Component testing scenarios

This guide explores common component testing use cases.

Component binding

In the example application, the Banner component presents static title text in the HTML template.

After a few changes, the Banner component presents a dynamic title by binding to the component's title property like this.

import {Component, signal} from '@angular/core';@Component({  selector: 'app-banner',  template: '<h1>{{ title() }}</h1>',  styles: ['h1 { color: green; font-size: 350%}'],})export class Banner {  title = signal('Test Tour of Heroes');}

As minimal as this is, you decide to add a test to confirm that component actually displays the right content where you think it should.

Query for the <h1>

You'll write a sequence of tests that inspect the value of the <h1> element that wraps the title property interpolation binding.

You update the beforeEach to find that element with a standard HTML querySelector and assign it to the h1 variable.

let component: Banner;let fixture: ComponentFixture<Banner>;let h1: HTMLElement;beforeEach(() => {  fixture = TestBed.createComponent(Banner);  component = fixture.componentInstance; // Banner test instance  h1 = fixture.nativeElement.querySelector('h1');});

createComponent() does not bind data

For your first test you'd like to see that the screen displays the default title. Your instinct is to write a test that immediately inspects the <h1> like this:

it('should display original title', () => {  expect(h1.textContent).toContain(component.title());});

That test fails with the message:

expected '' to contain 'Test Tour of Heroes'.

Binding happens when Angular performs change detection.

In production, change detection kicks in automatically when Angular creates a component or the user enters a keystroke, for example.

The TestBed.createComponent does not trigger change detection synchronously; a fact confirmed in the revised test:

it('no title in the DOM after createComponent()', () => {  expect(h1.textContent).toEqual('');});

whenStable()

You can tell the TestBed to wait for change detection to run with await fixture.whenStable(). Only then does the <h1> have the expected title.

it('should display original title', async () => {  await fixture.whenStable();  expect(h1.textContent).toContain(component.title());});

Delayed change detection is intentional and useful. It gives the tester an opportunity to inspect and change the state of the component before Angular initiates data binding and calls lifecycle hooks.

Here's another test that changes the component's title property before calling fixture.whenStable().

it('should display a different test title', async () => {  component.title.set('Test Title');  await fixture.whenStable();  expect(h1.textContent).toContain('Test Title');});

Binding signals to inputs

To reflect changes to inputs and listen to outputs you can dynamically bind signals to inputs and functions to outputs.

import {inputBinding, outputBinding} from '@angular/core';const fixture = TestBed.createComponent(ValueDisplay, {  bindings: [    inputBinding('value', value),    outputBinding('valueChange', () =>  (/* ... */) ),  ],});

Change an input value with dispatchEvent()

To simulate user input, find the input element and set its value property.

But there is an essential, intermediate step.

Angular doesn't know that you set the input element's value property. It won't read that property until you raise the element's input event by calling dispatchEvent().

The following example of a component using the TitleCasePipe demonstrates the proper sequence.

it('should convert hero name to Title Case', async () => {  const hostElement = fixture.nativeElement;  const nameInput: HTMLInputElement = hostElement.querySelector('input')!;  const nameDisplay: HTMLElement = hostElement.querySelector('span')!;  // simulate user entering a new name into the input box  nameInput.value = 'quick BROWN  fOx';  // Dispatch a DOM event so that Angular learns of input value change.  nameInput.dispatchEvent(new Event('input'));  // Wait for Angular to update the display binding through the title pipe  await fixture.whenStable();  expect(nameDisplay.textContent).toBe('Quick Brown  Fox');});

Component with a dependency

Components often have service dependencies.

The Welcome component displays a welcome message to the logged-in user. It knows who the user is based on a property of the injected UserAuthentication:

import {Component, inject, OnInit, signal} from '@angular/core';import {UserAuthentication} from '../model/user.authentication';@Component({  selector: 'app-welcome',  template: '<h3 class="welcome"><i>{{ welcome() }}</i></h3>',})export class Welcome {  private userAuth = inject(UserAuthentication);  welcome = signal(    this.userAuth.isLoggedIn() ? `Welcome, ${this.userAuth.user().name}` : 'Please log in.',  );}

The Welcome component has decision logic that interacts with the service, logic that makes this component worth testing.

Provide service test doubles

A component-under-test doesn't have to be provided with real services.

Injecting the real UserAuthentication could be difficult. The real service might ask the user for login credentials and attempt to reach an authentication server. These behaviors can be hard to intercept. Be aware that using test doubles makes the test behave differently from production so use them sparingly.

Get injected services

The tests need access to the UserAuthentication injected into the Welcome component.

Angular has a hierarchical injection system. There can be injectors at multiple levels, from the root injector created by the TestBed down through the component tree.

The safest way to get the injected service, the way that always works, is to get it from the injector of the component-under-test. The component injector is a property of the fixture's DebugElement.

// UserAuthentication actually injected into the componentuserAuth = fixture.debugElement.injector.get(UserAuthentication);

TestBed.inject()

This is easier to remember and less verbose than retrieving a service using the fixture's DebugElement.

In this test suite, the only provider of UserAuthentication is the root testing module, so it is safe to call TestBed.inject() as follows:

userAuth = TestBed.inject(UserAuthentication);

Final setup and tests

Here's the complete beforeEach(), using TestBed.inject():

let fixture: ComponentFixture<Welcome>;let comp: Welcome;let userAuth: UserAuthentication; // the TestBed injected servicelet el: HTMLElement; // the DOM element with the welcome messagebeforeEach(() => {  fixture = TestBed.createComponent(Welcome);  comp = fixture.componentInstance;  // UserAuthentication from the root injector  userAuth = TestBed.inject(UserAuthentication);  //  get the "welcome" element by CSS selector (e.g., by class name)  el = fixture.nativeElement.querySelector('.welcome');});

And here are some tests:

it('should welcome the user', async () => {  await fixture.whenStable();  const content = el.textContent;  expect(content, '"Welcome ..."').toContain('Welcome');  expect(content, 'expected name').toContain('Test User');});it('should welcome "Bubba"', async () => {  userAuth.user.set({name: 'Bubba'}); // welcome message hasn't been shown yet  await fixture.whenStable();  expect(el.textContent).toContain('Bubba');});it('should request login if not logged in', async () => {  userAuth.isLoggedIn.set(false); // welcome message hasn't been shown yet  await fixture.whenStable();  const content = el.textContent;  expect(content, 'not welcomed').not.toContain('Welcome');  expect(content, '"log in"').toMatch(/log in/i);});

The first is a sanity test; it confirms that the UserAuthentication is called and working.

The remaining tests confirm the logic of the component when the service returns different values. The second test validates the effect of changing the user name. The third test checks that the component displays the proper message when there is no logged-in user.

Component with async service

In this sample, the About component template hosts a Twain component. The Twain component displays Mark Twain quotes.

<p class="twain">  <i>{{ quote | async }}</i></p><button type="button" (click)="getQuote()">Next quote</button>@if (errorMessage()) {  <p class="error">{{ errorMessage() }}</p>}

In this example, the TwainQuotes.getQuote() method tells you that the quote property returns an Observable.

getQuote() {  this.errorMessage.set('');  this.quote = this.twainQuotes.getQuote().pipe(    startWith('...'),    catchError((err: any) => {      this.errorMessage.set(err.message || err.toString());      return of('...'); // reset message to placeholder    }),  );}

The Twain component gets quotes from an injected TwainQuotes. The component starts the returned Observable with a placeholder value ('...'), before the service can return its first quote.

The catchError intercepts service errors, prepares an error message, and returns the placeholder value on the success channel.

These are all features you'll want to test.

Testing by mocking http requests with the HttpTestingController.

When testing a component, only the service's public API should matter. In general, tests themselves should not make calls to remote servers. They should emulate such calls.

In the case your async service relies on the HttpClient to load remote data, it is recommended to return mock responses at the HTTP level with the HttpTestingController.

For more details on mocking the HttpBackend, refer to the dedicated guide.

Testing by providing a stubbed implementation of a service.

When mocking async request at the http level isn't possible, an alternative is to leverage spies.

The setup in this app/twain/twain-quotes.spec.ts shows one way to do that:

class TwainQuotesStub implements TwainQuotes {  private testQuote = 'Test Quote';  getQuote() {    return of(this.testQuote);  }  // ... Implement everything to conform to the API}beforeEach(async () => {  TestBed.configureTestingModule({    providers: [{provide: TwainQuotes, useClass: TwainQuotesStub}],  });  fixture = TestBed.createComponent(Twain);  component = fixture.componentInstance;  await fixture.whenStable();  quoteEl = fixture.nativeElement.querySelector('.twain');});

Focus on the how the stub implementation replaces the original one.

TestBed.configureTestingModule({  providers: [{provide: TwainQuotes, useClass: TwainQuotesStub}],});

The stub is designed in such a way that any component or service that injects it will receive the stubbed implementation. It means that any call to getQuote receives an observable with a test quote.

Unlike the real getQuote() method, this spy bypasses the server and returns a synchronous observable whose value is available immediately.

Async test with a Vitest fake timers

To mock async functions like setTimeout or Promises, you can leverage Vitest fake timers to controle whenever the fire.

it('should display error when TwainQuotes service fails', async () => {  class TwainQuotesStub implements TwainQuotes {    getQuote() {      return defer(() => {        return new Promise<string>((_, reject) => {          setTimeout(() => reject('TwainService test failure'));        });      });    }    // ... Implement everything to conform to the API  }  TestBed.configureTestingModule({    providers: [{provide: TwainQuotes, useClass: TwainQuotesStub}],  });  vi.useFakeTimers(); // setting up the fake timers  const fixture = TestBed.createComponent(TwainComponent);  // rendering isn't async, we need to flush  await vi.runAllTimersAsync();  await expect(fixture.nativeElement.querySelector('.error')!.textContent).toMatch(/test failure/);  expect(fixture.nativeElement.querySelector('.twain')!.textContent).toBe('...');  vi.useRealTimers(); // resets to regular async execution});

More async tests

With the stubbe service returning async observables, most of your tests will have to be async as well.

Here's a test that demonstrates the data flow you'd expect in the real world.

it('should show quote after getQuote', async () => {  class MockTwainQuotes implements TwainQuotes {    private subject = new Subject<string>();    getQuote() {      return this.subject.asObservable();    }    emit(val: string) {      this.subject.next(val);    }  }  it('should show quote after getQuote (success)', async () => {    vi.useFakeTimers();    TestBed.configureTestingModule({      providers: [{provide: TwainQuotes, useClass: MockTwainQuotes}],    });    const fixture = TestBed.createComponent(TwainComponent);    const twainQuotes = TestBed.inject(TwainQuotes) as MockTwainQuotes;    await vi.runAllTimersAsync(); // render before the quote is recieved    const quoteEl = fixture.nativeElement.querySelector('.twain');    expect(quoteEl.textContent).toBe('...');    twainQuotes.emit('Twain Quote'); // emits the quote    await vi.runAllTimersAsync(); // render with the quote received    expect(quoteEl.textContent).toBe('Twain Quote');    expect(fixture.nativeElement.querySelector('.error')).toBeNull();    vi.useRealTimers();  });});

Notice that the quote element displays the placeholder value ('...') on first rendering. The first quote hasn't arrived yet.

Then you can assert that the quote element displays the expected text.

Async tests with zone.js and fakeAsync

The fakeAsync helper function is another mock clock that relies on patching asynchronous APIs with zone.js. It was commonly used in zone.js based applications for testing. The use of fakeAsync is no longer recommended.

Component with inputs and outputs

A component with inputs and outputs typically appears inside the view template of a host component. The host uses a property binding to set the input property and an event binding to listen to events raised by the output property.

The testing goal is to verify that such bindings work as expected. The tests should set input values and listen for output events.

The DashboardHero component is a tiny example of a component in this role. It displays an individual hero provided by the Dashboard component. Clicking that hero tells the Dashboard component that the user has selected the hero.

The DashboardHero component is embedded in the Dashboard component template like this:

@for (hero of heroes; track hero) {  <dashboard-hero class="col-1-4" [hero]="hero" (selected)="gotoDetail($event)"/>}

The DashboardHero component appears in an @for block, which sets each component's hero input property to the looping value and listens for the component's selected event.

Here's the component's full definition:

@Component({  selector: 'dashboard-hero',  imports: [UpperCasePipe],  template: `    <button type="button" (click)="click()" class="hero">      {{ hero().name | uppercase }}    </button>  `,})export class DashboardHero {  readonly hero = input.required<Hero>();  readonly selected = output<Hero>();  click() {    this.selected.emit(this.hero());  }}

While testing a component this simple has little intrinsic value, it's worth knowing how. Use one of these approaches:

• Test it as used by the Dashboard component
• Test it as a standalone component
• Test it as used by a substitute for the Dashboard component

The immediate goal is to test the DashboardHero component, not the Dashboard component, so, try the second and third options.

Test the DashboardHero component standalone

Here's the meat of the spec file setup.

let fixture: ComponentFixture<DashboardHero>;let comp: DashboardHero;let heroDe: DebugElement;let heroEl: HTMLElement;let expectedHero: Hero;beforeEach(async () => {  fixture = TestBed.createComponent(DashboardHero);  comp = fixture.componentInstance;  // find the hero's DebugElement and element  heroDe = fixture.debugElement.query(By.css('.hero'));  heroEl = heroDe.nativeElement;  // mock the hero supplied by the parent component  expectedHero = {id: 42, name: 'Test Name'};  // simulate the parent setting the input property with that hero  fixture.componentRef.setInput('hero', expectedHero);  // wait for initial data binding  await fixture.whenStable();});

Notice how the setup code assigns a test hero (expectedHero) to the component's hero property, emulating the way the Dashboard would set it using the property binding in its repeater.

The following test verifies that the hero name is propagated to the template using a binding.

it('should display hero name in uppercase', () => {  const expectedPipedName = expectedHero.name.toUpperCase();  expect(heroEl.textContent).toContain(expectedPipedName);});

Because the template passes the hero name through the Angular UpperCasePipe, the test must match the element value with the upper-cased name.

Clicking

Clicking the hero should raise a selected event that the host component (Dashboard presumably) can hear:

it('should raise selected event when clicked (triggerEventHandler)', () => {  let selectedHero: Hero | undefined;  comp.selected.subscribe((hero: Hero) => (selectedHero = hero));  heroDe.triggerEventHandler('click');  expect(selectedHero).toBe(expectedHero);});

The component's selected property returns an EventEmitter, which looks like an RxJS synchronous Observable to consumers. The test subscribes to it explicitly just as the host component does implicitly.

If the component behaves as expected, clicking the hero's element should tell the component's selected property to emit the hero object.

The test detects that event through its subscription to selected.

triggerEventHandler

The heroDe in the previous test is a DebugElement that represents the hero <div>.

It has Angular properties and methods that abstract interaction with the native element. This test calls the DebugElement.triggerEventHandler with the "click" event name. The "click" event binding responds by calling DashboardHero.click().

The Angular DebugElement.triggerEventHandler can raise any data-bound event by its event name. The second parameter is the event object passed to the handler.

The test triggered a "click" event.

heroDe.triggerEventHandler('click');

In this case, the test correctly assumes that the runtime event handler, the component's click() method, doesn't care about the event object.

Click the element

The following test alternative calls the native element's own click() method, which is perfectly fine for this component.

it('should raise selected event when clicked (element.click)', () => {  let selectedHero: Hero | undefined;  comp.selected.subscribe((hero: Hero) => (selectedHero = hero));  heroEl.click();  expect(selectedHero).toBe(expectedHero);});

click() helper

Clicking a button, an anchor, or an arbitrary HTML element is a common test task.

Make that consistent and straightforward by encapsulating the click-triggering process in a helper such as the following click() function:

/** Button events to pass to `DebugElement.triggerEventHandler` for RouterLink event handler */export const ButtonClickEvents = {  left: {button: 0},  right: {button: 2},};/** Simulate element click. Defaults to mouse left-button click event. */export function click(  el: DebugElement | HTMLElement,  eventObj: any = ButtonClickEvents.left,): void {  if (el instanceof HTMLElement) {    el.click();  } else {    el.triggerEventHandler('click', eventObj);  }}

The first parameter is the element-to-click. If you want, pass a custom event object as the second parameter. The default is a partial left-button mouse event object accepted by many handlers including the RouterLink directive.

Here's the previous test, rewritten using the click helper.

it('should raise selected event when clicked (click helper with DebugElement)', () => {  let selectedHero: Hero | undefined;  comp.selected.subscribe((hero: Hero) => (selectedHero = hero));  click(heroDe); // click helper with DebugElement  expect(selectedHero).toBe(expectedHero);});

Component inside a test host

The previous tests played the role of the host Dashboard component themselves. But does the DashboardHero component work correctly when properly data-bound to a host component?

@Component({  imports: [DashboardHero],  template: ` <dashboard-hero [hero]="hero" (selected)="onSelected($event)" />`,})class TestHost {  hero: Hero = {id: 42, name: 'Test Name'};  selectedHero: Hero | undefined;  onSelected(hero: Hero) {    this.selectedHero = hero;  }}

The test host sets the component's hero input property with its test hero. It binds the component's selected event with its onSelected handler, which records the emitted hero in its selectedHero property.

Later, the tests will be able to check selectedHero to verify that the DashboardHero.selected event emitted the expected hero.

The setup for the test-host tests is similar to the setup for the stand-alone tests:

beforeEach(async () => {  // create TestHost instead of DashboardHero  fixture = TestBed.createComponent(TestHost);  testHost = fixture.componentInstance;  heroEl = fixture.nativeElement.querySelector('.hero');  await fixture.whenStable();});

This testing module configuration shows two important differences:

• It creates the TestHost component instead of the DashboardHero
• The TestHost component sets the DashboardHero.hero with a binding

The createComponent returns a fixture that holds an instance of TestHost instead of an instance of DashboardHero.

Creating the TestHost has the side effect of creating a DashboardHero because the latter appears within the template of the former. The query for the hero element (heroEl) still finds it in the test DOM, albeit at greater depth in the element tree than before.

The tests themselves are almost identical to the stand-alone version:

it('should display hero name', () => {  const expectedPipedName = testHost.hero.name.toUpperCase();  expect(heroEl.textContent).toContain(expectedPipedName);});it('should raise selected event when clicked', () => {  click(heroEl);  // selected hero should be the same data bound hero  expect(testHost.selectedHero).toBe(testHost.hero);});

Only the selected event test differs. It confirms that the selected DashboardHero hero really does find its way up through the event binding to the host component.

Routing component

A routing component is a component that tells the Router to navigate to another component. The Dashboard component is a routing component because the user can navigate to the HeroDetail component by clicking on one of the hero buttons on the dashboard.

Angular provides test helpers to reduce boilerplate and more effectively test code which depends on HttpClient. The provideRouter function can be used directly in the test module as well.

beforeEach(async () => {  TestBed.configureTestingModule({    providers: [      provideRouter([{path: '**', component: Dashboard}]),      provideHttpClientTesting(),      HeroService,    ],  });  harness = await RouterTestingHarness.create();  comp = await harness.navigateByUrl('/', Dashboard);  TestBed.inject(HttpTestingController).expectOne('api/heroes').flush(getTestHeroes());});

The following test clicks the displayed hero and confirms that we navigate to the expected URL.

it('should tell navigate when hero clicked', async () => {  // get first <dashboard-hero> DebugElement  const heroDe = harness.routeDebugElement!.query(By.css('dashboard-hero'));  heroDe.triggerEventHandler('selected', comp.heroes[0]);  // expecting to navigate to id of the component's first hero  const id = comp.heroes[0].id;  expect(TestBed.inject(Router).url, 'should nav to HeroDetail for first hero').toEqual(    `/heroes/${id}`,  );});

Routed components

A routed component is the destination of a Router navigation. It can be trickier to test, especially when the route to the component includes parameters. The HeroDetail is a routed component that is the destination of such a route.

When a user clicks a Dashboard hero, the Dashboard tells the Router to navigate to heroes/:id. The :id is a route parameter whose value is the id of the hero to edit.

The Router matches that URL to a route to the HeroDetail. It creates an ActivatedRoute object with the routing information and injects it into a new instance of the HeroDetail.

Here are the services injected into HeroDetail:

private heroDetailService = inject(HeroDetailService);private route = inject(ActivatedRoute);private router = inject(Router);

The HeroDetail component needs the id parameter so it can fetch the corresponding hero using the HeroDetailService. The component has to get the id from the ActivatedRoute.paramMap property which is an Observable.

It can't just reference the id property of the ActivatedRoute.paramMap. The component has to subscribe to the ActivatedRoute.paramMap observable and be prepared for the id to change during its lifetime.

constructor() {  // get hero when `id` param changes  this.route.paramMap    .pipe(takeUntilDestroyed())    .subscribe((pmap) => this.getHero(pmap.get('id')));}

Tests can explore how the HeroDetail responds to different id parameter values by navigating to different routes.

Nested component tests

Component templates often have nested components, whose templates might contain more components.

The component tree can be very deep and sometimes the nested components play no role in testing the component at the top of the tree.

The App component, for example, displays a navigation bar with anchors and their RouterLink directives.

<app-banner /><app-welcome /><nav>  <a routerLink="/dashboard">Dashboard</a>  <a routerLink="/heroes">Heroes</a>  <a routerLink="/about">About</a></nav><router-outlet />

To validate the links but not the navigation, you don't need the Router to navigate and you don't need the <router-outlet> to mark where the Router inserts routed components.

The Banner and Welcome components (indicated by <app-banner> and <app-welcome>) are also irrelevant.

Yet any test that creates the App component in the DOM also creates instances of these three components and, if you let that happen, you'll have to configure the TestBed to create them.

If you neglect to declare them, the Angular compiler won't recognize the <app-banner>, <app-welcome>, and <router-outlet> tags in the App template and will throw an error.

If you declare the real components, you'll also have to declare their nested components and provide for all services injected in any component in the tree.

This section describes two techniques for minimizing the setup. Use them, alone or in combination, to stay focused on testing the primary component.

Stubbing unneeded components

In the first technique, you create and declare stub versions of the components and directive that play little or no role in the tests.

@Component({selector: 'app-banner', template: ''})class BannerStub {}@Component({selector: 'router-outlet', template: ''})class RouterOutletStub {}@Component({selector: 'app-welcome', template: ''})class WelcomeStub {}

The stub selectors match the selectors for the corresponding real components. But their templates and classes are empty.

Then declare them by overriding the imports of your component using TestBed.overrideComponent.

let comp: App;let fixture: ComponentFixture<App>;beforeEach(() => {  TestBed.configureTestingModule({    providers: [provideRouter([]), UserAuthentication],  }).overrideComponent(App, {    set: {      imports: [RouterLink, BannerStub, RouterOutletStub, WelcomeStub],    },  });  fixture = TestBed.createComponent(App);  comp = fixture.componentInstance;});

NO_ERRORS_SCHEMA

In the second approach, add NO_ERRORS_SCHEMA to the metadata overrides of your component.

beforeEach(() => {  TestBed.configureTestingModule({    providers: [provideRouter([]), UserAuthentication],  }).overrideComponent(App, {    set: {      imports: [], // resets all imports      schemas: [NO_ERRORS_SCHEMA],    },  });});

The NO_ERRORS_SCHEMA tells the Angular compiler to ignore unrecognized elements and attributes.

The compiler recognizes the <app-root> element and the routerLink attribute because you declared a corresponding App component and RouterLink in the TestBed configuration.

But the compiler won't throw an error when it encounters <app-banner>, <app-welcome>, or <router-outlet>. It simply renders them as empty tags and the browser ignores them.

You no longer need the stub components.

Use both techniques together

These are techniques for Shallow Component Testing, so-named because they reduce the visual surface of the component to just those elements in the component's template that matter for tests.

The NO_ERRORS_SCHEMA approach is the easier of the two but don't overuse it.

The NO_ERRORS_SCHEMA also prevents the compiler from telling you about the missing components and attributes that you omitted inadvertently or misspelled. You could waste hours chasing phantom bugs that the compiler would have caught in an instant.

The stub component approach has another advantage. While the stubs in this example were empty, you could give them stripped-down templates and classes if your tests need to interact with them in some way.

In practice you will combine the two techniques in the same setup, as seen in this example.

beforeEach(() => {  TestBed.configureTestingModule({    providers: [provideRouter([]), UserAuthentication],  }).overrideComponent(App, {    remove: {imports: [RouterOutlet, Welcome]},    set: {schemas: [NO_ERRORS_SCHEMA]},  });});

The Angular compiler creates the BannerStub for the <app-banner> element and applies the RouterLink to the anchors with the routerLink attribute, but it ignores the <app-welcome> and <router-outlet> tags.

By.directive and injected directives

A little more setup triggers the initial data binding and gets references to the navigation links:

beforeEach(async () => {  await fixture.whenStable();  // find DebugElements with an attached RouterLinkStubDirective  linkDes = fixture.debugElement.queryAll(By.directive(RouterLink));  // get attached link directive instances  // using each DebugElement's injector  routerLinks = linkDes.map((de) => de.injector.get(RouterLink));});

Three points of special interest:

• Locate the anchor elements with an attached directive using By.directive
• The query returns DebugElement wrappers around the matching elements
• Each DebugElement exposes a dependency injector with the specific instance of the directive attached to that element

The App component links to validate are as follows:

<nav>  <a routerLink="/dashboard">Dashboard</a>  <a routerLink="/heroes">Heroes</a>  <a routerLink="/about">About</a></nav>

Here are some tests that confirm those links are wired to the routerLink directives as expected:

it('can get RouterLinks from template', () => {  expect(routerLinks.length, 'should have 3 routerLinks').toBe(3);  expect(routerLinks[0].href).toBe('/dashboard');  expect(routerLinks[1].href).toBe('/heroes');  expect(routerLinks[2].href).toBe('/about');});it('can click Heroes link in template', async () => {  const heroesLinkDe = linkDes[1]; // heroes link DebugElement  TestBed.inject(Router).resetConfig([{path: '**', children: []}]);  heroesLinkDe.triggerEventHandler('click', {button: 0});  await fixture.whenStable();  expect(TestBed.inject(Router).url).toBe('/heroes');});

Use a page object

The HeroDetail component is a simple view with a title, two hero fields, and two buttons.

But there's plenty of template complexity even in this simple form.

@if (hero) {  <div>    <h2>      <span>{{ hero.name | titlecase }}</span> Details    </h2>    <div><span>id: </span>{{ hero.id }}</div>    <div>      <label for="name">name: </label>      <input id="name" [(ngModel)]="hero.name" placeholder="name" />    </div>    <button type="button" (click)="save()">Save</button>    <button type="button" (click)="cancel()">Cancel</button>  </div>}

Tests that exercise the component need …

• To wait until a hero arrives before elements appear in the DOM
• A reference to the title text
• A reference to the name input box to inspect and set it
• References to the two buttons so they can click them

Even a small form such as this one can produce a mess of tortured conditional setup and CSS element selection.

Tame the complexity with a Page class that handles access to component properties and encapsulates the logic that sets them.

Here is such a Page class for the hero-detail.component.spec.ts

class Page {  // getter properties wait to query the DOM until called.  get buttons() {    return this.queryAll<HTMLButtonElement>('button');  }  get saveBtn() {    return this.buttons[0];  }  get cancelBtn() {    return this.buttons[1];  }  get nameDisplay() {    return this.query<HTMLElement>('span');  }  get nameInput() {    return this.query<HTMLInputElement>('input');  }  //// query helpers ////  private query<T>(selector: string): T {    return harness.routeNativeElement!.querySelector(selector)! as T;  }  private queryAll<T>(selector: string): T[] {    return harness.routeNativeElement!.querySelectorAll(selector) as any as T[];  }}

Now the important hooks for component manipulation and inspection are neatly organized and accessible from an instance of Page.

A createComponent method creates a page object and fills in the blanks once the hero arrives.

async function createComponent(id: number) {  harness = await RouterTestingHarness.create();  component = await harness.navigateByUrl(`/heroes/${id}`, HeroDetail);  page = new Page();  const request = TestBed.inject(HttpTestingController).expectOne(`api/heroes/?id=${id}`);  const hero = getTestHeroes().find((h) => h.id === Number(id));  request.flush(hero ? [hero] : []);  await harness.fixture.whenStable();}

Here are a few more HeroDetail component tests to reinforce the point.

it("should display that hero's name", () => {  expect(page.nameDisplay.textContent).toBe(expectedHero.name);});it('should navigate when click cancel', () => {  click(page.cancelBtn);  expect(TestBed.inject(Router).url).toEqual(`/heroes/${expectedHero.id}`);});it('should save when click save but not navigate immediately', () => {  click(page.saveBtn);  expect(TestBed.inject(HttpTestingController).expectOne({method: 'PUT', url: 'api/heroes'}));  expect(TestBed.inject(Router).url).toEqual('/heroes/41');});it('should navigate when click save and save resolves', async () => {  click(page.saveBtn);  await harness.fixture.whenStable();  expect(TestBed.inject(Router).url).toEqual('/heroes/41');});it('should convert hero name to Title Case', async () => {  // get the name's input and display elements from the DOM  const hostElement: HTMLElement = harness.routeNativeElement!;  const nameInput: HTMLInputElement = hostElement.querySelector('input')!;  const nameDisplay: HTMLElement = hostElement.querySelector('span')!;  // simulate user entering a new name into the input box  nameInput.value = 'quick BROWN  fOx';  // Dispatch a DOM event so that Angular learns of input value change.  nameInput.dispatchEvent(new Event('input'));  // Wait for Angular to update the display binding through the title pipe  await harness.fixture.whenStable();  expect(nameDisplay.textContent).toBe('Quick Brown  Fox');});

Override component providers

The HeroDetail provides its own HeroDetailService.

@Component({  /* ... */  providers: [HeroDetailService],})export class HeroDetail {  private heroDetailService = inject(HeroDetailService);  private route = inject(ActivatedRoute);  private router = inject(Router);}

It's not possible to stub the component's HeroDetailService in the providers of the TestBed.configureTestingModule. Those are providers for the testing module, not the component. They prepare the dependency injector at the fixture level.

Angular creates the component with its own injector, which is a child of the fixture injector. It registers the component's providers (the HeroDetailService in this case) with the child injector.

A test cannot get to child injector services from the fixture injector. And TestBed.configureTestingModule can't configure them either.

Angular has created new instances of the real HeroDetailService all along!

Fortunately, the HeroDetailService delegates responsibility for remote data access to an injected HeroService.

@Injectable({providedIn: 'root'})export class HeroDetailService {  private heroService = inject(HeroService);}

The previous test configuration replaces the real HeroService with a TestHeroService that intercepts server requests and fakes their responses.

What if you aren't so lucky. What if faking the HeroService is hard? What if HeroDetailService makes its own server requests?

The TestBed.overrideComponent method can replace the component's providers with easy-to-manage test doubles as seen in the following setup variation:

beforeEach(async () => {  await TestBed.configureTestingModule({    providers: [      provideRouter([        {path: 'heroes', component: HeroList},        {path: 'heroes/:id', component: HeroDetail},      ]),      // HeroDetailService at this level is IRRELEVANT!      {provide: HeroDetailService, useValue: {}},    ],  }).overrideComponent(HeroDetail, {    set: {providers: [{provide: HeroDetailService, useClass: HeroDetailServiceSpy}]},  });});

Notice that TestBed.configureTestingModule no longer provides a fake HeroService because it's not needed.

The overrideComponent method

Focus on the overrideComponent method.

.overrideComponent(HeroDetail, {  set: {providers: [{provide: HeroDetailService, useClass: HeroDetailServiceSpy}]},});

It takes two arguments: the component type to override (HeroDetail) and an override metadata object. The override metadata object is a generic defined as follows:

type MetadataOverride<T> = {  add?: Partial<T>;  remove?: Partial<T>;  set?: Partial<T>;};

A metadata override object can either add-and-remove elements in metadata properties or completely reset those properties. This example resets the component's providers metadata.

The type parameter, T, is the kind of metadata you'd pass to the @Component decorator:

selector?: string;template?: string;templateUrl?: string;providers?: any[];…

Provide a spy stub (HeroDetailServiceSpy)

This example completely replaces the component's providers array with a new array containing a HeroDetailServiceSpy.

The HeroDetailServiceSpy is a stubbed version of the real HeroDetailService that fakes all necessary features of that service. It neither injects nor delegates to the lower level HeroService so there's no need to provide a test double for that.

The related HeroDetail component tests will assert that methods of the HeroDetailService were called by spying on the service methods. Accordingly, the stub implements its methods as spies:

import {vi} from 'vitest';class HeroDetailServiceSpy {  testHero: Hero = {...testHero};  /* emit cloned test hero */  getHero = vi.fn(() => asyncData({...this.testHero}));  /* emit clone of test hero, with changes merged in */  saveHero = vi.fn((hero: Hero) => asyncData(Object.assign(this.testHero, hero)));}

The override tests

Now the tests can control the component's hero directly by manipulating the spy-stub's testHero and confirm that service methods were called.

let hdsSpy: HeroDetailServiceSpy;beforeEach(async () => {  harness = await RouterTestingHarness.create();  component = await harness.navigateByUrl(`/heroes/${testHero.id}`, HeroDetail);  page = new Page();  // get the component's injected HeroDetailServiceSpy  hdsSpy = harness.routeDebugElement!.injector.get(HeroDetailService) as any;  harness.detectChanges();});it('should have called `getHero`', () => {  expect(hdsSpy.getHero, 'getHero called once').toHaveBeenCalledTimes(1);});it("should display stub hero's name", () => {  expect(page.nameDisplay.textContent).toBe(hdsSpy.testHero.name);});it('should save stub hero change', async () => {  const origName = hdsSpy.testHero.name;  const newName = 'New Name';  page.nameInput.value = newName;  page.nameInput.dispatchEvent(new Event('input')); // tell Angular  expect(component.hero.name, 'component hero has new name').toBe(newName);  expect(hdsSpy.testHero.name, 'service hero unchanged before save').toBe(origName);  click(page.saveBtn);  expect(hdsSpy.saveHero, 'saveHero called once').toHaveBeenCalledTimes(1);  await harness.fixture.whenStable();  expect(hdsSpy.testHero.name, 'service hero has new name after save').toBe(newName);  expect(TestBed.inject(Router).url).toEqual('/heroes');});

More overrides

The TestBed.overrideComponent method can be called multiple times for the same or different components. The TestBed offers similar overrideDirective, overrideModule, and overridePipe methods for digging into and replacing parts of these other classes.

Explore the options and combinations on your own.