Testing Your Angular 2 Application: Part 2 of 2

Written by Seth Davenport

This post was last updated 03/17/2016. Please refer to our Angular 2 eBook for the most recent updates.

In part 1 of this series we defined a QuoteComponent that displays a random quote on a web page. We showed how to use Angular 2's testing utilities to mock out QuoteService and test the QuoteComponent's presentation logic in isolation.

In this post, I'll do the opposite: I'll isolate the QuoteService and show how to unit test the code at your application's service level. By the end of this post, you should have all the tools you need to test a non-trivial Angular 2 application.

For reference, here's the component again:

import { IQuote, QuoteService } from '../services/quote.service';  
import { Component } from 'angular2/core';

@Component({
  selector: 'my-quote',
  providers: [ QuoteService ],
  template: `
    <h3>Random Quote</h3>
    <blockquote>{{ quote.text }}</blockquote>
    <p>- {{ quote.attribution }}</p>
    <button (click)=getQuote()>Get a new quote</button>
  `
})
export class QuoteComponent {  
  quote: IQuote;

  constructor (private quoteService: QuoteService){
    this.getQuote();
  }

  getQuote() {
    this.quote = this.quoteService.getQuote();
  }
}

Testing QuoteService

Consider the following implementation of QuoteService:

import { Injectable, Inject } from 'angular2/core';  
import { quotes } from './quote.data';  
import { RandomNumberService } from './random-number.service';

export type IQuote = {  
  text: string,
  attribution: string
};

@Injectable()
export class QuoteService {  
  constructor(
    private randomNumberService: RandomNumberService,
    @Inject('QUOTE_DATA') private allQuotes: IQuote[]) {}

  getQuote() {
    const index = this.randomNumberService.pick(
      0,
      this.allQuotes.length);
    return this.allQuotes[index];
  }
};

This service uses a random number generator to pick a quote at random from a hard-coded list in quote.data.ts:

export const quotes = [  
  {
  text: 'Talk is cheap. Show me the code.',
  attribution: 'Linus Torvalds'
  },
  // ...
];

How would we get this under test?

Angular 2 Testing Tools

First, we can access the tools provided by the angular2/testing package:

import {  
  describe,
  it,
  inject,
  beforeEachProviders,
  expect
} from 'angular2/testing';

In effect, Angular 2 gives us a modified version of the excellent jasmine
testing framework.

You should remember these tools from part 1 of this series, but briefly:

  • describe: the describe function creates a set of related tests.
  • it: the it function defines a particular unit test.
  • inject allows you to invoke Angular 2's dependency injector to instantiate the services being used.
  • beforeEachProviders allows you to override the dependency injector to supply stubs or mocks for the parts of the system you're not explicitly testing right now.
  • expect allows us to check that conditions have been met, throwing errors if they have not been.

Isolate the Unit under Test

Next, we need to isolate the unit under test (in this case QuoteService).
We'll do this by replacing RandomNumberService and QUOTE_DATA with stubs or mocks.

Once again, we use our old friends provide and beforeEachProviders to do this:

class StubRandomNumberService {  
  pick: (min: number, max: number) => number;
}

beforeEachProviders(() => [  
  QuoteService,
  provide(RandomNumberService, {useClass: StubRandomNumberService}),
  provide('QUOTE_DATA', { useValue: [ {
    text: 'Testing is a good thing',
    attribution: 'Me'
  }]})
]);

Since QUOTE_DATA is just a list of hard-coded records, we can replace it with some data that's tailored to the behaviour we want to test. This is done using provide's useValue option: whenever Angular 2's dependency injector is asked for QUOTE_DATA, it will use our test data instead.

The RandomNumberService, on the other hand, can be stubbed out with an empty class that we'll manipulate later. Think of this as partial typings for the bits of RandomNumberService that we care about in this test.

Unlike a mock, a stub does not result in in complex mocking logic being shared across different test suites; we'll leave it up to each specific test to supply the relevant implementation of this simple service.

Whether to use a stub or a mock is largely a matter of taste; I tend to prefer stubs where possible because they decouple tests from each other and are in many cases easier to reason about.

Set up an Actual Test

Now that we've set up our test environment, we can write an actual test.

it('should use RandomNumberService to choose a quote',  
  inject(
    [QuoteService, RandomNumberService],
    (quoteService: QuoteService,
    stubRandomNumberService: RandomNumberService) => {

    // Specify the stub behaviour for this test.
    stubRandomNumberService.pick = jasmine.createSpy(
      'pick').and.returnValue(0);

    // Exercise the function under test.
    quoteService.getQuote();

    // Verify expectations.
    expect(stubRandomNumberService.pick).toHaveBeenCalledWith(0, 1);
  }));

This is a good example of stub-based unit testing: I'm beginning with an
empty stub for RandomNumberService and swapping in the minimum implementation for a meaningful test. This way I don't have to maintain a complex mock object in addition to the real one.

I verify behaviour using Jasmine's createSpy, which creates an instrumented, fake implementation of the pick method.

I personally find that this is a very clean, readable, and flexible testing style.

Putting it all Together

import { provide } from 'angular2/core';  
import {  
  describe,
  expect,
  it,
  inject,
  beforeEachProviders
} from 'angular2/testing';

import { QuoteService } from './quote.service';  
import { RandomNumberService } from './random-number.service';

class StubRandomNumberService {  
  pick: (min: number, max: number) => number;
}

describe('QuoteService', () => {

  beforeEachProviders(() => {
    return [
      QuoteService,
      provide(RandomNumberService, {useClass: StubRandomNumberService}),
      provide('QUOTE_DATA', { useValue: [ {
        text: 'Testing is a good thing',
        attribution: 'Me'
      }]})
    ];
  });

  it('should use RandomNumberService to choose a quote',
    inject(
      [QuoteService, RandomNumberService],
      (quoteService: QuoteService,
      stubRandomNumberService: RandomNumberService) => {

      stubRandomNumberService.pick = jasmine.createSpy(
        'pick').and.returnValue(0);

      quoteService.getQuote();

      expect(stubRandomNumberService.pick).toHaveBeenCalledWith(0, 1);
    }));
});

Asynchronous Service Calls

The example above is nice and simple. However in the real world, we're more likely to get the quote data from a remote API than from a hard-coded list.

Let's walk through the same exercise, but this time with an implementation that gets quotes from http://quotesondesign.com/.

First we'll beef up the QuoteComponent to handle async data using Observables and the built-in asyncPipe from Angular 2:

import { IQuote, QuoteService } from '../services/quote.service';  
import { Component } from 'angular2/core';  
import { Observable } from 'rxjs';

@Component({
  selector: 'my-quote',
  providers: [ QuoteService ],
  template: `
    <h3>Random Quote</h3>
    <blockquote [innerHtml]="text$ | async"></blockquote>
    <p>- {{ attribution$ | async }}</p>
    <button (click)=getQuote()>Get a new quote</button>
  `
})
export class QuoteComponent {  
  text$: Observable<String>;
  attribution$: Observable<String>;

  constructor (private quoteService: QuoteService){
    this.getQuote();
  }

  getQuote() {
    const quote$ = this.quoteService.getQuote();
    this.text$ = quote$.map(quote => quote.text);
    this.attribution$ = quote$.map(quote => quote.attribution);
  }
}

Next, we'll provide an HTTP-capable implementation of quote service:

import { Injectable } from 'angular2/core';  
import { Http, URLSearchParams } from 'angular2/http';  
import { Observable } from 'rxjs';  
import 'rxjs/add/operator/map';

export type IQuote = {  
  text: string,
  attribution: string
};

export type IAPIRecord = {  
  content: string,
  title: string
};

@Injectable()
export class QuoteService {  
  private static URL = 'http://quotesondesign.com/wp-json/posts';

  constructor(private http: Http) {}

  getQuote(): Observable<IQuote> {
    const search = new URLSearchParams();
    search.set('filter[orderby]', 'rand');

    return this.http.get(QuoteService.URL, { search })
      .map<IAPIRecord[]>(response => response.json())
      .map<IAPIRecord>(records => records[0])
      .map<IQuote>(record => ({
          text: record.content,
          attribution: record.title
        }));
  }
};

This implementation is straightforward: it uses Angular 2's http service to hit QuotesOnDesign's JSON endpoint, and then converts the response data to the IQuote interface used by our QuoteComponent.

However, this example has two aspects that complicate testing:

  1. It returns data asynchronously using an Observable.
  2. It relies on Angular 2's HTTP infrastructure, which is difficult to isolate.

Fortunately, Angular 2 gives us the tools to handle these cases as well.

Handling Asynchronicity

We'll import a new helper from angular2/testing called fakeAsync. This is a simple wrapper function that places your test in the "async zone": this causes asynchronous code to run in a synchronous way via the magic of zone.js.

Mocking out the HTTP Backend

The new implementation of QuoteService relies on the http service provided by Angular 2. However, because this is a unit test, we don't want to actually make HTTP calls - we need to be able to control the environment in which our tests run.

We could use a stub implementation of HTTP as we did above; and in some cases that's appropriate. However HTTP is a fairly complex interface. In this case, it's easier to use some mocks that Angular 2 has already made available to us:

import { MockBackend, MockConnection } from 'angular2/http/testing';  

Supplying these to Angular 2's dependency injector allows us to intercept outgoing HTTP calls and perform expectations them:

import { provide } from 'angular2/core';  
import {  
  ResponseOptions,
  Response,
  Http,
  BaseRequestOptions,
  RequestMethod
} from 'angular2/http';

import {  
  describe,
  expect,
  it,
  inject,
  fakeAsync,
  beforeEachProviders
} from 'angular2/testing';

import { MockBackend, MockConnection } from 'angular2/http/testing';

const mockHttpProvider = {  
  deps: [ MockBackend, BaseRequestOptions ],
  useFactory: (backend: MockBackend, defaultOptions: BaseRequestOptions) => {
    return new Http(backend, defaultOptions);
  }
}

describe('QuoteService', () => {  
  beforeEachProviders(() => {
    return [
      MockBackend,
      BaseRequestOptions,
      provide(Http, mockHttpProvider)
    ];
  });

  it('should use an HTTP call to obtain a quote',
    inject(
      [QuoteService, MockBackend],
      fakeAsync((service: QuoteService, backend: MockBackend) => {
        backend.connections.subscribe((connection: MockConnection) => {

          expect(connection.request.method).toBe(RequestMethod.Get);
          expect(connection.request.url).toBe(
            'http://quotesondesign.com/wp-json/posts?filter[orderby]=rand');
        });

        service.getQuote();
      })));
});

It also allows us to supply controlled response data:

it('should parse the server response correctly', inject(  
  [QuoteService, MockBackend],
  fakeAsync((service: QuoteService, backend: MockBackend) => {
    backend.connections.subscribe((connection: MockConnection) => {

      let mockResponseBody: IAPIRecord[] = [{
        title: 'Me',
        content: 'Testing is a good thing'
      }];

      let response = new ResponseOptions({body: JSON.stringify(mockResponseBody)});
      connection.mockRespond(new Response(response));
    });

    const parsedQuote$ = service.getQuote()
      .subscribe(quote => {
        expect(quote.text).toEqual('Testing is a good thing');
        expect(quote.attribution).toEqual('Me');
      });
  })));

Wrap-up

Angular 2 provides a great set of tools for testing everything from your UI
components down to your HTTP services. Between parts one and two of this series, you should be equipped to handle most scenarios you may encounter when unit testing your Angular 2 apps.

The full source code for these examples can be found on github.

If you want to learn more, check out some of the following resources:

Happy Testing!

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