angular-cn/aio/content/guide/testing-services.md

Testing services

To check that your services are working as you intend, you can write tests specifically for them.

For a hands-on experience you can run tests and explore the test code in your browser as your read this guide.

If you'd like to experiment with the application that this guide describes, you can run it in your browser or download and run it locally.

Services are often the easiest files to unit test. Here are some synchronous and asynchronous unit tests of the ValueService written without assistance from Angular testing utilities.

{@a services-with-dependencies}

Services with dependencies

Services often depend on other services that Angular injects into the constructor. In many cases, it's easy to create and inject these dependencies by hand while calling the service's constructor.

The MasterService is a simple example:

MasterService delegates its only method, getValue, to the injected ValueService.

Here are several ways to test it.

The first test creates a ValueService with new and passes it to the MasterService constructor.

However, injecting the real service rarely works well as most dependent services are difficult to create and control.

Instead you can mock the dependency, use a dummy value, or create a spy on the pertinent service method.

Prefer spies as they are usually the easiest way to mock services.

These standard testing techniques are great for unit testing services in isolation.

However, you almost always inject services into application classes using Angular dependency injection and you should have tests that reflect that usage pattern. Angular testing utilities make it easy to investigate how injected services behave.

Testing services with the TestBed

Your application relies on Angular dependency injection (DI) to create services. When a service has a dependent service, DI finds or creates that dependent service. And if that dependent service has its own dependencies, DI finds-or-creates them as well.

As service consumer, you don't worry about any of this. You don't worry about the order of constructor arguments or how they're created.

As a service tester, you must at least think about the first level of service dependencies but you can let Angular DI do the service creation and deal with constructor argument order when you use the TestBed testing utility to provide and create services.

{@a testbed}

Angular TestBed

The TestBed is the most important of the Angular testing utilities. The TestBed creates a dynamically-constructed Angular test module that emulates an Angular @NgModule.

The TestBed.configureTestingModule() method takes a metadata object that can have most of the properties of an @NgModule.

To test a service, you set the providers metadata property with an array of the services that you'll test or mock.

Then inject it inside a test by calling TestBed.inject() with the service class as the argument.

Note: TestBed.get() was deprecated as of Angular version 9. To help minimize breaking changes, Angular introduces a new function called TestBed.inject(), which you should use instead. For information on the removal of TestBed.get(), see its entry in the Deprecations index.

Or inside the beforeEach() if you prefer to inject the service as part of your setup.

When testing a service with a dependency, provide the mock in the providers array.

In the following example, the mock is a spy object.

The test consumes that spy in the same way it did earlier.

{@a no-before-each}

Testing without beforeEach()

Most test suites in this guide call beforeEach() to set the preconditions for each it() test and rely on the TestBed to create classes and inject services.

There's another school of testing that never calls beforeEach() and prefers to create classes explicitly rather than use the TestBed.

Here's how you might rewrite one of the MasterService tests in that style.

Begin by putting re-usable, preparatory code in a setup function instead of beforeEach().

The setup() function returns an object literal with the variables, such as masterService, that a test might reference. You don't define semi-global variables (for example, let masterService: MasterService) in the body of the describe().

Then each test invokes setup() in its first line, before continuing with steps that manipulate the test subject and assert expectations.

Notice how the test uses destructuring assignment to extract the setup variables that it needs.

Many developers feel this approach is cleaner and more explicit than the traditional beforeEach() style.

Although this testing guide follows the traditional style and the default CLI schematics generate test files with beforeEach() and TestBed, feel free to adopt this alternative approach in your own projects.

Testing HTTP services

Data services that make HTTP calls to remote servers typically inject and delegate to the Angular HttpClient service for XHR calls.

You can test a data service with an injected HttpClient spy as you would test any service with a dependency.

The HeroService methods return Observables. You must subscribe to an observable to (a) cause it to execute and (b) assert that the method succeeds or fails.

The subscribe() method takes a success (next) and fail (error) callback. Make sure you provide both callbacks so that you capture errors. Neglecting to do so produces an asynchronous uncaught observable error that the test runner will likely attribute to a completely different test.

HttpClientTestingModule

Extended interactions between a data service and the HttpClient can be complex and difficult to mock with spies.

The HttpClientTestingModule can make these testing scenarios more manageable.

While the code sample accompanying this guide demonstrates HttpClientTestingModule, this page defers to the Http guide, which covers testing with the HttpClientTestingModule in detail.