Testing

This guide offers tips and techniques for testing Angular applications. Though this page includes some general testing principles and techniques, the focus is on testing applications written with Angular.

Contents

• Live examples
  
  
• Introduction to Angular testing
  • Tools and technologies
  • Setup
  • Isolated unit tests vs. the Angular testing utilities
• The first karma test
  • Run with karma
  • Test debugging
  • Try the live example
• Test a component
  • TestBed
  • createComponent
  • ComponentFixture, DebugElement, and query(By.css)
  • The tests
  • detectChanges: Angular change detection within a test
  • Try the live example
  • Automatic change detection
• Test a component with an external template
  • The first asynchronous beforeEach
  • compileComponents
  • The second synchronous beforeEach
  • Waiting for compileComponents
  • Try the live example
• Test a component with a service dependency
  • Provide service test doubles
  • Get injected services
  • TestBed.get
  • Always get the service from an injector
  • Final setup and tests
• Test a component with an async service
  • Spying on the real service
  • Synchronous tests
  • The async funciton in it
  • whenStable
  • The fakeAsync function
  • The tick function
  • jasmine.done
• Test a component with inputs and outputs
  • Test DashboardHeroComponent stand-alone
  • triggerEventHandler
• Test a component inside a test host component
  
  
• Test a routed component
  • The inject helper function
  • Test a routed component with parameters
  • Create an Observable test double
  • Testing with the Observable test double
• Use a page object to simplify setup
• Set up with module imports
• Import the feature module
  
  
• Override a component's providers
  • The overrideComponent method
  • Provide a spy-stub (HeroDetailServiceSpy)
  • The override tests
  • More overrides
• Test a RouterOutlet component
  • Stubbing unneeded components
  • Stubbing the RouterLink
  • By.directive and injected directives
  • What good are these tests?
• "Shallow component tests" with NO_ERRORS_SCHEMA
  
  
• Test an attribute directive
  
  
• Isolated unit tests
  • Services
  • Services with dependencies
  • Pipes
  • Write Angular tests too
  • Components
• Angular testing utility APIs
  • TestBed class summary
  • The ComponentFixture
  • ComponentFixture properties
  • The ComponentFixture methods
  • DebugElement
• Test environment setup files
  • npm packages
• FAQ: Frequently asked questions

It’s a big agenda. Fortunately, you can learn a little bit at a time and put each lesson to use.

Live examples

This guide presents tests of a sample application that is much like the Tour of Heroes tutorial. The sample application and all tests in this guide are available as live examples for inspection, experiment, and download:

• A spec to verify the test environment.
• The first component spec with inline template.
• A component spec with external template.
• The QuickStart seed's AppComponent spec.
• The sample application to be tested.
• All specs that test the sample application.
• A grab bag of additional specs.

Introduction to Angular Testing

This page guides you through writing tests to explore and confirm the behavior of the application. Testing does the following:

1. Guards against changes that break existing code (“regressions”).

2. Clarifies what the code does both when used as intended and when faced with deviant conditions.

3. Reveals mistakes in design and implementation. Tests shine a harsh light on the code from many angles. When a part of the application seems hard to test, the root cause is often a design flaw, something to cure now rather than later when it becomes expensive to fix.

Tools and technologies

You can write and run Angular tests with a variety of tools and technologies. This guide describes specific choices that are known to work well.

Setup

There are two fast paths to getting started with unit testing.

1. Start a new project following the instructions in Setup.

2. Start a new project with the Angular CLI.

Both approaches install npm packages, files, and scripts pre-configured for applications built in their respective modalities. Their artifacts and procedures differ slightly but their essentials are the same and there are no differences in the test code.

In this guide, the application and its tests are based on the setup instructions. For a discussion of the unit testing setup files, see below.

Isolated unit tests vs. the Angular testing utilites

Isolated unit tests examine an instance of a class all by itself without any dependence on Angular or any injected values. The tester creates a test instance of the class with new, supplying test doubles for the constructor parameters as needed, and then probes the test instance API surface.

You should write isolated unit tests for pipes and services.

You can test components in isolation as well. However, isolated unit tests don't reveal how components interact with Angular. In particular, they can't reveal how a component class interacts with its own template or with other components.

Such tests require the Angular testing utilities. The Angular testing utilities include the TestBed class and several helper functions from @angular/core/testing. They are the main focus of this guide and you'll learn about them when you write your first component test. A comprehensive review of the Angular testing utilities appears later in this guide.

But first you should write a dummy test to verify that your test environment is set up properly and to lock in a few basic testing skills.

The first karma test

Start with a simple test to make sure that the setup works properly.

Create a new file called 1st.spec.ts in the application root folder, src/app/

Put spec files somewhere within the src/app/ folder. The karma.conf.js tells karma to look for spec files there, for reasons explained below.

Add the following code to src/app/1st.spec.ts.

describe('1st tests', () => {
  it('true is true', () => expect(true).toBe(true));
});

Run with karma

Compile and run it in karma from the command line using the following command:

npm test

The command compiles the application and test code and starts karma. Both processes watch pertinent files, write messages to the console, and re-run when they detect changes.

After a few moments, karma opens a browser and starts writing to the console.

Hide (don't close!) the browser and focus on the console output, which should look something like this:

> npm test
...
[0] 1:37:03 PM - Compilation complete. Watching for file changes.
...
[1] Chrome 51.0.2704: Executed 0 of 0 SUCCESS
    Chrome 51.0.2704: Executed 1 of 1 SUCCESS
SUCCESS (0.005 secs / 0.005 secs)

Both the compiler and karma continue to run. The compiler output is preceded by [0]; the karma output by [1].

Change the expectation from true to false.

The compiler watcher detects the change and recompiles.

[0] 1:49:21 PM - File change detected. Starting incremental compilation...
[0] 1:49:25 PM - Compilation complete. Watching for file changes.

The karma watcher detects the change to the compilation output and re-runs the test.

[1] Chrome 51.0.2704 1st tests true is true FAILED
[1] Expected false to equal true.
[1] Chrome 51.0.2704: Executed 1 of 1 (1 FAILED) (0.005 secs / 0.005 secs)

It fails of course.

Restore the expectation from false back to true. Both processes detect the change, re-run, and karma reports complete success.

Test debugging

Debug specs in the browser in the same way that you debug an application.

1. Reveal the karma browser window (hidden earlier).
2. Click the DEBUG button; it opens a new browser tab and re-runs the tests.
3. Open the browser's “Developer Tools” (Ctrl-Shift-I on windows; Command-Option-I in OSX).
4. Pick the "sources" section.
5. Open the 1st.spec.ts test file (Control/Command-P, then start typing the name of the file).
6. Set a breakpoint in the test.
7. Refresh the browser, and it stops at the breakpoint.

Try the live example

You can also try this test as a in plunker. All of the tests in this guide are available as live examples.

Test a component

An Angular component is the first thing most developers want to test. The BannerComponent in src/app/banner-inline.component.ts is the simplest component in this application and a good place to start. It presents the application title at the top of the screen within an <h1> tag.

import { Component } from '@angular/core';

@Component({
  selector: 'app-banner',
  template: '<h1>{{title}}</h1>'
})
export class BannerComponent {
  title = 'Test Tour of Heroes';
}

This version of the BannerComponent has an inline template and an interpolation binding. The component is probably too simple to be worth testing in real life but it's perfect for a first encounter with the Angular testing utilities.

The corresponding src/app/banner-inline.component.spec.ts sits in the same folder as the component, for reasons explained in the FAQ answer to "Why put specs next to the things they test?".

Start with ES6 import statements to get access to symbols referenced in the spec.

import { ComponentFixture, TestBed } from '@angular/core/testing';
import { By }              from '@angular/platform-browser';
import { DebugElement }    from '@angular/core';

import { BannerComponent } from './banner-inline.component';

Here's the describe and the beforeEach that precedes the tests:

describe('BannerComponent (inline template)', () => {

  let comp:    BannerComponent;
  let fixture: ComponentFixture<BannerComponent>;
  let de:      DebugElement;
  let el:      HTMLElement;

  beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [ BannerComponent ], // declare the test component
    });

    fixture = TestBed.createComponent(BannerComponent);

    comp = fixture.componentInstance; // BannerComponent test instance

    // query for the title <h1> by CSS element selector
    de = fixture.debugElement.query(By.css('h1'));
    el = de.nativeElement;
  });
});

TestBed

TestBed is the first and most important of the Angular testing utilities. It creates an Angular testing module—an @NgModule class—that you configure with the configureTestingModule method to produce the module environment for the class you want to test. In effect, you detach the tested component from its own application module and re-attach it to a dynamically-constructed Angular test module tailored specifically for this battery of tests.

The configureTestingModule method takes an @NgModule-like metadata object. The metadata object can have most of the properties of a normal Angular module.

This metadata object simply declares the component to test, BannerComponent. The metadata lack imports because (a) the default testing module configuration already has what BannerComponent needs and (b) BannerComponent doesn't interact with any other components.

Call configureTestingModule within a beforeEach so that TestBed can reset itself to a base state before each test runs.

The base state includes a default testing module configuration consisting of the declarables (components, directives, and pipes) and providers (some of them mocked) that almost everyone needs.

This default configuration is merely a foundation for testing an app. Later you'll call TestBed.configureTestingModule with more metadata that define additional imports, declarations, providers, and schemas to fit your application tests. Optional override methods can fine-tune aspects of the configuration.

createComponent

After configuring TestBed, you tell it to create an instance of the component-under-test. In this example, TestBed.createComponent creates an instance of BannerComponent and returns a component test fixture.

The createComponent method closes the current TestBed instance to further configuration. You cannot call any more TestBed configuration methods, not configureTestingModule nor any of the override... methods. If you try, TestBed throws an error.

ComponentFixture, DebugElement, and query(By.css)

The createComponent method returns a ComponentFixture, a handle on the test environment surrounding the created component. The fixture provides access to the component instance itself and to the DebugElement, which is a handle on the component's DOM element.

The title property value is interpolated into the DOM within <h1> tags. Use the fixture's DebugElement to query for the <h1> element by CSS selector.

The query method takes a predicate function and searches the fixture's entire DOM tree for the first element that satisfies the predicate. The result is a different DebugElement, one associated with the matching DOM element.

The By class is an Angular testing utility that produces useful predicates. Its By.css static method produces a standard CSS selector predicate that filters the same way as a jQuery selector.

Finally, the setup assigns the DOM element from the DebugElement nativeElement property to el. The tests assert that el contains the expected title text.

The tests

Jasmine runs the beforeEach function before each of these tests

it('should display original title', () => {
  fixture.detectChanges();
  expect(el.textContent).toContain(comp.title);
});

it('should display a different test title', () => {
  comp.title = 'Test Title';
  fixture.detectChanges();
  expect(el.textContent).toContain('Test Title');
});

These tests ask the DebugElement for the native HTML element to satisfy their expectations.

detectChanges: Angular change detection within a test

Each test tells Angular when to perform change detection by calling fixture.detectChanges(). The first test does so immediately, triggering data binding and propagation of the title property to the DOM element.

The second test changes the component's title property and only then calls fixture.detectChanges(); the new value appears in the DOM element.

In production, change detection kicks in automatically when Angular creates a component or the user enters a keystroke or an asynchronous activity (e.g., AJAX) completes.

The TestBed.createComponent does not trigger change detection. The fixture does not automatically push the component's title property value into the data bound element, a fact demonstrated in the following test:

it('no title in the DOM until manually call `detectChanges`', () => {
  expect(el.textContent).toEqual('');
});

This behavior (or lack of it) is intentional. It gives the tester an opportunity to inspect or change the state of the component before Angular initiates data binding or calls lifecycle hooks.

Try the live example

Take a moment to explore this component spec as a and lock in these fundamentals of component unit testing.

Automatic change detection

The BannerComponent tests frequently call detectChanges. Some testers prefer that the Angular test environment run change detection automatically.

That's possible by configuring the TestBed with the ComponentFixtureAutoDetect provider. First import it from the testing utility library:

import { ComponentFixtureAutoDetect } from '@angular/core/testing';

Then add it to the providers array of the testing module configuration:

TestBed.configureTestingModule({
  declarations: [ BannerComponent ],
  providers: [
    { provide: ComponentFixtureAutoDetect, useValue: true }
  ]
})

Here are three tests that illustrate how automatic change detection works.

it('should display original title', () => {
  // Hooray! No `fixture.detectChanges()` needed
  expect(el.textContent).toContain(comp.title);
});

it('should still see original title after comp.title change', () => {
  const oldTitle = comp.title;
  comp.title = 'Test Title';
  // Displayed title is old because Angular didn't hear the change :(
  expect(el.textContent).toContain(oldTitle);
});

it('should display updated title after detectChanges', () => {
  comp.title = 'Test Title';
  fixture.detectChanges(); // detect changes explicitly
  expect(el.textContent).toContain(comp.title);
});

The first test shows the benefit of automatic change detection.

The second and third test reveal an important limitation. The Angular testing environment does not know that the test changed the component's title. The ComponentFixtureAutoDetect service responds to asynchronous activities such as promise resolution, timers, and DOM events. But a direct, synchronous update of the component property is invisible. The test must call fixture.detectChanges() manually to trigger another cycle of change detection.

Test a component with an external template

The application's actual BannerComponent behaves the same as the version above but is implemented differently. It has external template and css files, specified in templateUrl and styleUrls properties.

import { Component } from '@angular/core';

@Component({
  selector: 'app-banner',
  templateUrl: './banner.component.html',
  styleUrls:  ['./banner.component.css']
})
export class BannerComponent {
  title = 'Test Tour of Heroes';
}

That's a problem for the tests. The TestBed.createComponent method is synchronous. But the Angular template compiler must read the external files from the file system before it can create a component instance. That's an asynchronous activity. The previous setup for testing the inline component won't work for a component with an external template.

The first asynchronous beforeEach

The test setup for BannerComponent must give the Angular template compiler time to read the files. The logic in the beforeEach of the previous spec is split into two beforeEach calls. The first beforeEach handles asynchronous compilation.

// async beforeEach
beforeEach(async(() => {
  TestBed.configureTestingModule({
    declarations: [ BannerComponent ], // declare the test component
  })
  .compileComponents();  // compile template and css
}));

Notice the async function called as the argument to beforeEach. The async function is one of the Angular testing utilities and has to be imported.

import { async } from '@angular/core/testing';

It takes a parameterless function and returns a function which becomes the true argument to the beforeEach.

The body of the async argument looks much like the body of a synchronous beforeEach. There is nothing obviously asynchronous about it. For example, it doesn't return a promise and there is no done function to call as there would be in standard Jasmine asynchronous tests. Internally, async arranges for the body of the beforeEach to run in a special async test zone that hides the mechanics of asynchronous execution.

All this is necessary in order to call the asynchronous TestBed.compileComponents method.

compileComponents

The TestBed.configureTestingModule method returns the TestBed class so you can chain calls to other TestBed static methods such as compileComponents.

The TestBed.compileComponents method asynchronously compiles all the components configured in the testing module. In this example, the BannerComponent is the only component to compile. When compileComponents completes, the external templates and css files have been "inlined" and TestBed.createComponent can create new instances of BannerComponent synchronously.

In this example, TestBed.compileComponents only compiles the BannerComponent. Tests later in the guide declare multiple components and a few specs import entire application modules that hold yet more components. Any of these components might have external templates and css files. TestBed.compileComponents compiles all of the declared components asynchronously at one time.

Calling compileComponents closes the current TestBed instance is further configuration. You cannot call any more TestBed configuration methods, not configureTestingModule nor any of the override... methods. The TestBed throws an error if you try.

The second synchronous beforeEach

A synchronous beforeEach containing the remaining setup steps follows the asynchronous beforeEach.

// synchronous beforeEach
beforeEach(() => {
  fixture = TestBed.createComponent(BannerComponent);

  comp = fixture.componentInstance; // BannerComponent test instance

  // query for the title <h1> by CSS element selector
  de = fixture.debugElement.query(By.css('h1'));
  el = de.nativeElement;
});

These are the same steps as in the original beforeEach. They include creating an instance of the BannerComponent and querying for the elements to inspect.

You can count on the test runner to wait for the first asynchronous beforeEach to finish before calling the second.

Waiting for compileComponents

The compileComponents method returns a promise so you can perform additional tasks immediately after it finishes. For example, you could move the synchronous code in the second beforeEach into a compileComponents().then(...) callback and write only one beforeEach.

Most developers find that hard to read. The two beforeEach calls are widely preferred.

Try the live example

Take a moment to explore this component spec as a .

Test a component with a dependency

Components often have service dependencies.

The WelcomeComponent displays a welcome message to the logged in user. It knows who the user is based on a property of the injected UserService:

import { Component, OnInit } from '@angular/core';
import { UserService }       from './model';

@Component({
  selector: 'app-welcome',
  template: '<h3 class="welcome" ><i>{{welcome}}</i></h3>'
})
export class WelcomeComponent  implements OnInit {
  welcome = '-- not initialized yet --';
  constructor(private userService: UserService) { }

  ngOnInit(): void {
    this.welcome = this.userService.isLoggedIn ?
      'Welcome, ' + this.userService.user.name :
      'Please log in.';
  }
}

The WelcomeComponent has decision logic that interacts with the service, logic that makes this component worth testing. Here's the testing module configuration for the spec file, src/app/welcome.component.spec.ts:

    TestBed.configureTestingModule({
       declarations: [ WelcomeComponent ],
    // providers:    [ UserService ]  // NO! Don't provide the real service!
                                      // Provide a test-double instead
       providers:    [ {provide: UserService, useValue: userServiceStub } ]
    });

This time, in addition to declaring the component-under-test, the configuration adds a UserService provider to the providers list. But not the real UserService.

Provide service test doubles

A component-under-test doesn't have to be injected with real services. In fact, it is usually better if they are test doubles (stubs, fakes, spies, or mocks). The purpose of the spec is to test the component, not the service, and real services can be trouble.

Injecting the real UserService could be a nightmare. The real service might ask the user for login credentials and attempt to reach an authentication server. These behaviors can be hard to intercept. It is far easier and safer to create and register a test double in place of the real UserService.

This particular test suite supplies a minimal UserService stub that satisfies the needs of the WelcomeComponent and its tests:

userServiceStub = {
  isLoggedIn: true,
  user: { name: 'Test User'}
};

Get injected services

The tests need access to the (stub) UserService injected into the WelcomeComponent.

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.

// UserService actually injected into the component
userService = fixture.debugElement.injector.get(UserService);

TestBed.get

You may also be able to get the service from the root injector via TestBed.get. This is easier to remember and less verbose. But it only works when Angular injects the component with the service instance in the test's root injector. Fortunately, in this test suite, the only provider of UserService is the root testing module, so it is safe to call TestBed.get as follows:

// UserService from the root injector
userService = TestBed.get(UserService);

Always get the service from an injector

Do not reference the userServiceStub object that's provided to the testing module in the body of your test. It does not work! The userService instance injected into the component is a completely different object, a clone of the provided userServiceStub.

it('stub object and injected UserService should not be the same', () => {
  expect(userServiceStub === userService).toBe(false);

  // Changing the stub object has no effect on the injected service
  userServiceStub.isLoggedIn = false;
  expect(userService.isLoggedIn).toBe(true);
});

Final setup and tests

Here's the complete beforeEach using TestBed.get:

  beforeEach(() => {
    // stub UserService for test purposes
    userServiceStub = {
      isLoggedIn: true,
      user: { name: 'Test User'}
    };

    TestBed.configureTestingModule({
       declarations: [ WelcomeComponent ],
       providers:    [ {provide: UserService, useValue: userServiceStub } ]
    });

    fixture = TestBed.createComponent(WelcomeComponent);
    comp    = fixture.componentInstance;

    // UserService from the root injector
    userService = TestBed.get(UserService);

    //  get the "welcome" element by CSS selector (e.g., by class name)
    de = fixture.debugElement.query(By.css('.welcome'));
    el = de.nativeElement;
  });

And here are some tests:

it('should welcome the user', () => {
  fixture.detectChanges();
  const content = el.textContent;
  expect(content).toContain('Welcome', '"Welcome ..."');
  expect(content).toContain('Test User', 'expected name');
});

it('should welcome "Bubba"', () => {
  userService.user.name = 'Bubba'; // welcome message hasn't been shown yet
  fixture.detectChanges();
  expect(el.textContent).toContain('Bubba');
});

it('should request login if not logged in', () => {
  userService.isLoggedIn = false; // welcome message hasn't been shown yet
  fixture.detectChanges();
  const content = el.textContent;
  expect(content).not.toContain('Welcome', 'not welcomed');
  expect(content).toMatch(/log in/i, '"log in"');
});

The first is a sanity test; it confirms that the stubbed UserService 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.

Test a component with an async service

Many services return values asynchronously. Most data services make an HTTP request to a remote server and the response is necessarily asynchronous.

The "About" view in this sample displays Mark Twain quotes. The TwainComponent handles the display, delegating the server request to the TwainService.

Both are in the src/app/shared folder because the author intends to display Twain quotes on other pages someday. Here is the TwainComponent.

@Component({
  selector: 'twain-quote',
  template: '<p class="twain"><i>{{quote}}</i></p>'
})
export class TwainComponent  implements OnInit {
  intervalId: number;
  quote = '...';
  constructor(private twainService: TwainService) { }

  ngOnInit(): void {
    this.twainService.getQuote().then(quote => this.quote = quote);
  }
}

The TwainService implementation is irrelevant for this particular test. It is sufficient to see within ngOnInit that twainService.getQuote returns a promise, which means it is asynchronous.

In general, tests should not make calls to remote servers. They should emulate such calls. The setup in this src/app/shared/twain.component.spec.ts shows one way to do that:

  beforeEach(() => {
    TestBed.configureTestingModule({
       declarations: [ TwainComponent ],
       providers:    [ TwainService ],
    });

    fixture = TestBed.createComponent(TwainComponent);
    comp    = fixture.componentInstance;

    // TwainService actually injected into the component
    twainService = fixture.debugElement.injector.get(TwainService);

    // Setup spy on the `getQuote` method
    spy = spyOn(twainService, 'getQuote')
          .and.returnValue(Promise.resolve(testQuote));

    // Get the Twain quote element by CSS selector (e.g., by class name)
    de = fixture.debugElement.query(By.css('.twain'));
    el = de.nativeElement;
  });

Spying on the real service

This setup is similar to the welcome.component.spec setup. But instead of creating a stubbed service object, it injects the real service (see the testing module providers) and replaces the critical getQuote method with a Jasmine spy.

spy = spyOn(twainService, 'getQuote')
      .and.returnValue(Promise.resolve(testQuote));

The spy is designed such that any call to getQuote receives an immediately resolved promise with a test quote. The spy bypasses the actual getQuote method and therefore does not contact the server.

Here are the tests with commentary to follow:

  it('should not show quote before OnInit', () => {
    expect(el.textContent).toBe('', 'nothing displayed');
    expect(spy.calls.any()).toBe(false, 'getQuote not yet called');
  });

  it('should still not show quote after component initialized', () => {
    fixture.detectChanges();
    // getQuote service is async => still has not returned with quote
    expect(el.textContent).toBe('...', 'no quote yet');
    expect(spy.calls.any()).toBe(true, 'getQuote called');
  });

  it('should show quote after getQuote promise (async)', async(() => {
    fixture.detectChanges();

    fixture.whenStable().then(() => { // wait for async getQuote
      fixture.detectChanges();        // update view with quote
      expect(el.textContent).toBe(testQuote);
    });
  }));

  it('should show quote after getQuote promise (fakeAsync)', fakeAsync(() => {
    fixture.detectChanges();
    tick();                  // wait for async getQuote
    fixture.detectChanges(); // update view with quote
    expect(el.textContent).toBe(testQuote);
  }));

Synchronous tests

The first two tests are synchronous. Thanks to the spy, they verify that getQuote is called after the first change detection cycle during which Angular calls ngOnInit.

Neither test can prove that a value from the service is displayed. The quote itself has not arrived, despite the fact that the spy returns a resolved promise.

This test must wait at least one full turn of the JavaScript engine before the value becomes available. The test must become asynchronous.

The async function in it

Notice the async in the third test.

it('should show quote after getQuote promise (async)', async(() => {
  fixture.detectChanges();

  fixture.whenStable().then(() => { // wait for async getQuote
    fixture.detectChanges();        // update view with quote
    expect(el.textContent).toBe(testQuote);
  });
}));

The async function is one of the Angular testing utilities. It simplifies coding of asynchronous tests by arranging for the tester's code to run in a special async test zone as discussed earlier when it was called in a beforeEach.

Although async does a great job of hiding asynchronous boilerplate, some functions called within a test (such as fixture.whenStable) continue to reveal their asynchronous behavior.

whenStable

The test must wait for the getQuote promise to resolve in the next turn of the JavaScript engine.

This test has no direct access to the promise returned by the call to twainService.getQuote because it is buried inside TwainComponent.ngOnInit and therefore inaccessible to a test that probes only the component API surface.

Fortunately, the getQuote promise is accessible to the async test zone, which intercepts all promises issued within the async method call no matter where they occur.

The ComponentFixture.whenStable method returns its own promise, which resolves when the getQuote promise finishes. In fact, the whenStable promise resolves when all pending asynchronous activities within this test complete—the definition of "stable."

Then the test resumes and kicks off another round of change detection (fixture.detectChanges), which tells Angular to update the DOM with the quote. The getQuote helper method extracts the display element text and the expectation confirms that the text matches the test quote.

The fakeAsync function

The fourth test verifies the same component behavior in a different way.

it('should show quote after getQuote promise (fakeAsync)', fakeAsync(() => {
  fixture.detectChanges();
  tick();                  // wait for async getQuote
  fixture.detectChanges(); // update view with quote
  expect(el.textContent).toBe(testQuote);
}));

Notice that fakeAsync replaces async as the it argument. The fakeAsync function is another of the Angular testing utilities.

Like async, it takes a parameterless function and returns a function that becomes the argument to the Jasmine it call.

The fakeAsync function enables a linear coding style by running the test body in a special fakeAsync test zone.

The principle advantage of fakeAsync over async is that the test appears to be synchronous. There is no then(...) to disrupt the visible flow of control. The promise-returning fixture.whenStable is gone, replaced by tick().

The tick function

The tick function is one of the Angular testing utilities and a companion to fakeAsync. You can only call it within a fakeAsync body.

Calling tick() simulates the passage of time until all pending asynchronous activities finish, including the resolution of the getQuote promise in this test case.

It returns nothing. There is no promise to wait for. Proceed with the same test code that appeared in the whenStable.then() callback.

Even this simple example is easier to read than the third test. To more fully appreciate the improvement, imagine a succession of asynchronous operations, chained in a long sequence of promise callbacks.

jasmine.done

While the async and fakeAsync functions greatly simplify Angular asynchronous testing, you can still fall back to the traditional Jasmine asynchronous testing technique.

You can still pass it a function that takes a done callback. Now you are responsible for chaining promises, handling errors, and calling done at the appropriate moment.

Here is a done version of the previous two tests:

it('should show quote after getQuote promise (done)', done => {
  fixture.detectChanges();

  // get the spy promise and wait for it to resolve
  spy.calls.mostRecent().returnValue.then(() => {
    fixture.detectChanges(); // update view with quote
    expect(el.textContent).toBe(testQuote);
    done();
  });
});

Although there is no direct access to the getQuote promise inside TwainComponent, the spy has direct access, which makes it possible to wait for getQuote to finish.

Writing test functions with done, while more cumbersome than async and fakeAsync, is a viable and occasionally necessary technique. For example, you can't call async or fakeAsync when testing code that involves the intervalTimer, as is common when testing async Observable methods.

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

The DashboardHeroComponent is embedded in the DashboardComponent template like this:

<dashboard-hero *ngFor="let hero of heroes"  class="col-1-4"
  [hero]=hero  (selected)="gotoDetail($event)" >
</dashboard-hero>

The DashboardHeroComponent appears in an *ngFor repeater, 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 definition:

@Component({
  selector:    'dashboard-hero',
  templateUrl: './dashboard-hero.component.html',
  styleUrls: [ './dashboard-hero.component.css' ]
})
export class DashboardHeroComponent {
  @Input() hero: Hero;
  @Output() selected = new EventEmitter<Hero>();
  click() { this.selected.emit(this.hero); }
}

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

• Test it as used by DashboardComponent.
• Test it as a stand-alone component.
• Test it as used by a substitute for DashboardComponent.

A quick look at the DashboardComponent constructor discourages the first approach:

constructor(
  private router: Router,
  private heroService: HeroService) {
}

The DashboardComponent depends on the Angular router and the HeroService. You'd probably have to replace them both with test doubles, which is a lot of work. The router seems particularly challenging.

The immediate goal is to test the DashboardHeroComponent, not the DashboardComponent, so, try the second and third options.

Test DashboardHeroComponent stand-alone

Here's the spec file setup.

  // async beforeEach
  beforeEach( async(() => {
    TestBed.configureTestingModule({
      declarations: [ DashboardHeroComponent ],
    })
    .compileComponents(); // compile template and css
  }));

  // synchronous beforeEach
  beforeEach(() => {
    fixture = TestBed.createComponent(DashboardHeroComponent);
    comp    = fixture.componentInstance;
    heroEl  = fixture.debugElement.query(By.css('.hero')); // find hero element

    // pretend that it was wired to something that supplied a hero
    expectedHero = new Hero(42, 'Test Name');
    comp.hero = expectedHero;
    fixture.detectChanges(); // trigger initial data binding
  });

The async beforeEach was discussed above. Having compiled the components asynchronously with compileComponents, the rest of the setup proceeds synchronously in a second beforeEach, using the basic techniques described earlier.

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

The first test follows:

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

It verifies that the hero name is propagated to template with a binding. Because the template passes the hero name through the Angular UpperCasePipe, the test must match the element value with the uppercased name:

<div (click)="click()" class="hero">
  {{hero.name | uppercase}}
</div>

The second test verifies click behavior. Clicking the hero should raise a selected event that the host component (DashboardComponent presumably) can hear:

  it('should raise selected event when clicked', () => {
    let selectedHero: Hero;
    comp.selected.subscribe((hero: Hero) => selectedHero = hero);

    heroEl.triggerEventHandler('click', null);
    expect(selectedHero).toBe(expectedHero);
  });

The component exposes an EventEmitter property. The test subscribes to it just as the host component would do.

The heroEl is a DebugElement that represents the hero <div>. The test calls triggerEventHandler with the "click" event name. The "click" event binding responds by calling DashboardHeroComponent.click().

If the component behaves as expected, click() tells the component's selected property to emit the hero object, the test detects that value through its subscription to selected, and the test should pass.

triggerEventHandler

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.

In this example, the test triggers a "click" event with a null event object.

heroEl.triggerEventHandler('click', null);

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

Other handlers are less forgiving. For example, the RouterLink directive expects an object with a button property that identifies which mouse button was pressed. This directive throws an error if the event object doesn't do this correctly.

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

Make that easy by encapsulating the click-triggering process in a helper such as the click function below:

/** 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 wish, you can 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 this click helper.

it('should raise selected event when clicked', () => {
  let selectedHero: Hero;
  comp.selected.subscribe((hero: Hero) => selectedHero = hero);

  click(heroEl);   // triggerEventHandler helper
  expect(selectedHero).toBe(expectedHero);
});

Test a component inside a test host component

In the previous approach, the tests themselves played the role of the host DashboardComponent. But does the DashboardHeroComponent work correctly when properly data-bound to a host component?

Testing with the actual DashboardComponent host is doable but seems more trouble than its worth. It's easier to emulate the DashboardComponent host with a test host like this one:

@Component({
  template: `
    <dashboard-hero  [hero]="hero"  (selected)="onSelected($event)"></dashboard-hero>`
})
class TestHostComponent {
  hero = new Hero(42, 'Test Name');
  selectedHero: Hero;
  onSelected(hero: Hero) { this.selectedHero = hero; }
}

The test host binds to DashboardHeroComponent as the DashboardComponent would but without the distraction of the Router, the HeroService, or even the *ngFor repeater.

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 check that property to verify that the DashboardHeroComponent.selected event emitted the right hero.

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

beforeEach( async(() => {
  TestBed.configureTestingModule({
    declarations: [ DashboardHeroComponent, TestHostComponent ], // declare both
  }).compileComponents();
}));

beforeEach(() => {
  // create TestHostComponent instead of DashboardHeroComponent
  fixture  = TestBed.createComponent(TestHostComponent);
  testHost = fixture.componentInstance;
  heroEl   = fixture.debugElement.query(By.css('.hero')); // find hero
  fixture.detectChanges(); // trigger initial data binding
});

This testing module configuration shows two important differences:

1. It declares both the DashboardHeroComponent and the TestHostComponent.
2. It creates the TestHostComponent instead of the DashboardHeroComponent.

The createComponent returns a fixture that holds an instance of TestHostComponent instead of an instance of DashboardHeroComponent.

Creating the TestHostComponent has the side-effect of creating a DashboardHeroComponent 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.nativeElement.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 DashboardHeroComponent hero really does find its way up through the event binding to the host component.

Test a routed component

Testing the actual DashboardComponent seemed daunting because it injects the Router.

constructor(
  private router: Router,
  private heroService: HeroService) {
}

It also injects the HeroService, but faking that is a familiar story. The Router has a complicated API and is entwined with other services and application preconditions.

Fortunately, the DashboardComponent isn't doing much with the Router

gotoDetail(hero: Hero) {
  let url = `/heroes/${hero.id}`;
  this.router.navigateByUrl(url);
}

This is often the case. As a rule you test the component, not the router, and care only if the component navigates with the right address under the given conditions. Stubbing the router with a test implementation is an easy option. This should do the trick:

class RouterStub {
  navigateByUrl(url: string) { return url; }
}

Now set up the testing module with the test stubs for the Router and HeroService, and create a test instance of the DashboardComponent for subsequent testing.

beforeEach( async(() => {
  TestBed.configureTestingModule({
    providers: [
      { provide: HeroService, useClass: FakeHeroService },
      { provide: Router,      useClass: RouterStub }
    ]
  })
  .compileComponents().then(() => {
    fixture = TestBed.createComponent(DashboardComponent);
    comp = fixture.componentInstance;
  });

The following test clicks the displayed hero and confirms (with the help of a spy) that Router.navigateByUrl is called with the expected url.

    it('should tell ROUTER to navigate when hero clicked',
      inject([Router], (router: Router) => { // ...

      const spy = spyOn(router, 'navigateByUrl');

      heroClick(); // trigger click on first inner <div class="hero">

      // args passed to router.navigateByUrl()
      const navArgs = spy.calls.first().args[0];

      // expecting to navigate to id of the component's first hero
      const id = comp.heroes[0].id;
      expect(navArgs).toBe('/heroes/' + id,
        'should nav to HeroDetail for first hero');
    }));

The inject function

Notice the inject function in the second it argument.

it('should tell ROUTER to navigate when hero clicked',
  inject([Router], (router: Router) => { // ...
}));

The inject function is one of the Angular testing utilities. It injects services into the test function where you can alter, spy on, and manipulate them.

The inject function has two parameters:

1. An array of Angular dependency injection tokens.
2. A test function whose parameters correspond exactly to each item in the injection token array.

This example injects the Router from the current TestBed injector. That's fine for this test because the Router is, and must be, provided by the application root injector.

If you need a service provided by the component's own injector, call fixture.debugElement.injector.get instead:

// UserService actually injected into the component
userService = fixture.debugElement.injector.get(UserService);

The inject function closes the current TestBed instance to further configuration. You cannot call any more TestBed configuration methods, not configureTestingModule nor any of the override... methods. The TestBed throws an error if you try.

Test a routed component with parameters

Clicking a Dashboard hero triggers navigation to heroes/:id, where :id is a route parameter whose value is the id of the hero to edit. That URL matches a route to the HeroDetailComponent.

The router pushes the :id token value into the ActivatedRoute.params Observable property, Angular injects the ActivatedRoute into the HeroDetailComponent, and the component extracts the id so it can fetch the corresponding hero via the HeroDetailService. Here's the HeroDetailComponent constructor:

constructor(
  private heroDetailService: HeroDetailService,
  private route:  ActivatedRoute,
  private router: Router) {
}

HeroDetailComponent subscribes to ActivatedRoute.params changes in its ngOnInit method.

ngOnInit(): void {
  // get hero when `id` param changes
  this.route.params.subscribe(p => this.getHero(p && p['id']));
}

A test can explore how the HeroDetailComponent responds to different id parameter values by manipulating the ActivatedRoute injected into the component's constructor.

By now you know how to stub the Router and a data service. Stubbing the ActivatedRoute follows the same pattern except for a complication: the ActivatedRoute.params is an Observable.

Create an Observable test double

The hero-detail.component.spec.ts relies on an ActivatedRouteStub to set ActivatedRoute.params values for each test. This is a cross-application, re-usable test helper class. Consider placing such helpers in a testing folder sibling to the app folder. This sample keeps ActivatedRouteStub in testing/router-stubs.ts:

import { BehaviorSubject } from 'rxjs/BehaviorSubject';

@Injectable()
export class ActivatedRouteStub {

  // ActivatedRoute.params is Observable
  private subject = new BehaviorSubject(this.testParams);
  params = this.subject.asObservable();

  // Test parameters
  private _testParams: {};
  get testParams() { return this._testParams; }
  set testParams(params: {}) {
    this._testParams = params;
    this.subject.next(params);
  }

  // ActivatedRoute.snapshot.params
  get snapshot() {
    return { params: this.testParams };
  }
}

Notable features of this stub are:

• The stub implements only two of the ActivatedRoute capabilities: params and snapshot.params.

• BehaviorSubject drives the stub's params Observable and returns the same value to every params subscriber until it's given a new value.

• The HeroDetailComponent chains its expressions to this stub params Observable which is now under the tester's control.

• Setting the testParams property causes the subject to push the assigned value into params. That triggers the HeroDetailComponent params subscription, described above, in the same way that navigation does.

• Setting the testParams property also updates the stub's internal value for the snapshot property to return.

Testing with the Observable test double

Here's a test demonstrating the component's behavior when the observed id refers to an existing hero:

  describe('when navigate to existing hero', () => {
    let expectedHero: Hero;

    beforeEach( async(() => {
      expectedHero = firstHero;
      activatedRoute.testParams = { id: expectedHero.id };
      createComponent();
    }));

    it('should display that hero\'s name', () => {
      expect(page.nameDisplay.textContent).toBe(expectedHero.name);
    });
  });

When the id cannot be found, the component should re-route to the HeroListComponent. The test suite setup provided the same RouterStub described above which spies on the router without actually navigating. This test supplies a "bad" id and expects the component to try to navigate.

describe('when navigate to non-existant hero id', () => {
  beforeEach( async(() => {
    activatedRoute.testParams = { id: 99999 };
    createComponent();
  }));

  it('should try to navigate back to hero list', () => {
    expect(page.gotoSpy.calls.any()).toBe(true, 'comp.gotoList called');
    expect(page.navSpy.calls.any()).toBe(true, 'router.navigate called');
  });
});

While this app doesn't have a route to the HeroDetailComponent that omits the id parameter, it might add such a route someday. The component should do something reasonable when there is no id.

In this implementation, the component should create and display a new hero. New heroes have id=0 and a blank name. This test confirms that the component behaves as expected:

describe('when navigate with no hero id', () => {
  beforeEach( async( createComponent ));

  it('should have hero.id === 0', () => {
    expect(comp.hero.id).toBe(0);
  });

  it('should display empty hero name', () => {
    expect(page.nameDisplay.textContent).toBe('');
  });
});

Use a page object to simplify setup

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

But there's already plenty of template complexity.

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

To fully exercise the component, the test needs a lot of setup:

• It must wait until a hero arrives before *ngIf allows any element in DOM.
• It needs references to the title <span> and the name <input> so it can inspect their values.
• It needs references to the two buttons so it can click them.
• It needs spies for some of the component and router methods.

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

Tame the madness with a Page class that simplifies access to component properties and encapsulates the logic that sets them. Here's the Page class for the hero-detail.component.spec.ts

class Page {
  gotoSpy:      jasmine.Spy;
  navSpy:       jasmine.Spy;

  saveBtn:      DebugElement;
  cancelBtn:    DebugElement;
  nameDisplay:  HTMLElement;
  nameInput:    HTMLInputElement;

  constructor() {
    const router = TestBed.get(Router); // get router from root injector
    this.gotoSpy = spyOn(comp, 'gotoList').and.callThrough();
    this.navSpy  = spyOn(router, 'navigate');
  }

  /** Add page elements after hero arrives */
  addPageElements() {
    if (comp.hero) {
      // have a hero so these elements are now in the DOM
      const buttons    = fixture.debugElement.queryAll(By.css('button'));
      this.saveBtn     = buttons[0];
      this.cancelBtn   = buttons[1];
      this.nameDisplay = fixture.debugElement.query(By.css('span')).nativeElement;
      this.nameInput   = fixture.debugElement.query(By.css('input')).nativeElement;
    }
  }
}

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.

/** Create the HeroDetailComponent, initialize it, set test variables  */
function createComponent() {
  fixture = TestBed.createComponent(HeroDetailComponent);
  comp    = fixture.componentInstance;
  page    = new Page();

  // 1st change detection triggers ngOnInit which gets a hero
  fixture.detectChanges();
  return fixture.whenStable().then(() => {
    // 2nd change detection displays the async-fetched hero
    fixture.detectChanges();
    page.addPageElements();
  });
}

The observable tests in the previous section demonstrate how createComponent and page keep the tests short and on message. There are no distractions: no waiting for promises to resolve and no searching the DOM for element values to compare.

Here are a few more HeroDetailComponent tests to drive the point home.

    it('should display that hero\'s name', () => {
      expect(page.nameDisplay.textContent).toBe(expectedHero.name);
    });

    it('should navigate when click cancel', () => {
      click(page.cancelBtn);
      expect(page.navSpy.calls.any()).toBe(true, 'router.navigate called');
    });

    it('should save when click save but not navigate immediately', () => {
      // Get service injected into component and spy on its`saveHero` method.
      // It delegates to fake `HeroService.updateHero` which delivers a safe test result.
      const hds = fixture.debugElement.injector.get(HeroDetailService);
      const saveSpy = spyOn(hds, 'saveHero').and.callThrough();

      click(page.saveBtn);
      expect(saveSpy.calls.any()).toBe(true, 'HeroDetailService.save called');
      expect(page.navSpy.calls.any()).toBe(false, 'router.navigate not called');
    });

    it('should navigate when click save and save resolves', fakeAsync(() => {
      click(page.saveBtn);
      tick(); // wait for async save to complete
      expect(page.navSpy.calls.any()).toBe(true, 'router.navigate called');
    }));

    it('should convert hero name to Title Case', () => {
      const inputName = 'quick BROWN  fox';
      const titleCaseName = 'Quick Brown  Fox';

      // simulate user entering new name into the input box
      page.nameInput.value = inputName;

      // dispatch a DOM event so that Angular learns of input value change.
      page.nameInput.dispatchEvent(newEvent('input'));

      // Tell Angular to update the output span through the title pipe
      fixture.detectChanges();

      expect(page.nameDisplay.textContent).toBe(titleCaseName);
    });

Setup with module imports

Earlier component tests configured the testing module with a few declarations like this:

// async beforeEach
beforeEach( async(() => {
  TestBed.configureTestingModule({
    declarations: [ DashboardHeroComponent ],
  })
  .compileComponents(); // compile template and css
}));

The DashboardComponent is simple. It needs no help. But more complex components often depend on other components, directives, pipes, and providers and these must be added to the testing module too.

Fortunately, the TestBed.configureTestingModule parameter parallels the metadata passed to the @NgModule decorator which means you can also specify providers and imports.

The HeroDetailComponent requires a lot of help despite its small size and simple construction. In addition to the support it receives from the default testing module CommonModule, it needs:

• NgModel and friends in the FormsModule to enable two-way data binding.
• The TitleCasePipe from the shared folder.
• Router services (which these tests are stubbing).
• Hero data access services (also stubbed).

One approach is to configure the testing module from the individual pieces as in this example:

beforeEach( async(() => {
   TestBed.configureTestingModule({
    imports:      [ FormsModule ],
    declarations: [ HeroDetailComponent, TitleCasePipe ],
    providers: [
      { provide: ActivatedRoute, useValue: activatedRoute },
      { provide: HeroService,    useClass: FakeHeroService },
      { provide: Router,         useClass: RouterStub},
    ]
  })
  .compileComponents();
}));

Because many app components need the FormsModule and the TitleCasePipe, the developer created a SharedModule to combine these and other frequently requested parts. The test configuration can use the SharedModule too as seen in this alternative setup:

beforeEach( async(() => {
  TestBed.configureTestingModule({
    imports:      [ SharedModule ],
    declarations: [ HeroDetailComponent ],
    providers: [
      { provide: ActivatedRoute, useValue: activatedRoute },
      { provide: HeroService,    useClass: FakeHeroService },
      { provide: Router,         useClass: RouterStub},
    ]
  })
  .compileComponents();
}));

It's a bit tighter and smaller, with fewer import statements (not shown).

Import the feature module

The HeroDetailComponent is part of the HeroModule Feature Module that aggregates more of the interdependent pieces including the SharedModule. Try a test configuration that imports the HeroModule like this one:

beforeEach( async(() => {
   TestBed.configureTestingModule({
    imports:   [ HeroModule ],
    providers: [
      { provide: ActivatedRoute, useValue: activatedRoute },
      { provide: HeroService,    useClass: FakeHeroService },
      { provide: Router,         useClass: RouterStub},
    ]
  })
  .compileComponents();
}));

That's really crisp. Only the test doubles in the providers remain. Even the HeroDetailComponent declaration is gone.

Override a component's providers

The HeroDetailComponent provides its own HeroDetailService.

@Component({
  selector:    'app-hero-detail',
  templateUrl: './hero-detail.component.html',
  styleUrls:  ['./hero-detail.component.css' ],
  providers:  [ HeroDetailService ]
})
export class HeroDetailComponent implements OnInit {
  constructor(
    private heroDetailService: HeroDetailService,
    private route:  ActivatedRoute,
    private router: 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 been creating new instances of the real HeroDetailService all along!

@Injectable()
export class HeroDetailService {
  constructor(private heroService: HeroService) {  }
  /* . . . */
}

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(() => {
    TestBed.configureTestingModule({
      imports:   [ HeroModule ],
      providers: [
        { provide: ActivatedRoute, useValue: activatedRoute },
        { provide: Router,         useClass: RouterStub},
      ]
    })

    // Override component's own provider
    .overrideComponent(HeroDetailComponent, {
      set: {
        providers: [
          { provide: HeroDetailService, useClass: HeroDetailServiceSpy }
        ]
      }
    })

    .compileComponents();
  }));

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

The overrideComponent method

Focus on the overrideComponent method.

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

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

type MetadataOverride = {
  add?: T;
  remove?: T;
  set?: 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 HeroDetailComponent tests will assert that methods of the HeroDetailService were called by spying on the service methods. Accordingly, the stub implements its methods as spies:

class HeroDetailServiceSpy {
  testHero = new Hero(42, 'Test Hero');

  getHero = jasmine.createSpy('getHero').and.callFake(
    () => Promise
      .resolve(true)
      .then(() => Object.assign({}, this.testHero))
  );

  saveHero = jasmine.createSpy('saveHero').and.callFake(
    (hero: Hero) => Promise
      .resolve(true)
      .then(() => 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(() => {
  createComponent();
  // get the component's injected HeroDetailServiceSpy
  hdsSpy = fixture.debugElement.injector.get(HeroDetailService);
}));

it('should have called `getHero`', () => {
  expect(hdsSpy.getHero.calls.count()).toBe(1, 'getHero called once');
});

it('should display stub hero\'s name', () => {
  expect(page.nameDisplay.textContent).toBe(hdsSpy.testHero.name);
});

it('should save stub hero change', fakeAsync(() => {
  const origName = hdsSpy.testHero.name;
  const newName = 'New Name';

  page.nameInput.value = newName;
  page.nameInput.dispatchEvent(newEvent('input')); // tell Angular

  expect(comp.hero.name).toBe(newName, 'component hero has new name');
  expect(hdsSpy.testHero.name).toBe(origName, 'service hero unchanged before save');

  click(page.saveBtn);
  expect(hdsSpy.saveHero.calls.count()).toBe(1, 'saveHero called once');

  tick(); // wait for async save to complete
  expect(hdsSpy.testHero.name).toBe(newName, 'service hero has new name after save');
  expect(page.navSpy.calls.any()).toBe(true, 'router.navigate called');
}));

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.

Test a RouterOutlet component

The AppComponent displays routed components in a <router-outlet>. It also displays a navigation bar with anchors and their RouterLink directives.

<app-banner></app-banner>
<app-welcome></app-welcome>

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

<router-outlet></router-outlet>

The component class does nothing.

import { Component } from '@angular/core';
@Component({
  selector: 'my-app',
  templateUrl: './app.component.html'
})
export class AppComponent { }

Unit tests can confirm that the anchors are wired properly without engaging the router. See why this is worth doing below.

Stubbing unneeded components

The test setup should look familiar.

beforeEach( async(() => {
  TestBed.configureTestingModule({
    declarations: [
      AppComponent,
      BannerComponent, WelcomeStubComponent,
      RouterLinkStubDirective, RouterOutletStubComponent
    ]
  })

  .compileComponents()
  .then(() => {
    fixture = TestBed.createComponent(AppComponent);
    comp    = fixture.componentInstance;
  });
}));

The AppComponent is the declared test subject.

The setup extends the default testing module with one real component (BannerComponent) and several stubs.

• BannerComponent is simple and harmless to use as is.

• The real WelcomeComponent has an injected service. WelcomeStubComponent is a placeholder with no service to worry about.

• The real RouterOutlet is complex and errors easily. The RouterOutletStubComponent (in testing/router-stubs.ts) is safely inert.

The component stubs are essential. Without them, the Angular compiler doesn't recognize the <app-welcome> and <router-outlet> tags and throws an error.

Stubbing the RouterLink

The RouterLinkStubDirective contributes substantively to the test:

@Directive({
  selector: '[routerLink]',
  host: {
    '(click)': 'onClick()'
  }
})
export class RouterLinkStubDirective {
  @Input('routerLink') linkParams: any;
  navigatedTo: any = null;

  onClick() {
    this.navigatedTo = this.linkParams;
  }
}

The host metadata property wires the click event of the host element (the <a>) to the directive's onClick method. The URL bound to the [routerLink] attribute flows to the directive's linkParams property. Clicking the anchor should trigger the onClick method which sets the telltale navigatedTo property. Tests can inspect that property to confirm the expected click-to-navigation behavior.

By.directive and injected directives

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

beforeEach(() => {
  // trigger initial data binding
  fixture.detectChanges();

  // find DebugElements with an attached RouterLinkStubDirective
  linkDes = fixture.debugElement
    .queryAll(By.directive(RouterLinkStubDirective));

  // get the attached link directive instances using the DebugElement injectors
  links = linkDes
    .map(de => de.injector.get(RouterLinkStubDirective) as RouterLinkStubDirective);
});

Two points of special interest:

1. You can locate elements by directive, using By.directive, not just by css selectors.

2. You can use the component's dependency injector to get an attached directive because Angular always adds attached directives to the component's injector.

Here are some tests that leverage this setup:

it('can get RouterLinks from template', () => {
  expect(links.length).toBe(3, 'should have 3 links');
  expect(links[0].linkParams).toBe('/dashboard', '1st link should go to Dashboard');
  expect(links[1].linkParams).toBe('/heroes', '1st link should go to Heroes');
});

it('can click Heroes link in template', () => {
  const heroesLinkDe = linkDes[1];
  const heroesLink = links[1];

  expect(heroesLink.navigatedTo).toBeNull('link should not have navigated yet');

  heroesLinkDe.triggerEventHandler('click', null);
  fixture.detectChanges();

  expect(heroesLink.navigatedTo).toBe('/heroes');
});

What good are these tests?

Stubbed RouterLink tests can confirm that a component with links and an outlet is setup properly, that the component has the links it should have, and that they are all pointing in the expected direction. These tests do not concern whether the app will succeed in navigating to the target component when the user clicks a link.

Stubbing the RouterLink and RouterOutlet is the best option for such limited testing goals. Relying on the real router would make them brittle. They could fail for reasons unrelated to the component. For example, a navigation guard could prevent an unauthorized user from visiting the HeroListComponent. That's not the fault of the AppComponent and no change to that component could cure the failed test.

A different battery of tests can explore whether the application navigates as expected in the presence of conditions that influence guards such as whether the user is authenticated and authorized.

"Shallow component tests" with NO_ERRORS_SCHEMA

The previous setup declared the BannerComponent and stubbed two other components for no reason other than to avoid a compiler error.

Without them, the Angular compiler doesn't recognize the <app-banner>, <app-welcome> and <router-outlet> tags in the app.component.html template and throws an error.

Add NO_ERRORS_SCHEMA to the testing module's schemas metadata to tell the compiler to ignore unrecognized elements and attributes. You no longer have to declare irrelevant components and directives.

These tests are shallow because they only "go deep" into the components you want to test.

Here is a setup, with import statements, that demonstrates the improved simplicity of shallow tests, relative to the stubbing setup.

import { NO_ERRORS_SCHEMA }          from '@angular/core';
import { AppComponent }              from './app.component';
import { RouterOutletStubComponent } from '../testing';

beforeEach( async(() => {
  TestBed.configureTestingModule({
    declarations: [ AppComponent, RouterLinkStubDirective ],
    schemas:      [ NO_ERRORS_SCHEMA ]
  })

  .compileComponents()
  .then(() => {
    fixture = TestBed.createComponent(AppComponent);
    comp    = fixture.componentInstance;
  });
}));

  import { Component }                 from '@angular/core';
  import { AppComponent }              from './app.component';
  import { BannerComponent }           from './banner.component';
  import { RouterLinkStubDirective }   from '../testing';
  import { RouterOutletStubComponent } from '../testing';

  @Component({selector: 'app-welcome', template: ''})
  class WelcomeStubComponent {}

  beforeEach( async(() => {
    TestBed.configureTestingModule({
      declarations: [
        AppComponent,
        BannerComponent, WelcomeStubComponent,
        RouterLinkStubDirective, RouterOutletStubComponent
      ]
    })

    .compileComponents()
    .then(() => {
      fixture = TestBed.createComponent(AppComponent);
      comp    = fixture.componentInstance;
    });
  }));

The only declarations are the component-under-test (AppComponent) and the RouterLinkStubDirective that contributes actively to the tests. The tests in this example are unchanged.

Test an attribute directive

An attribute directive modifies the behavior of an element, component or another directive. Its name reflects the way the directive is applied: as an attribute on a host element.

The sample application's HighlightDirective sets the background color of an element based on either a data bound color or a default color (lightgray). It also sets a custom property of the element (customProperty) to true for no reason other than to show that it can.

import { Directive, ElementRef, Input, OnChanges } from '@angular/core';

@Directive({ selector: '[highlight]' })
/** Set backgroundColor for the attached element to highlight color
 *  and set the element's customProperty to true */
export class HighlightDirective implements OnChanges {

  defaultColor =  'rgb(211, 211, 211)'; // lightgray

  @Input('highlight') bgColor: string;

  constructor(private el: ElementRef) {
    el.nativeElement.style.customProperty = true;
  }

  ngOnChanges() {
    this.el.nativeElement.style.backgroundColor = this.bgColor || this.defaultColor;
  }
}

It's used throughout the application, perhaps most simply in the AboutComponent:

import { Component } from '@angular/core';
@Component({
  template: `
  <h2 highlight="skyblue">About</h2>
  <twain-quote></twain-quote>
  <p>All about this sample</p>`
})
export class AboutComponent { }

Testing the specific use of the HighlightDirective within the AboutComponent requires only the techniques explored above (in particular the "Shallow test" approach).

beforeEach(() => {
  fixture = TestBed.configureTestingModule({
    declarations: [ AboutComponent, HighlightDirective],
    schemas:      [ NO_ERRORS_SCHEMA ]
  })
  .createComponent(AboutComponent);
  fixture.detectChanges(); // initial binding
});

it('should have skyblue <h2>', () => {
  const de = fixture.debugElement.query(By.css('h2'));
  const bgColor = de.nativeElement.style.backgroundColor;
  expect(bgColor).toBe('skyblue');
});

However, testing a single use case is unlikely to explore the full range of a directive's capabilities. Finding and testing all components that use the directive is tedious, brittle, and almost as unlikely to afford full coverage.

Isolated unit tests might be helpful, but attribute directives like this one tend to manipulate the DOM. Isolated unit tests don't touch the DOM and, therefore, do not inspire confidence in the directive's efficacy.

A better solution is to create an artificial test component that demonstrates all ways to apply the directive.

@Component({
  template: `
  <h2 highlight="yellow">Something Yellow</h2>
  <h2 highlight>The Default (Gray)</h2>
  <h2>No Highlight</h2>
  <input #box [highlight]="box.value" value="cyan"/>`
})
class TestComponent { }

Here are some tests of this component:

beforeEach(() => {
  fixture = TestBed.configureTestingModule({
    declarations: [ HighlightDirective, TestComponent ]
  })
  .createComponent(TestComponent);

  fixture.detectChanges(); // initial binding

  // all elements with an attached HighlightDirective
  des = fixture.debugElement.queryAll(By.directive(HighlightDirective));

  // the h2 without the HighlightDirective
  bareH2 = fixture.debugElement.query(By.css('h2:not([highlight])'));
});

// color tests
it('should have three highlighted elements', () => {
  expect(des.length).toBe(3);
});

it('should color 1st <h2> background "yellow"', () => {
  const bgColor = des[0].nativeElement.style.backgroundColor;
  expect(bgColor).toBe('yellow');
});

it('should color 2nd <h2> background w/ default color', () => {
  const dir = des[1].injector.get(HighlightDirective) as HighlightDirective;
  const bgColor = des[1].nativeElement.style.backgroundColor;
  expect(bgColor).toBe(dir.defaultColor);
});

it('should bind <input> background to value color', () => {
  // easier to work with nativeElement
  const input = des[2].nativeElement as HTMLInputElement;
  expect(input.style.backgroundColor).toBe('cyan', 'initial backgroundColor');

  // dispatch a DOM event so that Angular responds to the input value change.
  input.value = 'green';
  input.dispatchEvent(newEvent('input'));
  fixture.detectChanges();

  expect(input.style.backgroundColor).toBe('green', 'changed backgroundColor');
});


it('bare <h2> should not have a customProperty', () => {
  expect(bareH2.properties['customProperty']).toBeUndefined();
});

A few techniques are noteworthy:

• The By.directive predicate is a great way to get the elements that have this directive when their element types are unknown.

• The :not pseudo-class in By.css('h2:not([highlight])') helps find <h2> elements that do not have the directive. By.css('*:not([highlight])') finds any element that does not have the directive.

• Angular adds a directive to the injector of the element to which it is applied. The test for the default color uses the injector of the second <h2> to get its HighlightDirective instance and its defaultColor.

Isolated Unit Tests

Testing applications with the help of the Angular testing utilities is the main focus of this guide.

However, it's often more productive to explore the inner logic of application classes with isolated unit tests that don't depend upon Angular. Such tests are often smaller and easier to read, write, and maintain.

They don't carry extra baggage:

• Import from the Angular test libraries.
• Configure a module.
• Prepare dependency injection providers.
• Call inject or async or fakeAsync.

They follow patterns familiar to test developers everywhere:

• Exhibit standard, Angular-agnostic testing techniques.
• Create instances directly with new.
• Substitute test doubles (stubs, spys, and mocks) for the real dependencies.

Services

Services are good candidates for isolated unit testing. Here are some synchronous and asynchronous unit tests of the FancyService written without assistance from Angular testing utilities.

// Straight Jasmine - no imports from Angular test libraries

describe('FancyService without the TestBed', () => {
  let service: FancyService;

  beforeEach(() => { service = new FancyService(); });

  it('#getValue should return real value', () => {
    expect(service.getValue()).toBe('real value');
  });

  it('#getAsyncValue should return async value', done => {
    service.getAsyncValue().then(value => {
      expect(value).toBe('async value');
      done();
    });
  });

  it('#getTimeoutValue should return timeout value',  done => {
    service = new FancyService();
    service.getTimeoutValue().then(value => {
      expect(value).toBe('timeout value');
      done();
    });
  });

  it('#getObservableValue should return observable value', done => {
    service.getObservableValue().subscribe(value => {
      expect(value).toBe('observable value');
      done();
    });
  });

});

A rough line count suggests that these isolated unit tests are about 25% smaller than equivalent Angular tests. That's telling but not decisive. The benefit comes from reduced setup and code complexity.

Compare these equivalent tests of FancyService.getTimeoutValue.

it('#getTimeoutValue should return timeout value',  done => {
  service = new FancyService();
  service.getTimeoutValue().then(value => {
    expect(value).toBe('timeout value');
    done();
  });
});

beforeEach(() => {
  TestBed.configureTestingModule({ providers: [FancyService] });
});

it('test should wait for FancyService.getTimeoutValue',
  async(inject([FancyService], (service: FancyService) => {

  service.getTimeoutValue().then(
    value => expect(value).toBe('timeout value')
  );
})));

They have about the same line-count, but the Angular-dependent version has more moving parts including a couple of utility functions (async and inject). Both approaches work and it's not much of an issue if you're using the Angular testing utilities nearby for other reasons. On the other hand, why burden simple service tests with added complexity?

Pick the approach that suits you.

Services with dependencies

Services often depend on other services that Angular injects into the constructor. You can test these services without the TestBed. In many cases, it's easier to create and inject dependencies by hand.

The DependentService is a simple example:

@Injectable()
export class DependentService {
  constructor(private dependentService: FancyService) { }
  getValue() { return this.dependentService.getValue(); }
}

It delegates its only method, getValue, to the injected FancyService.

Here are several ways to test it.

describe('DependentService without the TestBed', () => {
  let service: DependentService;

  it('#getValue should return real value by way of the real FancyService', () => {
    service = new DependentService(new FancyService());
    expect(service.getValue()).toBe('real value');
  });

  it('#getValue should return faked value by way of a fakeService', () => {
    service = new DependentService(new FakeFancyService());
    expect(service.getValue()).toBe('faked value');
  });

  it('#getValue should return faked value from a fake object', () => {
    const fake =  { getValue: () => 'fake value' };
    service = new DependentService(fake as FancyService);
    expect(service.getValue()).toBe('fake value');
  });

  it('#getValue should return stubbed value from a FancyService spy', () => {
    const fancy = new FancyService();
    const stubValue = 'stub value';
    const spy = spyOn(fancy, 'getValue').and.returnValue(stubValue);
    service = new DependentService(fancy);

    expect(service.getValue()).toBe(stubValue, 'service returned stub value');
    expect(spy.calls.count()).toBe(1, 'stubbed method was called once');
    expect(spy.calls.mostRecent().returnValue).toBe(stubValue);
  });
});

The first test creates a FancyService with new and passes it to the DependentService constructor.

However, it's rarely that simple. The injected service can be difficult to create or control. You can mock the dependency, use a dummy value, or stub the pertinent service method with a substitute method that's easy to control.

These isolated unit testing techniques are great for exploring the inner logic of a service or its simple integration with a component class. Use the Angular testing utilities when writing tests that validate how a service interacts with components within the Angular runtime environment.

Pipes

Pipes are easy to test without the Angular testing utilities.

A pipe class has one method, transform, that manipulates the input value into a transformed output value. The transform implementation rarely interacts with the DOM. Most pipes have no dependence on Angular other than the @Pipe metadata and an interface.

Consider a TitleCasePipe that capitalizes the first letter of each word. Here's a naive implementation with a regular expression.

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({name: 'titlecase', pure: false})
/** Transform to Title Case: uppercase the first letter of the words in a string.*/
export class TitleCasePipe implements PipeTransform {
  transform(input: string): string {
    return input.length === 0 ? '' :
      input.replace(/\w\S*/g, (txt => txt[0].toUpperCase() + txt.substr(1).toLowerCase() ));
  }
}

Anything that uses a regular expression is worth testing thoroughly. Use simple Jasmine to explore the expected cases and the edge cases.

describe('TitleCasePipe', () => {
  // This pipe is a pure, stateless function so no need for BeforeEach
  let pipe = new TitleCasePipe();

  it('transforms "abc" to "Abc"', () => {
    expect(pipe.transform('abc')).toBe('Abc');
  });

  it('transforms "abc def" to "Abc Def"', () => {
    expect(pipe.transform('abc def')).toBe('Abc Def');
  });

  // ... more tests ...
});

Write Angular tests too

These are tests of the pipe in isolation. They can't tell if the TitleCasePipe is working properly as applied in the application components.

Consider adding component tests such as this one:

it('should convert hero name to Title Case', () => {
  const inputName = 'quick BROWN  fox';
  const titleCaseName = 'Quick Brown  Fox';

  // simulate user entering new name into the input box
  page.nameInput.value = inputName;

  // dispatch a DOM event so that Angular learns of input value change.
  page.nameInput.dispatchEvent(newEvent('input'));

  // Tell Angular to update the output span through the title pipe
  fixture.detectChanges();

  expect(page.nameDisplay.textContent).toBe(titleCaseName);
});

Components

Component tests typically examine how a component class interacts with its own template or with collaborating components. The Angular testing utilities are specifically designed to facilitate such tests.

Consider this ButtonComp component.

@Component({
  selector: 'button-comp',
  template: `
    <button (click)="clicked()">Click me!</button>
    <span>{{message}}</span>`
})
export class ButtonComponent {
  isOn = false;
  clicked() { this.isOn = !this.isOn; }
  get message() { return `The light is ${this.isOn ? 'On' : 'Off'}`; }
}

The following Angular test demonstrates that clicking a button in the template leads to an update of the on-screen message.

it('should support clicking a button', () => {
  const fixture = TestBed.createComponent(ButtonComponent);
  const btn  = fixture.debugElement.query(By.css('button'));
  const span = fixture.debugElement.query(By.css('span')).nativeElement;

  fixture.detectChanges();
  expect(span.textContent).toMatch(/is off/i, 'before click');

  click(btn);
  fixture.detectChanges();
  expect(span.textContent).toMatch(/is on/i, 'after click');
});

The assertions verify that the data values flow from one HTML control (the <button>) to the component and from the component back to a different HTML control (the <span>). A passing test means the component and its template are wired correctly.

Isolated unit tests can more rapidly probe a component at its API boundary, exploring many more conditions with less effort.

Here are a set of unit tests that verify the component's outputs in the face of a variety of component inputs.

describe('ButtonComp', () => {
  let comp: ButtonComponent;
  beforeEach(() => comp = new ButtonComponent());

  it('#isOn should be false initially', () => {
    expect(comp.isOn).toBe(false);
  });

  it('#clicked() should set #isOn to true', () => {
    comp.clicked();
    expect(comp.isOn).toBe(true);
  });

  it('#clicked() should set #message to "is on"', () => {
    comp.clicked();
    expect(comp.message).toMatch(/is on/i);
  });

  it('#clicked() should toggle #isOn', () => {
    comp.clicked();
    expect(comp.isOn).toBe(true);
    comp.clicked();
    expect(comp.isOn).toBe(false);
  });
});

Isolated component tests offer a lot of test coverage with less code and almost no setup. This is even more of an advantage with complex components, which may require meticulous preparation with the Angular testing utilities.

On the other hand, isolated unit tests can't confirm that the ButtonComp is properly bound to its template or even data bound at all. Use Angular tests for that.

Angular testing utility APIs

This section takes inventory of the most useful Angular testing features and summarizes what they do.

The Angular testing utilities include the TestBed, the ComponentFixture, and a handful of functions that control the test environment. The TestBed and ComponentFixture classes are covered separately.

Here's a summary of the stand-alone functions, in order of likely utility:

TestBed class summary

The TestBed class is one of the principal Angular testing utilities. Its API is quite large and can be overwhelming until you've explored it, a little at a time. Read the early part of this guide first to get the basics before trying to absorb the full API.

The module definition passed to configureTestingModule is a subset of the @NgModule metadata properties.

type TestModuleMetadata = {
  providers?: any[];
  declarations?: any[];
  imports?: any[];
  schemas?: Array<SchemaMetadata | any[]>;
};

Each override method takes a MetadataOverride<T> where T is the kind of metadata appropriate to the method, that is, the parameter of an @NgModule, @Component, @Directive, or @Pipe.

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

The TestBed API consists of static class methods that either update or reference a global instance of theTestBed.

Internally, all static methods cover methods of the current runtime TestBed instance, which is also returned by the getTestBed() function.

Call TestBed methods within a beforeEach() to ensure a fresh start before each individual test.

Here are the most important static methods, in order of likely utility.

service = TestBed.get(FancyService, null);

A few of the TestBed instance methods are not covered by static TestBed class methods. These are rarely needed.

The ComponentFixture

The TestBed.createComponent<T> creates an instance of the component T and returns a strongly typed ComponentFixture for that component.

The ComponentFixture properties and methods provide access to the component, its DOM representation, and aspects of its Angular environment.

ComponentFixture properties

Here are the most important properties for testers, in order of likely utility.

ComponentFixture methods

The fixture methods cause Angular to perform certain tasks on the component tree. Call these method to trigger Angular behavior in response to simulated user action.

Here are the most useful methods for testers.

DebugElement

The DebugElement provides crucial insights into the component's DOM representation.

From the test root component's DebugElement returned by fixture.debugElement, you can walk (and query) the fixture's entire element and component subtrees.

Here are the most useful DebugElement members for testers, in approximate order of utility:

The DebugElement.query(predicate) and DebugElement.queryAll(predicate) methods take a predicate that filters the source element's subtree for matching DebugElement.

The predicate is any method that takes a DebugElement and returns a truthy value. The following example finds all DebugElements with a reference to a template local variable named "content":

// Filter for DebugElements with a #content reference
const contentRefs = el.queryAll( de => de.references['content']);

The Angular By class has three static methods for common predicates:

• By.all - return all elements.
• By.css(selector) - return elements with matching CSS selectors.
• By.directive(directive) - return elements that Angular matched to an instance of the directive class.

// Can find DebugElement either by css selector or by directive
const h2        = fixture.debugElement.query(By.css('h2'));
const directive = fixture.debugElement.query(By.directive(HighlightDirective));

Test environment setup files

Unit testing requires some configuration and bootstrapping that is captured in setup files. The setup files for this guide are provided for you when you follow the Setup instructions. The CLI delivers similar files with the same purpose.

Here's a brief description of this guide's setup files:

/** App specific SystemJS configuration */
System.config({
  packages: {
    // barrels
    'app/model': {main:'index.js', defaultExtension:'js'},
    'app/model/testing': {main:'index.js', defaultExtension:'js'}
  }
});

npm packages

The sample tests are written to run in Jasmine and karma. The two "fast path" setups added the appropriate Jasmine and karma npm packages to the devDependencies section of the package.json. They're installed when you run npm install.

Why put specs next to the things they test?

It's a good idea to put unit test spec files in the same folder as the application source code files that they test:

• Such tests are easy to find.
• You see at a glance if a part of your application lacks tests.
• Nearby tests can reveal how a part works in context.
• When you move the source (inevitable), you remember to move the test.
• When you rename the source file (inevitable), you remember to rename the test file.

When would I put specs in a test folder?

Application integration specs can test the interactions of multiple parts spread across folders and modules. They don't really belong to any part in particular, so they don't have a natural home next to any one file.

It's often better to create an appropriate folder for them in the tests directory.

Of course specs that test the test helpers belong in the test folder, next to their corresponding helper files.