angular-docs-cn/public/docs/ts/latest/guide/testing.jade

block includes
  include ../_util-fns
  - var _JavaScript = 'JavaScript';
  //- Double underscore means don't escape var, use !{__var}.
  - var __chaining_op = '<code>;</code> or <code>,</code>';
  - var __new_op = '<code>new</code>';
  - var __objectAsMap = 'object';

:marked
  This chapter offers tips and techniques for testing Angular applications. 
  Along the way you will learn some general testing principles and techniques but the focus is on
  Angular testing.

#top
:marked
  # Table of Contents
  1. [Introduction to Angular Testing](#testing-intro)
  <br><br>
  1. [Setup](#setup)
    - [setup files](#setup-files): `karma.conf`, `karma-test-shim`, `systemjs.config`
    - [npm packages](#npm-packages)
  1. [The first karma test](#1st-karma-test)
  <br><br>
  1. [The Angular Testing Platform (ATP) ](#atp-intro)
  <br><br>
  1. [The sample application and its tests](#sample-app)
  <br><br>
  1. [A simple component test](#simple-component-test)
    - [_configureTestingModule_](#configure-testing-module)
    - [_createComponent_](#create-component)
    - [_detectChanges_](#detect-changes)
    - [_autoDetectChanges_](#auto-detect-changes)
  1. [Test a component with a service dependency](#component-with-dependency)
  <br><br>
  1. [Test a component with an async service](#component-with-async-service)
    - [spies](#service-spy)
    - [_async_](#async)
    - [_whenStable_](#when-stable)
    - [_fakeAsync_](#async)
    - [_tick_](#tick)
    - [_jasmine.done_](#jasmine-done)
  1. [Test a component with an external template](#component-with-external-template)
    - [_async_](#async-in-before-each) in `beforeEach`
    - [_compileComponents_](#compile-components)
  1. [Test a component with inputs and outputs](#component-with-inputs-output)
    - [_triggerEventHandler_](#trigger-event-handler)
  1. [Test a component inside a test host component](#component-inside-test-host)
  <br><br>
  1. [Test a routed component](#routed-component)
    - [_inject_](#inject)
  1. [Test a routed component with parameters](#routed-component-w-param)
    - [_Observable_ test double](#stub-observable)
  1. [Use a _page_ object to simplify setup](#page-object)
  <br><br>
  1. [Setup with module imports](#import-module)
  <br><br>
  1. [Override component providers](#component-override)
  <br><br>
  1. [Test a _RouterOutlet_ component](#router-outlet-component)
    - [stubbing unneeded components](#stub-component)
    - [Stubbing the _RouterLink_](#router-link-stub)
    - [_By.directive_ and injected directives](#by-directive)
  1. ["Shallow" component tests with  *NO\_ERRORS\_SCHEMA*](#shallow-component-test)
  <br><br>
  1. [Test an attribute directive](#attribute-directive)
  <br><br>  
  1. [Isolated tests](#testing-without-atp "Testing without the Angular Testing Platform")
    - [Services](#isolated-service-tests)
    - [Pipes](#isolated-pipe-tests)
    - [Components](#isolated-component-tests)
  1. [_Angular Testing Platform_ API](#atp-api)
    - [Stand-alone functions](#atp-api): `async`, `fakeAsync`, etc.
    - [_TestBed_](#testbed-class-summary)
    - [_ComponentFixture_](#component-fixture-class-summary)
    - [_DebugElement_](#debug-element-details)
  1. [FAQ](#faq "Frequently asked questions")
:marked
  It’s a big agenda. Fortunately, you can learn a little bit at a time and put each lesson to use.

  # Live examples
  The chapter sample code is available as live examples for inspection, experiment, and download.
  
  * <live-example>The sample application</live-example>
  * <live-example plnkr="1st-specs">The first spec</live-example>
  * <live-example plnkr="app-specs">The complete application specs</live-example>
  * <live-example plnkr="bag-specs">A grab bag of demonstration specs</live-example>
a(href="#top").to-top Back to top

.l-hr
#testing-intro
:marked
  # Introduction to Angular Testing

  You write tests to explore and confirm the behavior of the application.

  1. They **guard** against changes that break existing code (“regressions”).

  1. They **clarify** what the code does both when used as intended and when faced with deviant conditions.

  1. They **reveal** 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.

  This chapter assumes that you know something about testing. Don't worry if you don't. 
  There are plenty of books and online resources to get up to speed.

  <!-- TODO
  :marked
  ## Learn more
  Learn more about basic Jasmine testing here
  [Resources TBD](./#)
  -->


  ## Tools and Technologies

  You can write and run Angular tests with a variety of tools and technologies. 
  This chapter describes specific choices that are known to work well.
  
table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th Technology
    th Purpose
  tr(style=top)
    td(style="vertical-align: top") Jasmine
    td
      :marked
        The [Jasmine test framework](http://jasmine.github.io/2.4/introduction.html).
        provides everything needed to write basic tests. 
        It ships with an HTML test runner that executes tests in the browser.
  tr(style=top)
    td(style="vertical-align: top") Angular Testing Platform
    td
      :marked
        The Angular Testing Platform creates a test environment and harness
        for the application code under test. 
        Use it to condition and control parts of the application as they 
        interact _within_ the Angular environment.
  tr(style=top)
    td(style="vertical-align: top") Karma
    td
      :marked
        The [karma test runner](https://karma-runner.github.io/1.0/index.html)
        is ideal for writing and running tests while developing the application. 
        It can be an integral part of the application build process.
        This chapter describes how to setup and run tests with karma.
  tr(style=top)
    td(style="vertical-align: top") Protractor
    td
      :marked
        Use protractor to write and run _end-to-end_ (e2e) tests.
        End-to-end tests explore the application _as users experience it_.
        In e2e testing, one process runs the real application
        and a second process runs protractor tests that simulate user behavior 
        and assert that the application responds in the browser as expected.

.l-hr
#setup
:marked
  # Setup

  Many think writing tests is fun. 
  Few enjoy setting up the test environment.
  To get to the fun as quickly as possible, 
  the deep details of setup appear later in the chapter (_forthcoming_).
  A bare minimum of discussion plus the downloadable source code must suffice for now.

  There are two fast paths to getting started.
  1. Start a new project following the instructions in the 
  [QuickStart github repository](https://github.com/angular/quickstart/blob/master/README.md).

  1. Start a new project with the 
  [Angular CLI](https://github.com/angular/angular-cli/blob/master/README.md).

  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 chapter, the application and its tests are based on the QuickStart repo.

.alert.is-helpful
  :marked
    If your application was based on the QuickStart repository,
    you can skip the rest of this section and get on with your first test.
    The QuickStart repo provides all necessary setup.

#setup-files
:marked
  ### Setup files
  Here's brief description of the setup files.
 
table(width="100%")
  col(width="20%")
  col(width="80%") 
  tr
    th File
    th Description
  tr
    td(style="vertical-align: top") <code>karma.conf.js</code>
    td
      :marked
        The karma configuration file that specifies which plug-ins to use, 
        which application and test files to load, which browser(s) to use,
        and how to report test results.

        It loads three other setup files:
        * `systemjs.config.js` 
        * `systemjs.config.extras.js`
        * `karma-test-shim.js`
  tr
    td(style="vertical-align: top") <code>karma-test-shim.js</code>
    td
      :marked
        This shim prepares karma specifically for the Angular test environment 
        and launches karma itself. 
        It loads the `systemjs.config.js` file as part of that process.
  tr
    td(style="vertical-align: top") <code>systemjs.config.js</code>
    td
      :marked
        [SystemJS](https://github.com/systemjs/systemjs/blob/master/README.md) 
        loads the application and test files.
        This script tells SystemJS where to find those files and how to load them.
        It's the same version of `systemjs.config.js` used by QuickStart-based applications.
  tr
    td(style="vertical-align: top") <code>systemjs.config.extras.js</code>
    td
      :marked
        An optional file that supplements the SystemJS configuration in `systemjs.config.js` with
        configuration for the specific needs of the application itself.

        A stock `systemjs.config.js` can't anticipate those needs. 
        You fill the gaps here.

        The sample version for this chapter adds the **model barrel**
        to the SystemJs `packages` configuration.
  tr
    td(colspan="2") 
      +makeExample('testing/ts/systemjs.config.extras.js', '', 'systemjs.config.extras.js')(format='.')
 
:marked
  ### 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 were installed when you ran `npm install`.

.l-hr
#1st-karma-test
:marked
  # The first karma test

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

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

.alert.is-important
  :marked
    Tests written in Jasmine are called _specs_ . 
    **The filename extension must be `.spec.ts`**, 
    the convention adhered to by  `karma.conf.js` and other tooling.

:marked
  **Put spec files somewhere within the `app/` folder.**
  The `karma.conf.js` tells karma to look for spec files there,
  for reasons explained [below](#spec-file-location).

  Add the following code to `app/1st.spec.ts`.
+makeExample('testing/ts/app/1st.spec.ts', '', 'app/1st.spec.ts')(format='.')
:marked
  ## Run karma
  Compile and run it in karma from the command line.

.l-sub-section
  :marked
    The QuickStart repo adds the following command to the `scripts` section in  `package.json`.

  code-example(format="." language="bash").
    "test": "tsc && concurrently \"tsc -w\" \"karma start karma.conf.js\"",
  :marked
    Add that to your `package.json` if it's not there already.

:marked
  Open a terminal or command window and enter
code-example(format="." language="bash").
  npm test
:marked
  The command compiles the application and test code a first time. 
  If the compile fails, the command aborts.

  If it succeeds, the command re-compiles (this time in watch mode) in one process
  and starts karma in another.
  Both processes watch pertinent files and re-run when they detect changes.

  After a few moments, karma opens a browser ...
figure.image-display
  img(src='/resources/images/devguide/testing/karma-browser.png' style="width:400px;" alt="Karma browser")
:marked
  ... and starts writing to the console.

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

code-example(format="." language="bash").
  > npm test
  > tsc && concurrently "tsc -w" "karma start karma.conf.js"

  [0] 1:37:03 PM - Compilation complete. Watching for file changes.
  [1] 24 07 2016 13:37:09.310:WARN [karma]: No captured browser, open http://localhost:9876/
  [1] 24 07 2016 13:37:09.361:INFO [karma]: Karma v0.13.22 server started at http://localhost:9876/
  [1] 24 07 2016 13:37:09.370:INFO [launcher]: Starting browser Chrome
  [1] 24 07 2016 13:37:10.974:INFO [Chrome 51.0.2704]: Connected on socket /#Cf6A5PkvMzjbbtn1AAAA with id 24600087
  [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)

:marked
  Both the compiler and karma continue to run. The compiler output is preceeded by `[0]`; 
  the karma output by `[1]`.

  Change the expectation from `true` to `false`.

  The _compiler_ watcher detects the change and recompiles.

code-example(format="." language="bash").
  [0] 1:49:21 PM - File change detected. Starting incremental compilation...
  [0] 1:49:25 PM - Compilation complete. Watching for file changes.

:marked
  The _karma_ watcher detects the change to the compilation output and re-runs the test.
code-example(format="." language="bash").
  [1] Chrome 51.0.2704: Executed 0 of 1 SUCCESS
      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)

:marked
  It failed of course.

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

.alert.is-helpful
  :marked
    The console log can be quite long. Keep your eye on the last line.
    It says `SUCCESS` when all is well.
    
    If it says `FAILED`, scroll up to look for the error or, if that's too painful,
    pipe the console output to a file and inspect with your favorite editor.
  code-example(format="." language="json").
    npm test > spec-output.txt

:marked
  ## Test debugging
  
  Debug specs in the browser in the same way you debug an application.

    - Reveal the karma browser window (hidden earlier).
    - Open the browser's “Developer Tools” (F12 or Ctrl-Shift-I).
    - Pick the “sources” section
    - Open the `1st.spec.ts` test file (Ctrl-P, then start typing the name of the file).
    - Set a breakpoint in the test
    - Refresh the browser … and it stops at the breakpoint.
 
figure.image-display
  img(src='/resources/images/devguide/testing/karma-1st-spec-debug.png' style="width:700px;" alt="Karma debugging")

a(href="#top").to-top Back to top

.l-hr
#atp-intro
:marked
  # The Angular Testing Platform (ATP)

  Many tests explore how applications classes interact with Angular and the DOM while under Angular's control.

  Such tests are easy to write with the help of the _Angular Testing Platform_ (ATP)
  which consists of the `TestBed` class and some helper functions.

  Tests written with the _Angular Testing Platform_ are the main focus of this chapter. 
  But they are not the only tests you should write.  

  ### Isolated unit tests

  You can and should write [isolated unit tests](#testing-without-atp "Testing without the Angular Testing Platform") 
  for components, directives, pipes, and services.
  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.

  Isolated tests don't reveal how the class interacts with Angular. 
  In particular, they can't reveal how a component class interacts with its own template or with other components.

  Those tests require the Angular Testing Platform.

  ### Testing with the _ Angular Testing Platform_

  The  _Angular Testing Platform_ consists of the `TestBed` class and some helper functions from `@angular/core/testing`.

  The `TestBed` creates an Angular testing module &mdash; an `@NgModule` class &mdash;
  that you configure to produce the module environment for the class you want to test. 
  You tell the `TestBed` to create an instance of the test component and probe that instance with tests.

  That's the `TestBed` in a nutshell.

  In practice, you work with the static methods of the `TestBed` class. 
  These static methods create and update a fresh hidden `TestBed` instance before each Jasmine `it`.
.l-sub-section
  :marked
    You can access that hidden instance anytime by calling `getTestBed()`;
:marked
  Thanks to initialization in the [testing shims](#setup), 
  the default `TestBed` instance is pre-configured with a baseline of default providers and declarables (components, directives, and pipes)
  that almost everyone needs. 

  The shims in this chapter are designed for testing a browser application so the default configuration includes the `CommonModule` declarables from `@angular/common`
  and the `BrowserModule` providers (some of them mocked) from `@angular/platform-browser`.

  This default testing module configuration is a _foundation_ for testing _any_ browser app.
  You call `TestBed.configureTestingModule` with an object that defines additional imports, declarations, providers and schemas
  to reshape the testing module to fit your application tests.
  Optional `override...` methods can fine-tune aspects of the configuration.

  After configuring the `TestBed`, tell it to create an instance of the test component and the test fixture
  that you'll need to inspect and control the component's immediate environment.
  
+makeExample('testing/ts/app/banner.component.spec.ts', 'simple-example-before-each', 'app/banner.component.spec.ts (simplified)')(format='.')
:marked
  Angular tests can interact with the HTML in the test DOM, 
  simulate user activity, tell Angular to perform specific task (such as change detection), 
  and see the effects of these actions both in the test component and in the test DOM.
+makeExample('testing/ts/app/banner.component.spec.ts', 'simple-example-it', 'app/banner.component.spec.ts (simplified)')(format='.')
:marked
  A comprehensive review of the _Angular Testing Platform_ APIs appears [later in the chapter](#atp-api).
  Let's dive right into Angular testing, starting with with the components of a sample application.

a(href="#top").to-top Back to top

.l-hr

#sample-app
:marked
  # The sample application and its tests

  This chapter tests a cut-down version of the _Tour of Heroes_ [tutorial app](../tutorial).
  
  The following live example shows how it works and provides the complete source code.
  Give it some time to load and warm up.
<live-example embedded img="devguide/testing/app-plunker.png"></live-example>
<br><br>
:marked
  The following live example runs all the tests of this application
  inside the browser, using the Jasmine Test Runner instead of karma.

  It includes the tests discussed in this chapter and additional tests for you to explore.
  This live example contains both application and test code. 
  It is large and can take up to a minute to start. Please be patient.
<live-example plnkr="app-specs" embedded img="devguide/testing/app-specs-plunker.png"></live-example>

a(href="#top").to-top Back to top
.l-hr

#simple-component-test
:marked
  # Test a component

  The top of the screen displays application title, presented by the `BannerComponent` in `app/banner.component.ts`.
+makeExample('testing/ts/app/banner.component.ts', '', 'app/banner.component.ts')(format='.')
:marked
  `BannerComponent` has an inline template and an interpolation binding, about as simple as it gets.
  Probably too simple to be worth testing in real life but perfect for a first encounter with the `TestBed`.

  The corresponding `app/banner-component.spec.ts` sits in the same folder as the component,
  for reasons explained [here](#q-spec-file-location);

  Start with ES6 import statements to get access to symbols referenced in the spec.
+makeExample('testing/ts/app/banner.component.spec.ts', 'imports', 'app/banner.component.spec.ts (imports)')(format='.')

#configure-testing-module
:marked
  Here's the setup for the tests followed by observations about the `beforeEach`:
+makeExample('testing/ts/app/banner.component.spec.ts', 'setup', 'app/banner.component.spec.ts (setup)')(format='.')
:marked
  `TestBed.configureTestingModule` takes an `@NgModule`-like metadata object.
  This one simply declares the component to test, `BannerComponent`.
  
  It lacks `imports` because (a) it extends the default testing module configuration which 
  already has what `BannerComponent` needs
  and (b) `BannerComponent` doesn't interact with any other components.

  The configuration could have imported `AppModule` (which declares `BannerComponent`).
  But that would lead to tons more configuration in order to support the other components within `AppModule` 
  that have nothing to do with `BannerComponent`.

#create-component
:marked
  `TestBed.createComponent` creates an instance of `BannerComponent` to test.
  The 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.

  Query the `DebugElement` by CSS selector for the `<h1>` sub-element that holds the actual title.

  ### _createComponent_ closes configuration
  `TestBed.createComponent` closes the current `TestBed` instance to further configuration.
    You cannot call any more `TestBed` configuration methods, not `configureTestModule` 
    nor any of the `override...` methods. The `TestBed` throws an error if you try.

.alert.is-important
  :marked
    Do not configure the `TestBed` after calling `createComponent`. 
:marked
  ### The tests
  Jasmine runs this `beforeEach` before each test of which there are two
+makeExample('testing/ts/app/banner.component.spec.ts', 'tests', 'app/banner.component.spec.ts (tests)')(format='.')
:marked
  These tests ask the `DebugElement` for the native HTML element to satisfy their expectations.

#detect-changes
:marked
  ### _detectChanges_: Angular change detection under 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:
+makeExample('testing/ts/app/banner.component.spec.ts', 'test-w-o-detect-changes', 'app/banner.component.spec.ts (no detectChanges)')(format='.')
:marked
  This behavior (or lack of it) is intentional.
  It gives the tester an opportunity to investigate the state of
  the component _before Angular initiates data binding or calls lifecycle hooks_.

#auto-detect-changes
:marked
  ### Automatic change detection
  Some testers prefer that the Angular test environment run change detection automatically.
  That's possible by configuring the `TestBed` with the _AutoDetect_ provider:
+makeExample('testing/ts/app/banner.component.spec.ts', 'auto-detect', 'app/banner.component.spec.ts (AutoDetect)')(format='.')
:marked
  Here are three tests that illustrate how _auto-detect_ works.
+makeExample('testing/ts/app/banner.component.spec.ts', 'auto-detect-tests', 'app/banner.component.spec.ts (AutoDetect Tests)')(format='.')
:marked
  The first test shows the benefit of automatic change detection.

  The second and third test remind us that Angular does _not_ know about changes to component property
  values unless Angular itself (or some asynchronous process) makes the change.
  This is as true in production as it is in test. 
  
  In production, external forces rarely change component properties like this,
  whereas these kinds of probing changes are typical in unit tests.
  The tester will have to call `fixture.detectChanges()` quite often 
  despite having opted into auto detect.

.alert.is-helpful
  :marked
    Rather than wonder when the test fixture will or won't perform change detection,
    the samples in this chapter _always call_ `detectChanges()` _explicitly_.

a(href="#top").to-top Back to top

.l-hr

#component-with-dependency
:marked
  # 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`:
+makeExample('testing/ts/app/welcome.component.ts', '', 'app/welcome.component.ts')(format='.')
:marked
  The `WelcomeComponent` has decision logic that interacts with the service;
  such logic makes this component worth testing.
  Here's the testing module configuration for the spec file, `app/welcome.component.spec.ts`:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'config-test-module', 'app/welcome.component.spec.ts')(format='.')
:marked
  This time, in addition to declaring the component under test,
  the configurations sets the `providers` list with the dependent `UserService`.

  This example configures the testing module with a stub `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 try to ask the user for login credentials and 
  try to reach an authentication server.
  These behaviors could be hard to intercept. 
  It is far easier 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:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'user-service-stub')(format='.')

#injected-service-reference
:marked
  ## Referencing injected services
  The tests need access to the injected (stubbed) `UserService`.

  You cannot reference the `userServiceStub` object provided to the testing module. 
  **It does not work!** 
  Surprisingly, the instance actually injected into the component is _not the same_ object
  as the provided `userServiceStub`.

.alert.is-important
  :marked
    Always use an injector to get a reference to an injected service.
:marked
  Where do you get the injector? 
  Angular has an hierarchical injection system.
  In a test there can be injectors at multiple levels.
  The current `TestBed` injector creates a top-level injector.
  The `WelcomeComponent` injector is a child of that injector created specifically for the component.

  You can get a `UserService` from the current `TestBed` injector by calling `TestBed.get`.
+makeExample('testing/ts/app/welcome.component.spec.ts', 'inject-from-testbed', 'TestBed injector')(format='.')
.l-sub-section
  :marked
    The [inject](#inject) function is another way to inject one or more services into a test.
:marked
  That happens to work for testing the `WelcomeComponent` because the `UserService` instance from the `TestBed`
  is the same as the `UserService` instance injected into the component.

  That won't always be the case.
  Be absolutely sure to reference the service instance that the component is _actually receiving_,
  Call `get` on the component's injector which is `fixture.debugElement.injector`:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'injected-service', 'Component\'s injector')(format='.')
.alert.is-important
  :marked
    Use the component's own injector to get the component's injected service.
#welcome-spec-setup
:marked
  Here's the complete, preferred `beforeEach`:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'setup', 'app/welcome.component.spec.ts')(format='.')
:marked
  And here are some tests:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'tests', 'app/welcome.component.spec.ts')(format='.')
:marked
  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.

a(href="#top").to-top Back to top

.l-hr

#component-with-async-service
:marked
  # 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 `app/shared` folder because the author intends to display Twain quotes on other pages someday.
  Here is the `TwainComponent`.
+makeExample('testing/ts/app/shared/twain.component.ts', 'component', 'app/shared/twain.component.ts')(format='.')
:marked
  The `TwainService` implementation is irrelevant at this point.
  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 `app/shared/twain.component.spec.ts` shows one way to do that: 
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'setup', 'app/shared/twain.component.spec.ts (setup)')(format='.')

#service-spy
:marked
  ### Spying on the real service

  This setup is similar to the [`welcome.component.spec` setup](#welcome-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.
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'spy')(format='.')
:marked
  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 will not contact the server.

.l-sub-section
  :marked
    Faking a service instance and spying on the real service are _both_ great options. 
    Pick the one that seems easiest for the current test suite. Don't be afraid to change your mind.
:marked
  Here are the tests with commentary to follow:
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'tests', 'app/shared/twain.component.spec.ts (tests)')
:marked
  ### Synchronous tests
  The first two tests are synchronous. 
  Neither test can prove that a value from the service will be displayed.
  
  Thanks to the spy, the second test verifies that `getQuote` is called. 
  But 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, a least one "tick", before the
  value becomes available. By that time, the test runner has moved on to the next test in the suite.

  The test must become an "async test" ... like the third test

#async
:marked
  ## The _async_ function in _it_

  Notice the `async` in the third test.
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'async-test', 'app/shared/twain.component.spec.ts (async test)')(format='.')
:marked
  The `async` function is an independent feature of the _Angular Testing Platform_. 

  It simplifyies coding of asynchronous tests by arranging for the tester's code to run in a special _async test zone_.

  The `async` function _takes_ a parameterless function and _returns_ a parameterless function
  which becomes the argument to the  Jasmine `it` call. 

  The body of the `async` argument looks much like the body of a normal `it` argument.
  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 is in standard Jasmine asynchronous tests.

  Some functions called within a test (such as `fixture.whenStable`) continue to reveal their asynchronous behavior.
  Consider also the [_fakeAsync_](#fake-async) alternative which affords a more linear coding experience.

#when-stable
:marked
  ## _whenStable_
  The test must wait for the `getQuote` promise to resolve. 
 
  The `getQuote` promise promise resolves in the next turn of the JavaScript engine, thanks to the spy.
  But a different test implementation of  `getQuote` could take longer.
  An integration test might call the _real_ `getQuote`, resulting in an XHR request
  that took many seconds to respond.

  This test has no direct access to the promise returned by the call to `testService.getQuote` 
  which is private and inaccessible inside `TwainComponent`.

  Fortunately, the `getQuote` promise is accessible to the _async test zone_ 
  which intercepts all promises issued within the _async_ method call.
  
  The `ComponentFixture.whenStable` method returns its own promise which resolves when the `getQuote` promise completes.
  In fact, the _whenStable_ promise resolves when _all pending asynchronous activities_ complete ... the definition of "stable".

  Then the testing continues. 
  The test kicks off another round of change detection (`fixture.detechChanges`) 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.

#fakeAsync
#fake-async
:marked
  ## The _fakeAsync_ function

  The fourth test verifies the same component behavior in a different way.
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'fake-async-test', 'app/shared/twain.component.spec.ts (fakeAsync test)')(format='.')
:marked
  Notice that `fakeAsync` replaces `async` as the `it` argument.
  The `fakeAsync` function is another, independent feature of the _Angular Testing Platform_.

  Like [async](#), it _takes_ a parameterless function and _returns_ a parameterless function
  which 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 are no promises at all. 
  No `then(...)` chains to disrupt the visible flow of control.

  There are limitations. For example, you cannot make an XHR call from within a `fakeAsync`.

#tick
#tick-first-look
:marked
  ## The _tick_ function
  Compare the third and fourth tests. Notice that `fixture.whenStable` is gone, replaced by `tick()`.

  The `tick` function is a part of the _Angular Testing Platform_ and a companion to `fakeAsync`.
  It can only be called within a `fakeAsync` body.

  Calling `tick()` simulates the passage of time until all pending asynchronous activities complete,
  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 as formerly appeared within 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
:marked
  ## _jasmine.done_

  While `fakeAsync` and even `async` function greatly simplify Angular asynchronous testing,
  you can still fallback to the traditional Jasmine asynchronous testing technique.

  You can still pass `it` a function that takes a 
  [`done` callback](http://jasmine.github.io/2.0/introduction.html#section-Asynchronous_Support).
  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:
+makeExample('testing/ts/app/shared/twain.component.spec.ts', 'done-test', 'app/shared/twain.component.spec.ts (done test)')(format='.')
:marked
  Although we have no direct access to the `getQuote` promise inside `TwainComponent`,
  the spy does and that makes it possible to wait for `getQuote` to finish.
  
  The `jasmine.done` technique, while discouraged, may become necessary when neither `async` nor `fakeAsync`
  can tolerate a particular asynchronous activity. That's rare but it happens.

a(href="#top").to-top Back to top

.l-hr

#component-with-external-template
:marked
  # Test a component with an external template
  The `TestBed.createComponent` is a synchronous method. 
  It assumes that everything it could need is already in memory.

  That has been true so far. 
  Each tested component's `@Component` metadata has a `template` property specifying an _inline templates_.
  Neither component had a `styleUrls` property.
  Everything necessary to compile them was in memory at test runtime.

  The `DashboardHeroComponent` is different. 
  It has an external template and external css file, specified in `templateUrl` and `styleUrls` properties.
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.ts', 'component', 'app/dashboard/dashboard-hero.component.ts (component)')(format='.')
:marked
  The compiler must read these files from a file system before it can create a component instance.

  The `TestBed.compileComponents` method asynchronously compiles all the components configured in its
  current testing module. After it completes, external templates and css files, have been "inlined"
  and `TestBed.createComponent` can do its job synchronously.
.l-sub-section
  :marked
    WebPack developers need not call `compileComponents` because it inlines templates and css
    as part of the automated build process that precedes running the test.
:marked
  The `app/dashboard/dashboard-hero.component.spec.ts` demonstrates the pre-compilation process:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'compile-components', 'app/dashboard/dashboard-hero.component.spec.ts (compileComponents)')(format='.')

#async-in-before-each
:marked
  ## The _async_ function in _beforeEach_

  Notice the `async` call in the `beforeEach`. 
  The `async` function arranges for the tester's code to run in a special _async test zone_
  that hides the mechanics of asynchronous execution, just as it does when passed to an [_it_ test)(#async).

#compile-components
:marked
  ## _compileComponents_ 
  In this example, `Testbed.compileComponents` compiles one component, the `DashboardComponent`. 
  It's the only declared component in this testing module.

  Tests later in this chapter have more declared components and some of them import application
  modules that declare yet more components.
  Some or all of these components could have external templates and css files.
  `TestBed.compileComponents` compiles them all asynchonously at one time.
  
  The `compileComponents` method returns a promise so you can perform additional tasks _after_ it finishes.

  ### _compileComponents_ closes configuration
  After `compileComponents` runs, the current `TestBed` instance is closed to further configuration.
  You cannot call any more `TestBed` configuration methods, not `configureTestModule` 
  nor any of the `override...` methods. The `TestBed` throws an error if you try.

.alert.is-important
  :marked
    Do not configure the `TestBed` after calling `compileComponents`. 
    Make `compileComponents` the last step 
    before calling `TestBed.createInstance` to instantiate the test component.
:marked
  The `DashboardHeroComponent` spec follows the asynchonous `beforeEach` with a
  _synchronous_ `beforeEach` that completes the setup steps and runs tests ... as described in the next section.

.l-hr

#component-with-inputs-outputs
:marked
  # 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 uses 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 tiny example of a component in this role.
  It displays an individual heroe provided by the `DashboardComponent`. 
  Clicking that hero tells the the `DashboardComponent` that the user has selected the hero.

  The `DashboardHeroComponent` is embedded in the `DashboardComponent` template like this:
+makeExample('testing/ts/app/dashboard/dashboard.component.html', 'dashboard-hero', 'app/dashboard/dashboard.component.html (excerpt)')(format='.')
:marked
  The `DashboardHeroComponent` appears in an `*ngFor` repeater which sets each component's `hero` input property
  to the iteration value and listens for the components `selected` event.
  
  Here's the component's definition again:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.ts', 'component', 'app/dashboard/dashboard-hero.component.ts (component)')(format='.')
:marked
  While testing a component this simple has little intrinsic value, it's worth knowing how.
  Three approaches come to mind:
  1. Test it as used by `DashboardComponent`
  1. Test it as a stand-alone component
  1. Test it as used by a substitute for `DashboardComponent`

  A quick look at the `DashboardComponent` constructor discourages the first approach:
+makeExample('testing/ts/app/dashboard/dashboard.component.ts', 'ctor', 'app/dashboard/dashboard.component.ts (constructor)')(format='.')
:marked
  The `DashboardComponent` depends upon the Angular router and the `HeroService`.
  You'd probably have to replace them both with test doubles and that looks like a lot of work. 
  The router seems particularly challenging.
.l-sub-section
  :marked
    The [discussion below](#routed-component) covers testing components that requre the router.
:marked
  The immediate goal is to test the `DashboardHeroComponent`, not the `DashboardComponent`, and there's no need
  to work hard unnecessarily. Let's try the second and third options.

  ## Test _DashboardHeroComponent_ stand-alone

  Here's the spec file setup.
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'setup', 'app/dashboard/dashboard-hero.component.spec.ts (setup)')(format='.')

:marked
  The async `beforeEach` was discussed [above](#component-with-external-template).
  Having compiled the components asynchronously with `compileComponents`, the rest of the setup
  proceeds _synchronously_ in a _second_ `beforeEach`, using the basic techniques described [earlier](#simple-component-test).

  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:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'name-test', 'app/dashboard/dashboard-hero.component.spec.ts (name test)')(format='.')
:marked
  It verifies that the hero name is propagated through to template with a binding.
  There's a twist. The template passes the hero name through the Angular `UpperCasePipe` so the
  test must match the element value with the uppercased name:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.html')(format='.')
:marked
.alert.is-helpful
  :marked
    This small test demonstrates how Angular tests can verify a component's visual representation
    &mdash; something not possible with [isolated unit tests](#isolated-component-tests) &mdash;
    at low cost and without resorting to much slower and more complicated end-to-end tests.

:marked
  The second test verifies click behavior. Clicking the hero should raise a `selected` event that the
  host component (`DashboardComponent` presumably) can hear:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'click-test', 'app/dashboard/dashboard-hero.component.spec.ts (click test)')(format='.')
:marked
  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.
  
#trigger-event-handler
:marked
  ### _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.

+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'trigger-event-handler')(format='.')
:marked
  The test assumes (correctly in this case) that the runtime event handler &mdash; the component's `click()` method &mdash;
  doesn't care about the event object.
  
  Other handlers will be less forgiving. 
  For example, the `RouterLink` directive expects an object with a `button` property indicating the mouse button that was pressed. 
  The directive throws an error if the event object doesn't do this correctly.

#click-helper
:marked
  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:
+makeExample('testing/ts/testing/index.ts', 'click-event', 'testing/index.ts (click helper)')(format='.')
:marked
  The first parameter is the _element-to-click_. You can pass a custom event object as the second parameter if you wish. The default is a (partial) 
  <a href="https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/button" target="_blank">left-button mouse event object</a> 
  accepted by many handlers including the `RouterLink` directive.

.callout.is-critical
  header click() is not an ATP function
  :marked
    The `click()` helper function is **not** part of the _Angular Testing Platform_. 
    It's a function defined in _this chapter's sample code_ and used by all of the sample tests. 
    If you like it, add it to your own collection of helpers.
:marked
  Here's the previous test, rewritten using this click helper.
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'click-test-2', 'app/dashboard/dashboard-hero.component.spec.ts (click test revised)')(format='.')


.l-hr

#component-inside-test-host
:marked
  # Test a component inside a test host component

  In the previous approach the tests themselves played the role of the host `DashboardComponent`.
  A nagging suspicion remains. 
  Will the `DashboardHeroComponent` work properly 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:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'test-host', 'app/dashboard/dashboard-hero.component.spec.ts (test host)')(format='.')
:marked
  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 that records the emitted hero
  in its `selectedHero` property. Later the tests check that property to verify that the
  `DashboardHeroComponent.selected` event really did emit the right hero.

  The setup for the test-host tests is similar to the setup for the stand-alone tests:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'test-host-setup', 'app/dashboard/dashboard-hero.component.spec.ts (test host setup)')(format='.')
:marked
  This testing module configuration shows two important differences:
  1. It _declares_ both the `DashboardHeroComponent` and the `TestHostComponent`.
  1. It _creates_ the `TestHostComponent` instead of the `DashboardHeroComponent`.

  The `fixture` returned by `createComponent` holds an instance of `TestHostComponent` instead of an instance of `DashboardHeroComponent`.
  
  Of course 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
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'test-host-tests', 'app/dashboard/dashboard-hero.component.spec.ts (test-host)')(format='.')
:marked
  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.

a(href="#top").to-top Back to top

.l-hr

#routed-component
:marked
  # Test a routed component

  Testing the actual `DashboardComponent` seemed daunting because it injects the `Router`.
+makeExample('testing/ts/app/dashboard/dashboard.component.ts', 'ctor', 'app/dashboard/dashboard.component.ts (constructor)')(format='.')
:marked
  It also injects the `HeroService` but faking that is a [familiar story](#component-with-async-servic).
  The `Router` has a complicated API and is entwined with other services and application pre-conditions.

  Fortunately, the `DashboardComponent` isn't doing much with the `Router`
+makeExample('testing/ts/app/dashboard/dashboard.component.ts', 'goto-detail', 'app/dashboard/dashboard.component.ts (goToDetail)')(format='.')
:marked
  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:
+makeExample('testing/ts/app/dashboard/dashboard.component.spec.ts', 'router-stub', 'app/dashboard/dashboard.component.spec.ts (Router Stub)')(format='.')
:marked
  Now we setup the testing module with the test stubs for the `Router` and `HeroService` and 
  create a test instance of the `DashbaordComponent` for subsequent testing.
+makeExample('testing/ts/app/dashboard/dashboard.component.spec.ts', 'compile-and-create-body', 'app/dashboard/dashboard.component.spec.ts (compile and create)')(format='.')
:marked
  The following test clicks the displayed hero and confirms (with the help of a spy) that `Router.navigateByUrl` is called with the expected url.
+makeExample('testing/ts/app/dashboard/dashboard.component.spec.ts', 'navigate-test', 'app/dashboard/dashboard.component.spec.ts (navigate test)')(format='.')

#inject
:marked
  ## The _inject_ function

  Notice the `inject` function in the second `it` argument.
+makeExample('testing/ts/app/dashboard/dashboard.component.spec.ts', 'inject')(format='.')
:marked
  The `inject` function is an independent feature of the _Angular Testing Platform_.
  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
  1. a test function whose parameters correspond exactly to each item in the injection token array

.callout.is-important
  header inject uses the TestBed Injector
  :marked
    The `inject` function uses the current `TestBed` injector and can only return services provided at that level.
    It does not return services from component providers.

:marked
  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:
+makeExample('testing/ts/app/welcome.component.spec.ts', 'injected-service', 'Component\'s injector')(format='.')
.alert.is-important
  :marked
    Use the component's own injector to get the service actually injected into the component.

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

.alert.is-important
  :marked
    Do not configure the `TestBed` after calling `inject`.
a(href="#top").to-top Back to top

.l-hr

#routed-component-w-param
:marked
  # 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:
+makeExample('testing/ts/app/hero/hero-detail.component.ts', 'ctor', 'app/hero/hero-detail.component.ts (constructor)')(format='.')
:marked
  `HeroDetailComponent`  listens for changes to the `ActivatedRoute.params` in its `ngOnInit` method.
+makeExample('testing/ts/app/hero/hero-detail.component.ts', 'ng-on-init', 'app/hero/hero-detail.component.ts (ngOnInit)')(format='.')
.l-sub-section
  :marked
    The expression after `route.params` chains an _Observable_ operator that _plucks_ the `id` from the `params` 
    and then chains a `forEach` operator to subscribes to `id`-changing events. 
    The `id` changes every time the user navigates to a different hero.
    
    The `forEach` passes the new `id` value to the component's `getHero` method (not shown)
    which fetches a hero and sets the component's `hero` property. 
    If the`id` parameter is missing, the `pluck` operator fails and the `catch` treats failure as a request to edit a new hero.
    
    The [Router](router.html#route-parameters) chapter covers `ActivatedRoute.params` in more detail.
:marked
  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` would follow the same pattern except for a complication:
  the `ActivatedRoute.params` is an _Observable_.
#stub-observable
:marked
  ### _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_. 
  We recommend locating such helpers in a `testing` folder sibling to the `app` folder.
  This sample keeps `ActivatedRouteStub` in `testing/router-stubs.ts`:
  
+makeExample('testing/ts/testing/router-stubs.ts', 'activated-route-stub', 'testing/router-stubs.ts (ActivatedRouteStub)')(format='.')
:marked
  Notable features of this stub:

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

  * <a href="https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/subjects/behaviorsubject.md" target="_blank">_BehaviorSubject_</a> 
  drives the stub's `params` _Observable_ and returns the same value to every `params` subscriber until it's given a new value.

  * The `HeroDetailComponent` chain 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.
.l-sub-section(style="margin-left:30px")
  :marked
    The [_snapshot_](router.html#snapshot "Router Chapter: snapshot") is another popular way for components to consume route parameters.
.callout.is-helpful
  :marked
    The router stubs in this chapter are meant to inspire you. Create your own stubs to fit your testing needs.

#observable-tests
:marked
  ### _Observable_ tests
  Here's a test demonstrating the component's behavior when the observed `id` refers to an existing hero:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'route-good-id', 'app/hero/hero-detail.component.spec.ts (existing id)')(format='.')
.l-sub-section
  :marked
    The `createComponent` method and `page` object are discussed [in the next section](#page-object).
    Rely on your intuition for now.
:marked
  When the `id` cannot be found, the component should re-route to the `HeroListComponent`.
  The test suite setup provided the same `RouterStub` [described above](#routed-component) which spies on the router without actually navigating.
  This test supplies a "bad" id and expects the component to try to navigate.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'route-bad-id', 'app/hero/hero-detail.component.spec.ts (bad id)')(format='.')
:marked
:marked
  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:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'route-no-id', 'app/hero/hero-detail.component.spec.ts (no id)')(format='.')
:marked
.callout.is-helpful
  :marked
    Inspect and download _all_ of the chapter's application test code with this <live-example plnkr="app-specs">live example</live-example>.

.l-hr

#page-object
:marked
  # Use a _page_ object to simplify setup

  The `HeroDetailComponent` is a simple view with a title, two hero fields, and two buttons.
figure.image-display
  img(src='/resources/images/devguide/testing/hero-detail.component.png' alt="HeroDetailComponent in action")
:marked
  But there's already plenty of template complexity.
+makeExample('testing/ts/app/hero/hero-detail.component.html', '', 'app/hero/hero-detail.component.html')(format='.')
:marked
  To fully exercise the component, the test needs ...
  * to wait until a `hero` arrives before `*ngIf` allows any element in DOM
  * element references for the title name span and name input-box to inspect their values
  * two button references to click
  * spies on services and component 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`
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'page', 'app/hero/hero-detail.component.spec.ts (Page)')(format='.')
:marked
  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` and fills in the blanks once the `hero` arrives.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'create-component', 'app/hero/hero-detail.component.spec.ts (createComponent)')(format='.')
:marked
  The [observable tests](#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.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'selected-tests', 'app/hero/hero-detail.component.spec.ts (selected tests)')(format='.')

a(href="#top").to-top Back to top
.l-hr

#import-module
:marked
  # Setup with module imports
  Earlier component tests configured the testing module with a few `declarations` like this:
+makeExample('testing/ts/app/dashboard/dashboard-hero.component.spec.ts', 'compile-components', 'app/dashboard/dashboard-hero.component.spec.ts (config)')(format='.')
:marked
  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` 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:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'setup-forms-module', 'app/hero/hero-detail.component.spec.ts (FormsModule setup)')(format='.')
:marked
  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:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'setup-shared-module', 'app/hero/hero-detail.component.spec.ts (SharedModule setup)')(format='.')
:marked
  It's a bit tighter and smaller, with fewer import statements (not shown).

#feature-module-import
:marked
  ### Import the feature module
  The `HeroDetailComponent` is part of the `HeroModule` [Feature Module](ngmodule.html#feature-modules) that aggregates more of the interdependent pieces
  including the `SharedModule`.
  Try a test configuration that imports the `HeroModule` like this one:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'setup-hero-module', 'app/hero/hero-detail.component.spec.ts (HeroModule setup)')(format='.')
:marked
  That's _really_ crisp. Only the _test doubles_ in the `providers` remain. Even the `HeroDetailComponent` declaration is gone.
.l-sub-section
  :marked
    In fact, if you try to declare it, Angular throws an error because
    `HeroDetailComponent` is declared in both the `HeroModule` and the `DynamicTestModule` (the testing module).

.alert.is-helpful
  :marked
    Importing the component's feature module is often the easiest way to configure the tests,
    especially when the feature module is small and mostly self-contained ... as feature modules should be.
:marked

a(href="#top").to-top Back to top
.l-hr

#component-override
:marked
  # Override component providers

  The `HeroDetailComponent` provides its own `HeroDetailService`.
+makeExample('testing/ts/app/hero/hero-detail.component.ts', 'prototype', 'app/hero/hero-detail.component.ts (prototype)')(format='.')
:marked
  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!

.l-sub-section
  :marked
    These tests could fail or timeout if the `HeroDetailService` made its own XHR calls to a remote server.
    There might not be a remote server to call.

    Fortunately, the `HeroDetailService` delegates responsibility for remote data access to an injected `HeroService`.

  +makeExample('testing/ts/app/hero/hero-detail.service.ts', 'prototype', 'app/hero/hero-detail.service.ts (prototype)')(format='.')
  :marked
    The [previous test configuration](#feature-module-import) replaces the real `HeroService` with a `FakeHeroService` 
    that intercepts server requests and fakes their responses.

:marked
  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:
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'setup-override', 'app/hero/hero-detail.component.spec.ts (Override setup)')(format='.')
:marked
  Notice that `TestBed.configureTestingModule` no longer provides a (fake) `HeroService` because it's [not needed](#stub-hero-detail-service).

#override-component-method
:marked
  ### The _overrideComponent_ method

  Focus on the `overrideComponent` method.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'override-component-method', 'app/hero/hero-detail.component.spec.ts (overrideComponent)')(format='.')
:marked
  It takes two arguments: the component type to override (`HeroDetailComponent`) and an override metadata object.
  The [overide metadata object](#metadata-override-object) is a generic defined as follows:

code-example(format="." language="javascript").
  type MetadataOverride<T> = {
    add?: T;
    remove?: T;
    set?: T;
  };
:marked
  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:
code-example(format="." language="javascript").
    selector?: string;
    template?: string;
    templateUrl?: string;
    providers?: any[];
    ...

#stub-hero-detail-service
:marked
  ### _StubHeroDetailService_

  This example completely replaces the component's `providers` with an array containing the `StubHeroDetailService`.
  The `StubHeroDetailService` is dead simple. It doesn't need a `HeroService` (fake or otherwise).
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'stub-hds', 'app/hero/hero-detail.component.spec.ts (StubHeroDetailService)')(format='.')
:marked
  ### The override tests

  Now the tests can control the component's hero directly by manipulating the stub's `testHero`.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'override-tests', 'app/hero/hero-detail.component.spec.ts (override tests)')(format='.')
:marked
  ### 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.

a(href="#top").to-top Back to top
.l-hr

#router-outlet-component
:marked
  # 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-component-html
+makeExample('testing/ts/app/app.component.html', '', 'app/app.component.html')(format='.')
:marked
  The component class does nothing.
+makeExample('testing/ts/app/app.component.ts', '', 'app/app.component.ts')(format='.')

:marked
  Unit tests can confirm that the anchors are wired properly without engaging the router.
  See why this is worth doing [below](#why-stubbed-routerlink-tests).

#stub-component
:marked
  ### Stubbing unneeded components

  The test setup should look familiar
+makeExample('testing/ts/app/app.component.spec.ts', 'setup-stubs', 'app/app.component.spec.ts (Stub Setup)')(format='.')
:marked
  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. 
#router-link-stub
:marked
  ### Stubbing the _RouterLink_

  The `RouterLinkStubDirective` contributes substantively to the test
+makeExample('testing/ts/testing/router-stubs.ts', 'router-link', 'testing/router-stubs.ts (RouterLinkStubDirective)')(format='.')
:marked
  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
#inject-directive
:marked
  ### _By.directive_ and injected directives

  A little more setup triggers the initial data binding and gets references to the navigation links:
+makeExample('testing/ts/app/app.component.spec.ts', 'test-setup', 'app/app.component.spec.ts (test setup)')(format='.')
:marked
  Two points of special interest:

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

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

#app-component-tests
:marked
  Here are some tests that leverage this setup:
+makeExample('testing/ts/app/app.component.spec.ts', 'tests', 'app/app.component.spec.ts (selected tests)')(format='.')
.l-sub-section
  :marked
    The "click" test _in this example_ is worthless.
    It works hard to appear useful when in fact it 
    tests the `RouterLinkStubDirective` rather than the _component_.
    This is a common failing of directive stubs.
    
    It has a legitimate purpose in this chapter. 
    It demonstrates how to find a `RouterLink` element, click it, and inspect a result,
    without engaging the full router machinery.
    This is a skill you may need to test a more sophisticated component, one that changes the display,
    re-calculates parameters, or re-arranges navigation options when the user clicks the link.

#why-stubbed-routerlink-tests
:marked
  ### 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.
  A future chapter update will explain how to write such tests with the `RouterTestingModule`.
a(href="#top").to-top Back to top
.l-hr

#shallow-component-test
:marked
  # "Shallow component tests" with *NO\_ERRORS\_SCHEMA*

  The [previous setup](#stub-component) 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_](#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.
+makeTabs('testing/ts/app/app.component.spec.ts, testing/ts/app/app.component.spec.ts', 
  'setup-schemas, setup-stubs-w-imports', 
  'app/app.component.spec.ts (NO_ERRORS_SCHEMA), app/app.component.spec.ts (Stubs)')(format='.')
:marked
  The _only_ declarations are the _component-under-test_ (`AppComponent`) and the `RouterLinkStubDirective` 
  that contributes actively to the tests.
  The [tests in this example](#app-component-tests) are unchanged.

.alert.is-important
  :marked
    _Shallow component tests_ with `NO_ERRORS_SCHEMA` greatly simplify unit testing of complex templates.
    However, the compiler no longer alerts you to mistakes 
    such as misspelled or misused components and directives.

a(href="#top").to-top Back to top

.l-hr

#attribute-directive
:marked
  # 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.
+makeExample('testing/ts/app/shared/highlight.directive.ts', '', 'app/shared/highlight.directive.ts')(format='.')
:marked
  It's used throughout the application, perhaps most simply in the `AboutComponent`:
+makeExample('testing/ts/app/about.component.ts', '', 'app/about.component.ts')(format='.')
:marked
  Testing the specific use of the `HighlightDirective` within the `AboutComponent` requires only the
  techniques explored above (in particular the ["Shallow test"](#shallow-component-test) approach).
+makeExample('testing/ts/app/about.component.spec.ts', 'tests', 'app/about.component.spec.ts')(format='.')
:marked
  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](#isolated-tests) might be helpful. 
  But attribute directives like this one tend to manipulate the DOM. 
  Isolated tests don't and therefore don't 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.
+makeExample('testing/ts/app/shared/highlight.directive.spec.ts', 'test-component', 'app/shared/highlight.directive.spec.ts (test component)')(format='.')
figure.image-display
  img(src='/resources/images/devguide/testing/highlight-directive-spec.png' width="200px" alt="HighlightDirective spec in action")
.l-sub-section
  :marked
    The `<input>` case binds the `HighlightDirective` to the name of a color value in the input box.
    The initial value is the word "cyan" which should be the background color of the input box.
:marked
  Here are some tests of this component:
+makeExample('testing/ts/app/shared/highlight.directive.spec.ts', 'selected-tests', 'app/shared/highlight.directive.spec.ts (selected tests)')
:marked
  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 <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/:not" target="_blank">`:not` pseudo-class</a> 
  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.

  * `DebugElement.styles` affords access to element styles even in the absence of a real browser, thanks to the `DebugElement` abstraction.
  But feel free to exploit the `nativeElement` when that seems easier or more clear than the abstraction.

  * 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 2nd `<h2>` to get its `HighlightDirective` instance
  and its `defaultColor`.
  
  * `DebugElement.properties` affords access to the artificial custom property that is set by the directive.

a(href="#top").to-top Back to top

.l-hr

#isolated-tests
#testing-without-atp
:marked
  # Testing without the Angular Testing Platform

  Testing applications with the help of the Angular Testing Platform (ATP) is the main focus of this chapter.

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

  They don't
  * import from the Angular test libraries
  * configure a module
  * prepare dependency injection `providers`
  * call `inject` or `async` or `fakeAsync`

  They do
  * exhibit standard, Angular-agnostic testing techniques
  * create instances directly with `new`
  * substitute test doubles (stubs, spys, and mocks) for the real dependencies.

.callout.is-important
  header Write both kinds of tests
  :marked
    Good developers write both kinds of tests for the same application part, often in the same spec file.
    Write simple _isolated_ unit tests to validate the part in isolation.
    Write _Angular_ tests to validate the part as it interacts with Angular,
    updates the DOM, and collaborates with the rest of the application.

#isolated-service-tests
:marked
  ## 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 Platform.

+makeExample('testing/ts/app/bag/bag.no-testbed.spec.ts', 'FancyService', 'app/bag/bag.no-testbed.spec.ts')
:marked
  A rough line count suggests that these tests are about 25% smaller than equivalent ATP tests. 
  That's telling but not decisive. 
  The benefit comes from reduced setup and code complexity.

  Compare these equivalent tests of `FancyService.getTimeoutValue`.
+makeTabs(
  `testing/ts/app/bag/bag.no-testbed.spec.ts, testing/ts/app/bag/bag.spec.ts`, 
  'getTimeoutValue, getTimeoutValue', 
  `app/bag/bag.no-testbed.spec.ts, app/bag/bag.spec.ts (with ATP)`)
:marked
  They have about the same line-count. 
  The ATP version has more moving parts, including a couple of helper functions (`async` and `inject`).
  Both work and it's not much of an issue if you're using the Angular Testing Platform nearby for other reasons. 
  On the other hand, why burden simple service tests with ATP 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
+makeExample('testing/ts/app/bag/bag.ts', 'DependentService', 'app/bag/bag.ts')(format='.')
:marked
  It delegates it's only method, `getValue`, to the injected `FancyService`.

  Here are several ways to test it.
+makeExample('testing/ts/app/bag/bag.no-testbed.spec.ts', 'DependentService', 'app/bag/bag.no-testbed.spec.ts')
:marked
  The first test creates a `FancyService` with `new` and passes it to the `DependentService` constructor.
  
  It's rarely that simple. The injected service can be difficult to create or control.
  You can mock the dependency, or use a dummy value, or stub the pertinent service method
  with a substitute method that is 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 Platform when writing tests that validate how a service interacts with components
  _within the Angular runtime environment_.

#isolated-pipe-tests
:marked
  ## Pipes
  Pipes are easy to test without the Angular Testing Platform (ATP).

  A pipe class has one method, `transform`, that turns an input to an output. 
  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 implemented with a regular expression.
+makeExample('testing/ts/app/shared/title-case.pipe.ts', '', 'app/shared/title-case.pipe.ts')(format='.')
:marked
  Anything that uses a regular expression is worth testing thoroughly.
  Use simple Jasmine to explore the expected cases and the edge cases.
+makeExample('testing/ts/app/shared/title-case.pipe.spec.ts', 'excerpt', 'app/shared/title-case.pipe.spec.ts')
:marked
  ### Write ATP 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 ATP component tests such as this one.
+makeExample('testing/ts/app/hero/hero-detail.component.spec.ts', 'title-case-pipe', 'app/hero/hero-detail.component.spec.ts (pipe test)')

#isolated-component-tests
:marked
  ## Components

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

  Consider this `ButtonComp` component.
+makeExample('testing/ts/app/bag/bag.ts', 'ButtonComp', 'app/bag/bag.ts (ButtonComp)')(format='.')
:marked
  The following ATP test demonstrates that clicking a button in the template leads
  to an update of the on-screen message.
+makeExample('testing/ts/app/bag/bag.spec.ts', 'ButtonComp', 'app/bag/bag.spec.ts (ButtonComp)')(format='.')
:marked
  The assertions verify the data binding 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 up correctly.

  Tests _without_ the ATP 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.
+makeExample('testing/ts/app/bag/bag.no-testbed.spec.ts', 'ButtonComp', 'app/bag/bag.no-testbed.spec.ts (ButtonComp)')(format='.')
:marked
  Isolated component tests offer a lot of test coverage with less code and almost no setup.
  This advantage is even more pronounced with complex components that 
  require meticulous preparation with the Angular Testing Platform.

  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 ATP tests for that.

a(href="#top").to-top Back to top

.l-hr

#atp-api
:marked
  # Angular Testing Platform APIs

  This section takes inventory of the most useful _Angular Testing Platform_ features and summarizes what they do.

  The _Angular Testing Platform_ consists of the `TestBed` and `ComponentFixture` classes plus a handful of functions in the test environment.
  The [_TestBed_](#testbed-api-summary) and [_ComponentFixture_](#component-fixture-api-summary) classes are covered separately.

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

table
  tr
    th Function
    th Description
  tr
    td(style="vertical-align: top") <code>async</code>
    td
      :marked
        Runs the body of a test (`it`) or setup (`beforeEach`) function within a special _async test zone_.
        See [discussion above](#async).
  tr
    td(style="vertical-align: top") <code>fakeAsync</code>
    td
      :marked
        Runs the body of a test (`it`) within a special _fakeAsync test zone_, enabling
        a linear control flow coding style. See [discussion above](#fake-async).
  tr
    td(style="vertical-align: top") <code>tick</code>
    td
      :marked
        Simulates the passage of time and the completion of pending asynchronous activities
        by flushing both _timer_ and _micro-task_ queues within the _fakeAsync test zone_.

      .l-sub-section
        :marked
          The curious, dedicated reader might enjoy this lengthy blog post, 
          "<a href="https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/" 
          target="_blank">_Tasks, microtasks, queues and schedules_</a>".
      :marked
        Accepts an optional argument that moves the virtual clock forward
        the specified number of milliseconds, 
        clearing asynchronous activities scheduled within that timeframe.
        See [discussion bove](#tick).

  tr
    td(style="vertical-align: top")  <code>inject</code>
    td
      :marked
        Injects one or more services from the current `TestBed` injector into a test function.
        See [above](#inject).

  tr
    td(style="vertical-align: top") <code>discardPeriodicTasks</code>
    td
      :marked
        When a `fakeAsync` test ends with pending timer event _tasks_ (queued `setTimeOut` and `setInterval` callbacks),
        the test fails with a clear error message.
        
        In general, a test should end with no queued tasks. 
        When pending timer tasks are expected, call `discardPeriodicTasks` to flush the _task_ queue
        and avoid the error.

  tr
    td(style="vertical-align: top") <code>flushMicrotasks</code>
    td
      :marked
        When a `fakeAsync` test ends with pending _micro-tasks_ such as unresolved promises,
        the test fails with a clear error message.
        
        In general, a test should wait for micro-tasks to finish.
        When pending microtasks are expected, call `flushMicrotasks` to flush the  _micro-task_ queue
        and avoid the error.

  tr
    td(style="vertical-align: top") <code>ComponentFixtureAutoDetect</code>
    td
      :marked
        A provider token for setting the default _auto-changeDetect_ from its default of `false`.
        See [automatic change detection](#automatic-change-detection)

  tr
    td(style="vertical-align: top") <code>getTestBed</code>
    td
      :marked
        Gets the current instance of the `TestBed`. 
        Usually unnecessary because the static class methods of the `TestBed` class are typically sufficient.
        The `TestBed` instance exposes a few rarely used members that are not available as
        static methods.

.l-hr

#testbed-class-summary
:marked
  # _TestBed_ Class Summary
  The `TestBed` class is a principle feature of the _Angular Testing Platform_. 
  Its API is quite large and can be overwhelming until you've explored it first
  a little at a time. Read the early part of this chapter 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.
code-example(format="." language="javascript").
  type TestModuleMetadata = {
    providers?: any[];
    declarations?: any[];
    imports?: any[];
    schemas?: Array&lt;SchemaMetadata | any[]&gt;;
  };

#metadata-override-object
:marked
  Each overide method takes a `MetadataOverride<T>` where `T` is the kind of metadata
  appropriate to the method, the parameter of an `@NgModule`, `@Component`, `@Directive`, or `@Pipe`.

code-example(format="." language="javascript").
  type MetadataOverride<T> = {
    add?: T;
    remove?: T;
    set?: T;
  };

:marked
#testbed-methods
:marked
  The `TestBed` API consists of static class methods that either update or reference a _global_ instance of the`TestBed`.
  
  Internally, all static methods cover methods of the current runtime `TestBed` instance that 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.
table
  tr
    th Methods
    th Description
  tr
    td(style="vertical-align: top") <code>configureTestingModule</code>
    td
      :marked
        The testing shims (`karma-test-shim`, `browser-test-shim`) 
        establish the [initial test environment](##testbed-initTestEnvironment) and a default testing module.
        The default testing module is configured with basic declaratives and some Angular service substitutes (e.g. `DebugDomRender`) 
        that every tester needs.
        
        Call `configureTestingModule` to refine the testing module configuration for a particular set of tests
        by adding and removing imports, declarations (of components, directives, and pipes), and providers.
  tr
    td(style="vertical-align: top") <code>compileComponents</code>
    td
      :marked
        Compile the testing module asynchronously after you've finished configuring it.
        You **must** call this method if _any_ of the testing module components have a `templateUrl`
        or `styleUrls` because fetching component template and style files is necessarily asynchronous.
        See [above](#compile-components).

        Once called, the `TestBed` configuration is frozen for the duration of the current spec.

  tr
    td(style="vertical-align: top") <code>createComponent<T></code>
    td
      :marked
        Create an instance of a component of type `T` based on the current `TestBed` configuration.
        Once called, the `TestBed` configuration is frozen for the duration of the current spec.
  tr
    td(style="vertical-align: top") <code>overrideModule</code>
    td
      :marked
        Replace metadata for the given `NgModule`. Recall that modules can import other modules.
        The `overrideModule` method can reach deeply into the current testing module to
        modify one of these inner modules.
  tr
    td(style="vertical-align: top") <code>overrideComponent</code>
    td
      :marked
        Replace metadata for the given component class which could be nested deeply 
        within an inner module.
  tr
    td(style="vertical-align: top") <code>overrideDirective</code>
    td
      :marked
        Replace metadata for the given directive class which could be nested deeply 
        within an inner module.
  tr
    td(style="vertical-align: top") <code>overridePipe</code>
    td
      :marked
        Replace metadata for the given pipe class which could be nested deeply 
        within an inner module.
  tr
    td(style="vertical-align: top").
      <a id="testbed-get"></a>
      <code>get</code>
    td
      :marked
        Retrieve a service from the current `TestBed` injector.

        The `inject` function is often adequate for this purpose. 
        But `inject` throws an error if it can't provide the service. 
        What if the service is optional?

        The `TestBed.get` method takes an optional second parameter, 
        the object to return if Angular can't find the provider
        (`null` in this example):
      +makeExample('testing/ts/app/bag/bag.spec.ts', 'testbed-get')(format=".")
      :marked
        Once called, the `TestBed` configuration is frozen for the duration of the current spec.

  tr
    td(style="vertical-align: top").
      <a id="testbed-initTestEnvironment"></a>
      <code>initTestEnvironment</code>
    td
      :marked
        Initialize the testing environment for the entire test run.

        The testing shims (`karma-test-shim`, `browser-test-shim`) call it for you
        so there is rarely a reason for you to call it yourself.

        This method may be called _exactly once_. Call `resetTestEnvironment` first
        if you absolutely need to change this default in the middle of your test run.

        Specify the Angular compiler factory, a `PlatformRef`, and a default Angular testing module.
        Alternatives for non-browser platforms are available in the general form
        `@angular/platform-<platform_name>/testing/<platform_name>`.
  tr
    td(style="vertical-align: top") <code>resetTestEnvironment</code>
    td
      :marked
        Reset the initial test environment including the default testing module.

:marked
  A few of the `TestBed` instance methods are not covered by static `TestBed` _class_ methods.
  These are rarely needed.

#component-fixture-api-summary
:marked
  ## 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. 

#component-fixture-properties
:marked
  ### _ComponentFixture_ properties

  Here are the most important properties for testers, in order of likely utility.
  
table
  tr
    th Properties
    th Description
  tr
    td(style="vertical-align: top") <code>componentInstance</code>
    td
      :marked
        The instance of the component class created by `TestBed.createComponent`.
  tr
    td(style="vertical-align: top") <code>debugElement</code>
    td
      :marked
        The `DebugElement` associated with the root element of the component.

        The `debugElement` provides insight into the component and its DOM element during test and debugging.
        It's a critical property for testers. The most interesting members are covered [below](#debug-element-details).
  tr
    td(style="vertical-align: top") <code>nativeElement</code>
    td
      :marked
        The native DOM element at the root of the component.
  tr
    td(style="vertical-align: top") <code>changeDetectorRef</code>
    td
      :marked
        The `ChangeDetectorRef` for the component.

        The `ChangeDetectorRef` is most valuable when testing a
        component that has the `ChangeDetectionStrategy.OnPush`
        or the component's change detection is under your programmatic control.

#component-fixture-methods
:marked
  ### _ComponentFixture_ methods

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

  Here are the most useful methods for testers.
table
  tr
    th Methods
    th Description
  tr
    td(style="vertical-align: top") <code>detectChanges</code>
    td
      :marked
        Trigger a change detection cycle for the component.

        Call it to initialize the component (it calls `ngOnInit`) and after your 
        test code change the component's data bound property values.
        Angular can't see that you've changed `personComponent.name` and won't update the `name`
        binding until you call `detectChanges`.

        Runs `checkNoChanges`afterwards to confirm there are no circular updates unless
        called as `detectChanges(false)`;
  tr
    td(style="vertical-align: top") <code>autoDetectChanges</code>
    td
      :marked
        Set whether the fixture should try to detect changes automatically.

        When autodetect is true, the test fixture listens for _zone_ events and calls `detectChanges`. 
        You probably still have to call `fixture.detectChanges` to trigger data binding updates 
        when your test code modifies component property values directly.

        The default is `false` and testers who prefer fine control over test behavior
        tend to keep it `false`. 

        Calls `detectChanges` immediately which detects existing changes
        and will trigger `ngOnInit` if the component has not yet been initialized.
  tr
    td(style="vertical-align: top") <code>checkNoChanges</code>
    td
      :marked
        Do a change detection run to make sure there are no pending changes.
        Throws an exceptions if there are.
  tr
    td(style="vertical-align: top") <code>isStable</code>
    td
      :marked
        Return `true` if the fixture is currently _stable_.
        Returns `false` if there are async tasks that have not completed.
  tr
    td(style="vertical-align: top") <code>whenStable</code>
    td
      :marked
        Returns a promise that resolves when the fixture is stable.
        
        Hook that promise to resume testing after completion of asynchronous activity or 
        asynchronous change detection.
        See [above](#when-stable)
  tr
    td(style="vertical-align: top") <code>destroy</code>
    td
      :marked
        Trigger component destruction.

#debug-element-details
:marked
  ### _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 sub-trees.

  Here are the most useful `DebugElement` members for testers in approximate order of utility.

table
  tr
    th Member
    th Description
  tr
    td(style="vertical-align: top") <code>nativeElement</code>
    td
      :marked
        The corresponding DOM element in the browser (null for WebWorkers).
  tr
    td(style="vertical-align: top") <code>query</code>
    td
      :marked
        Calling `query(predicate: Predicate<DebugElement>)` returns the first `DebugElement`
        that matches the [predicate](#query-predicate) at any depth in the subtree.
  tr
    td(style="vertical-align: top") <code>queryAll</code>
    td
      :marked
        Calling `queryAll(predicate: Predicate<DebugElement>)` returns all `DebugElements`
        that matches the [predicate](#query-predicate) at any depth in subtree.
  tr
    td(style="vertical-align: top") <code>injector</code>
    td
      :marked
        The host dependency injector. 
        For example, the root element's component instance injector.
  tr
    td(style="vertical-align: top") <code>componentInstance</code>
    td
      :marked
        The element's own component instance, if it has one.
   tr
    td(style="vertical-align: top") <code>context</code>
    td
      :marked
        An object that provides parent context for this element.
        Often an ancestor component instance that governs this element.

        When an element is repeated with in `*ngFor`, the context is an `NgForRow` whose `$implicit`
        property is the value of the row instance value. 
        For example, the `hero` in `*ngFor="let hero of heroes"`.
  tr
    td(style="vertical-align: top") <code>children</code>
    td
      :marked
        The immediate `DebugElement` children. Walk the tree by descending through `children`.

      .l-sub-section
        :marked
          `DebugElement` also has `childNodes`, a list of `DebugNode` objects. 
          `DebugElement` derives from `DebugNode` objects and there are often 
          more nodes than elements. Testers can usually ignore plain nodes.
  tr
    td(style="vertical-align: top") <code>parent</code>
    td
      :marked
        The `DebugElement` parent. Null if this is the root element.

  tr
    td(style="vertical-align: top") <code>name</code>
    td
      :marked
        The element tag name, if it is an element.

  tr
    td(style="vertical-align: top") <code>triggerEventHandler</code>
    td
      :marked
        Triggers the event by its name if there is a corresponding listener 
        in the element's `listeners` collection.
        The second parameter is the _event object_ expected by the handler.
        See [above](#trigger-event-handler).

        If the event lacks a listener or there's some other problem, 
        consider calling `nativeElement.dispatchEvent(eventObject)`

  tr
    td(style="vertical-align: top") <code>listeners</code>
    td
      :marked
        The callbacks attached to the component's `@Output` properties and/or the element's event properties.

  tr
    td(style="vertical-align: top") <code>providerTokens</code>
    td
      :marked
        This component's injector lookup tokens.
        Includes the component itself plus the tokens that the component lists in its `providers` metadata.
  tr
    td(style="vertical-align: top") <code>source</code>
    td
      :marked
        Where to find this element in the source component template.

  tr
    td(style="vertical-align: top") <code>references</code>
    td
      :marked
        Dictionary of objects associated with template local variables (e.g. `#foo`),
        keyed by the local variable name.

#query-predicate
:marked
  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":
+makeExample('testing/ts/app/bag/bag.spec.ts', 'custom-predicate')(format=".")

:marked
  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. 

+makeExample('testing/ts/app/hero/hero-list.component.spec.ts', 'by', 'app/hero/hero-list.component.spec.ts')(format=".")

#renderer-tests
:marked
  Many custom application directives inject the `Renderer` and call one of its `set...` methods.

  The test environment substitutes the `DebugDomRender` for the runtime `Renderer`.
  The `DebugDomRender` updates additional dictionary properties of the `DebugElement`
  when something calls a `set...` method.
 
  These dictionary properties are primarily of interest to authors of Angular DOM inspection tools
  but they may provide useful insights to testers as well.

table
  tr
    th Dictionary
    th Description
  tr
    td(style="vertical-align: top") <code>properties</code>
    td
      :marked
        Updated by `Renderer.setElementProperty`.
        Many Angular directives call it, including `NgModel`.
  tr
    td(style="vertical-align: top") <code>attributes</code>
    td
      :marked
        Updated by `Renderer.setElementAttribute`.
        Angular `[attribute]` bindings call it.
  tr
    td(style="vertical-align: top") <code>classes</code>
    td
      :marked
        Updated by `Renderer.setElementClass`.
        Angular `[class]` bindings call it.
  tr
    td(style="vertical-align: top") <code>styles</code>
    td
      :marked
        Updated by `Renderer.setElementStyle`.
        Angular `[style]` bindings call it.
:marked
  Here's an example of `Renderer` tests from the <live-example plnkr="bag-specs">live "Specs Bag" sample</live-example>.

+makeExample('testing/ts/app/bag/bag.spec.ts', 'debug-dom-renderer')(format=".")

a(href="#top").to-top Back to top

.l.hr

  #faq 
  .l-main-section
  :marked
    ## FAQ: Frequently Asked Questions
//
  :marked
    General
    * [When are end-to-end (e2e) tests a good choice?](#q-when-e2e)
    * [When to use the _TestBed_?](#q-why-testbed)
    * [When to write isolated unit tests without the _TestBed_?](#q-when-no-testbed)
    * [When can I skip _TestBed.compileComponents_?](#q-when-no-compile-components)
    * [Why must _TestBed.compileComponents_ be called last?](#q-why-compile-components-is-last)
    * [Why must _inject_ be called last?](#q-why-last-last)  
    * [What's the difference between _async_ and _fakeAsync_?](#q-async-vs-fake-async)
    * [What's the difference between _whenStable_ and _tick_?](#q-when-stable-vs-tick)
    * [How do I get something from the component's injector?](#q-component-injector)
    * [Why do feature modules make testing easier?](#q-why-feature-modules)  
    * [When should I prefer the _DynamicTestModule_?](#q-dynamic-test-module)
    * [How do I know if an injected service method was called?](#q-spy-on-service)
    * [When must I call _detectChanges_ and why?](#q-detect-changes)
    * [What's the difference between _triggerEventHandler_ and _dispatchEvent_?](#q-trigger-event-handler-vs-dispatch-event)
    * [How do I find an element by directive?](#q-by-directive)
    * [How do I extend Jasmine matchers?](#q-jasmine-matchers)
    * [Why would I add a test folder and how?](#q-test-folder)
    * [Why put specs next to the things they test?](#q-spec-file-location)
    * [When would I put specs in a test folder?](#q-specs-in-test-folder)
    * [How do I use the Jasmine HTML TestRunner in the browser?](#q-jasmine-browser-test-runner)

    Resources
    * [Where can I learn more about unit testing in JavaScript?](#q-js-unit-testing-resources)  
    * [Where can I learn more about testing with Jasmine?](#q-jasmine-resources)
    * [Where can I learn more about testing with karma?](#q-karma-resources)
    * [Where can I learn more about e2e testing with protractor?](#q-protractor-resources)

  a(href="#top").to-top Back to top

  .l-hr

#q-spec-file-location
:marked
  ### Why put specs next to the things they test?

  We recommend putting unit test spec files in the same folder 
  as the application source code files that they test because
  - Such tests are easy to find
  - You see at a glance if a part of our 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.

.l-hr

#q-specs-in-test-folder
:marked
  ### 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 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.