# 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](https://jasmine.github.io/tutorials/your_first_suite#section-Spies)
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)](guide/dependency-injection)
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](guide/ngmodules).
The `TestBed.configureTestingModule()` method takes a metadata object that can have most of the properties of an [@NgModule](guide/ngmodules).
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](guide/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_](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/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](https://github.com/angular/angular-cli)
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`](guide/http) 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](guide/http#testing-http-requests),
which covers testing with the `HttpClientTestingModule` in detail.