Basics of testing components

A component, unlike all other parts of an Angular application, combines an HTML template and a TypeScript class. The component truly is the template and the class working together. To adequately test a component, you should test that they work together as intended.

Such tests require creating the component's host element in the browser DOM, as Angular does, and investigating the component class's interaction with the DOM as described by its template.

The Angular TestBed facilitates this kind of testing as you'll see in the following sections. But in many cases, testing the component class alone, without DOM involvement, can validate much of the component's behavior in a straightforward, more obvious way.

Component DOM testing

A component is more than just its class. A component interacts with the DOM and with other components. Classes alone cannot tell you if the component is going to render properly, respond to user input and gestures, or integrate with its parent and child components.

Is Lightswitch.clicked() bound to anything such that the user can invoke it?
Is the Lightswitch.message displayed?
Can the user actually select the hero displayed by DashboardHeroComponent?
Is the hero name displayed as expected (such as uppercase)?
Is the welcome message displayed by the template of WelcomeComponent?

These might not be troubling questions for the preceding simple components illustrated. But many components have complex interactions with the DOM elements described in their templates, causing HTML to appear and disappear as the component state changes.

To answer these kinds of questions, you have to create the DOM elements associated with the components, you must examine the DOM to confirm that component state displays properly at the appropriate times, and you must simulate user interaction with the screen to determine whether those interactions cause the component to behave as expected.

To write these kinds of test, you'll use additional features of the TestBed as well as other testing helpers.

CLI-generated tests

The CLI creates an initial test file for you by default when you ask it to generate a new component.

For example, the following CLI command generates a BannerComponent in the app/banner folder (with inline template and styles):

      ng generate component banner --inline-template --inline-style --module app

It also generates an initial test file for the component, banner-external.component.spec.ts, that looks like this:

app/banner/banner-external.component.spec.ts (initial)

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

HELPFUL: Because compileComponents is asynchronous, it uses the waitForAsync utility function imported from @angular/core/testing.

Refer to the waitForAsync section for more details.

Reduce the setup

Only the last three lines of this file actually test the component and all they do is assert that Angular can create the component.

The rest of the file is boilerplate setup code anticipating more advanced tests that might become necessary if the component evolves into something substantial.

You'll learn about these advanced test features in the following sections. For now, you can radically reduce this test file to a more manageable size:

app/banner/banner-initial.component.spec.ts (minimal)

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

In this example, the metadata object passed to TestBed.configureTestingModule simply declares BannerComponent, the component to test.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

HELPFUL: There's no need to declare or import anything else. The default test module is pre-configured with something like the BrowserModule from @angular/platform-browser.

Later you'll call TestBed.configureTestingModule() with imports, providers, and more declarations to suit your testing needs. Optional override methods can further fine-tune aspects of the configuration.

`createComponent()`

After configuring TestBed, you call its createComponent() method.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

TestBed.createComponent() creates an instance of the BannerComponent, adds a corresponding element to the test-runner DOM, and returns a ComponentFixture.

IMPORTANT: Do not re-configure TestBed after calling createComponent.

The createComponent method freezes the current TestBed definition, closing it to further configuration.

You cannot call any more TestBed configuration methods, not configureTestingModule(), nor get(), nor any of the override... methods. If you try, TestBed throws an error.

`ComponentFixture`

The ComponentFixture is a test harness for interacting with the created component and its corresponding element.

Access the component instance through the fixture and confirm it exists with a Jasmine expectation:

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

`beforeEach()`

You will add more tests as this component evolves. Rather than duplicate the TestBed configuration for each test, you refactor to pull the setup into a Jasmine beforeEach() and some supporting variables:

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

Now add a test that gets the component's element from fixture.nativeElement and looks for the expected text.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

`nativeElement`

The value of ComponentFixture.nativeElement has the any type. Later you'll encounter the DebugElement.nativeElement and it too has the any type.

Angular can't know at compile time what kind of HTML element the nativeElement is or if it even is an HTML element. The application might be running on a non-browser platform, such as the server or a Web Worker, where the element might have a diminished API or not exist at all.

The tests in this guide are designed to run in a browser so a nativeElement value will always be an HTMLElement or one of its derived classes.

Knowing that it is an HTMLElement of some sort, use the standard HTML querySelector to dive deeper into the element tree.

Here's another test that calls HTMLElement.querySelector to get the paragraph element and look for the banner text:

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

`DebugElement`

The Angular fixture provides the component's element directly through the fixture.nativeElement.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

This is actually a convenience method, implemented as fixture.debugElement.nativeElement.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

There's a good reason for this circuitous path to the element.

The properties of the nativeElement depend upon the runtime environment. You could be running these tests on a non-browser platform that doesn't have a DOM or whose DOM-emulation doesn't support the full HTMLElement API.

Angular relies on the DebugElement abstraction to work safely across all supported platforms. Instead of creating an HTML element tree, Angular creates a DebugElement tree that wraps the native elements for the runtime platform. The nativeElement property unwraps the DebugElement and returns the platform-specific element object.

Because the sample tests for this guide are designed to run only in a browser, a nativeElement in these tests is always an HTMLElement whose familiar methods and properties you can explore within a test.

Here's the previous test, re-implemented with fixture.debugElement.nativeElement:

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

The DebugElement has other methods and properties that are useful in tests, as you'll see elsewhere in this guide.

You import the DebugElement symbol from the Angular core library.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

`By.css()`

Although the tests in this guide all run in the browser, some applications might run on a different platform at least some of the time.

For example, the component might render first on the server as part of a strategy to make the application launch faster on poorly connected devices. The server-side renderer might not support the full HTML element API. If it doesn't support querySelector, the previous test could fail.

The DebugElement offers query methods that work for all supported platforms. These query methods take a predicate function that returns true when a node in the DebugElement tree matches the selection criteria.

You create a predicate with the help of a By class imported from a library for the runtime platform. Here's the By import for the browser platform:

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

The following example re-implements the previous test with DebugElement.query() and the browser's By.css method.

      import {DebugElement} from '@angular/core';import {ComponentFixture, TestBed, waitForAsync} from '@angular/core/testing';import {By} from '@angular/platform-browser';import {BannerComponent} from './banner-initial.component';/*import { BannerComponent } from './banner.component';describe('BannerComponent', () => {*/describe('BannerComponent (initial CLI generated)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(waitForAsync(() => {    TestBed.configureTestingModule({imports: [BannerComponent]}).compileComponents();  }));  beforeEach(() => {    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;    fixture.detectChanges();  });  it('should create', () => {    expect(component).toBeDefined();  });});describe('BannerComponent (minimal)', () => {  it('should create', () => {    TestBed.configureTestingModule({imports: [BannerComponent]});    const fixture = TestBed.createComponent(BannerComponent);    const component = fixture.componentInstance;    expect(component).toBeDefined();  });});describe('BannerComponent (with beforeEach)', () => {  let component: BannerComponent;  let fixture: ComponentFixture<BannerComponent>;  beforeEach(() => {    TestBed.configureTestingModule({imports: [BannerComponent]});    fixture = TestBed.createComponent(BannerComponent);    component = fixture.componentInstance;  });  it('should create', () => {    expect(component).toBeDefined();  });  it('should contain "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    expect(bannerElement.textContent).toContain('banner works!');  });  it('should have <p> with "banner works!"', () => {    const bannerElement: HTMLElement = fixture.nativeElement;    const p = bannerElement.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.nativeElement)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const bannerEl: HTMLElement = bannerDe.nativeElement;    const p = bannerEl.querySelector('p')!;    expect(p.textContent).toEqual('banner works!');  });  it('should find the <p> with fixture.debugElement.query(By.css)', () => {    const bannerDe: DebugElement = fixture.debugElement;    const paragraphDe = bannerDe.query(By.css('p'));    const p: HTMLElement = paragraphDe.nativeElement;    expect(p.textContent).toEqual('banner works!');  });});

Some noteworthy observations:

The By.css() static method selects DebugElement nodes with a standard CSS selector.
The query returns a DebugElement for the paragraph.
You must unwrap that result to get the paragraph element.

When you're filtering by CSS selector and only testing properties of a browser's native element, the By.css approach might be overkill.

It's often more straightforward and clear to filter with a standard HTMLElement method such as querySelector() or querySelectorAll().