angular-docs-cn/aio/content/guide/testing-components-basics.md

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 sections below. But in many cases, testing the component class alone, without DOM involvement, can validate much of the component's behavior in an easier, more obvious way.

For the sample app that the testing guides describe, see the sample app.

For the tests features in the testing guides, see tests.

{@a component-class-testing}

Component class testing

Test a component class on its own as you would test a service class.

Component class testing should be kept very clean and simple. It should test only a single unit. At first glance, you should be able to understand what the test is testing.

Consider this LightswitchComponent which toggles a light on and off (represented by an on-screen message) when the user clicks the button.

You might decide only to test that the clicked() method toggles the light's on/off state and sets the message appropriately.

This component class has no dependencies. To test these types of classes, follow the same steps as you would for a service that has no dependencies:

1. Create a component using the new keyword.
2. Poke at its API.
3. Assert expectations on its public state.

Here is the DashboardHeroComponent from the Tour of Heroes tutorial.

It appears within the template of a parent component, which binds a hero to the @Input property and listens for an event raised through the selected @Output property.

You can test that the class code works without creating the DashboardHeroComponent or its parent component.

When a component has dependencies, you may wish to use the TestBed to both create the component and its dependencies.

The following WelcomeComponent depends on the UserService to know the name of the user to greet.

You might start by creating a mock of the UserService that meets the minimum needs of this component.

Then provide and inject both the component and the service in the TestBed configuration.

Then exercise the component class, remembering to call the lifecycle hook methods as Angular does when running the app.

Component DOM testing

Testing the component class is as easy as testing a service.

But a component is more than just its class. A component interacts with the DOM and with other components. The class-only tests can tell you about class behavior. They 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.

None of the class-only tests above can answer key questions about how the components actually behave on screen.

• 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 (i.e, in uppercase)?
• Is the welcome message displayed by the template of WelcomeComponent?

These may not be troubling questions for the simple components illustrated above. 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:

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

Please refer to the async 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 below. For now, you can radically reduce this test file to a more manageable size:

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

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.

{@a create-component}

createComponent()

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

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

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.

{@a component-fixture}

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:

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:

<code-example path="testing/src/app/banner/banner-initial.component.spec.ts" region="v3"

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

{@a native-element}

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 app might be running on a non-browser platform, such as the server or a Web Worker, where the element may 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, you can 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:

{@a debug-element}

DebugElement

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

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

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:

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.

{@a by-css}

By.css()

Although the tests in this guide all run in the browser, some apps 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:

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

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 may be overkill.

It's often easier and more clear to filter with a standard HTMLElement method such as querySelector() or querySelectorAll(), as you'll see in the next set of tests.