create a TestHostComponent so I could pass bindings into my actual component.
  • My TestBed boilerplate code-length exceeded my actual test code-length.

The Solution

We should mock everything we can except for the component in test and that should be EASY. Our modules already define the environment in which our components live. They should be reused, not rebuilt in our specs.

Here's the same specs using shallow-render:

describe('MyComponent', () => {
  let shallow: Shallow<MyComponent>;
  beforeEach(() => {
    shallow = new Shallow(MyComponent, MyModule);

  it('renders a link with the provided label text', async () => {
    const {find} = await shallow.render(`<my-component linkText="my text"></my-component>`);

    expect(find('a').nativeElement.innerText).toBe('my text');

  it('sends "foo" to bound click events', async () => {
    const {element, bindings} = await shallow.render(
      `<my-component (click)="handleClick($event)"></my-component>`,
      { bind: { handleClick: () => {} } }


Here's the difference:

  • Reuses (and verifies) MyModule contains your component and all its dependencies.
  • All components inside MyModule are mocked using the awesome ng-mocks library. This is what makes the rendering "shallow".
  • The tests have much less boilerplate which makes the specs easier to follow.
  • The HTML used to render the component is IN THE SPEC and easy to find.
    • This means specs now double examples of how to use your component.