Unit Testing Angular Services with HttpTestingController

This tutorial is part of a tutorial series titled ¨Ionic 3, Angular 4.3+ and RxJS Observables: Building an HTTP Service to Communicate with A REST API¨ that contains the following tutorials:

• Introduction and Building the API Back-End
• Building an HTTP Service to Communicate with A REST API
• Unit Testing Angular Services with HttpTestingController (this one)

Unit Testing Angular Services with HttpTestingController

The HttpClientTestingModule allows you to easily mock HTTP requests by providing you with the HttpTestingController service. In this section we’ll see how you can create tests for the previously created service using the HttpTestingController service to mock requests instead of making real API requests to our API back-end when testing.

Before you can use HttpClientTestingModule and its HttpTestingController service you first need to import and provide them in your TestBed alongside the service we are testing.

So go ahead and create src/providers/rest/rest.spec.ts, which will hold code for testing the Rest service, then add the following code.

import{TestBed,getTestBed}from'@angular/core/testing';import{HttpClientTestingModule,HttpTestingController}from'@angular/common/http/testing';import{RestProvider}from'./rest';describe('RestProvider',()=>{letinjector:TestBed;letmyProvider:RestProvider;lethttpMock:HttpTestingController;beforeEach(()=>{TestBed.configureTestingModule({imports:[HttpClientTestingModule],providers:[RestProvider]});testBed=getTestBed();myProvider=testBed.get(RestProvider);httpMock=testBed.get(HttpTestingController);});});

We are storing the provider and an instance of the HttpTestingController (httpMock) in variables so we can have access to them in each test that we run by using the beforEach(()=>{}) API.

Now let’s test the getProducts() method as an example:

describe('getProducts',()=>{it('should return an Observable<Product[]>',()=>{constsomeProducts=[{id:1,name:'Product001',cost:10,quantity:100},{id:2,name:'Product002',cost:100,quantity:200},{id:3,name:'Product003',cost:200,quantity:300},];myProvider.getProducts().subscribe((products)=>{expect(products.length).toBe(3);expect(products).toEqual(someProducts);});constrequest=httpMock.expectOne('${myProvider.baseUrl}/products');expect(req.request.method).toBe("GET");request.flush(someProducts);httpMock.verify();});});

Inside it(‘should return an Observable’)[]> we first define a varibale which holds some testing data then we call the provider’s method (in this case .getProducts()) as we normally do. In the subscribe handler we tell Angular that we are expecting the retrun values which is products to equal to our someProducts array and that the length should equal to 3 (that’s because we we are not using the real HttpClient but a mock based on HttpTestingController).

Next we tell the httpMock what’s the HTTP method we expect to be sent and the endpoint’sURL.

Finally we fire the request with the data we use as a mock then we verify that there are no outstanding http requests.

You can follow the same steps for testing other HTTP methods i.e POST, PUT and DELETE or more accurately their corresponding operations in the service provider.

Conclusion

So we have implemented all the required methods to create a CRUD app with Ionic 3 and Angular 4.3+ HttpClient. ALl you need now is to link these methods to the HTML interfaces using list, form controls and buttons.