Simple Observable testing in Angular2

data.service.ts

import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';
import { Observable } from 'rxjs/Observable';

import { Data } from '../models';

@Injectable()

export class DataService {
  dataUrl: string = 'data.json';
  constructor ( http: Http ) { }

  get(): Observable<Data[]> {
    return this.http.get(this.dataUrl)
      .map(this.extractData)
      .catch(this.handleError)
  }

  // ...

  private extractData(res: Response) {
    let body = res.json();
    return body || { };
  }

  private handleError(error: any) {
    let errMsg = (error.message) ? error.message :
    error.status ? `${error.status} - ${error.statusText}` : this.errorMsg;
    console.error(error, errMsg); // log to console instead
    return Observable.throw(errMsg);
  }
}

data.stub.ts

import { Observable } from 'rxjs/Observable';

import { Data, testData } from '../models';

export class DataStub {
  public get(url: string): Observable<Data[]> {
    return Observable.of(testData);
  }

  // ...
}

my.component.ts

import { Component, OnInit } from '@angular/core';
import { Observable } from 'rxjs/Observable';

import { Data } from '../models';
import { DataService } from '../services';

@Component {
  selector: 'my-comp',
  templateUrl: '/myComp/myComp.html'
}

export class MyComponent implements OnInit {
  data: Data[];
    
  constructor ( dataSvc : DataService ) { }

  ngOnInit() {
    this.dataSvc.get().subscribe(
      data => this.data = data
    );
  }
}

my.spec.ts

import {
  inject,
  tick,
  TestBed,
  getTestBed,
  async,
  fakeAsync,
  ComponentFixture
} from '@angular/core/testing';
import { Observable } from 'rxjs/Rx';

import { DataStub } from '../mocks';
import { testData } from '../models';
import { MyComponent } from './my.component';
import { DataService } from '../services';
 
let comp: MyComponent;
let fixture: ComponentFixture<MyComponent>;
let dataStub: Datastub;

describe('My Component', () => {

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ MyComponent ]
    }).overrideComponent(MyComponent, {
      set: {
        providers: [
          { provide: DataService, useClass: DataStub },
        ]
      }
    }).compileComponents()
    .then(() => {
      fixture = TestBed.createComponent(MyComponent);
      comp = fixture.componentInstance;
      dataStub = fixture.debugElement.injector.get(DataService);
    });
  }));
  it('should resolve test data', fakeAsync(() => {
    const spy = spyOn(dataStub, 'get').and.returnValue(
      Observable.of(testData)
    );
    comp.ngOnInit();
    fixture.detectChanges();
    expect(comp.data).toEqual(testData);
    expect(spy.calls.any()).toEqual(true);
  }));
});

Awesome! thanks for sharing.

elias-garcia: Returning another mock value is very usefull when you need to test with different return values and not the same value all the time.

How I test directly the data from arrow function? You are testing the this.data, but and If I want test directly the data? This is a dillema to me, I can't do.