Creating Angular Tests and Mocking with Jasmine

When creating Angular Components, by default a spec.ts file is created.  By definition, a unit test should not call any files, databases, or services.  We use a mock, or a stand in for services.  

Mocking Services

To mock services, the jasmine.createSpyObj is used. Example:

loggingService = jasmine.createSpyObj('LoggingService', ['Info','Error']); 

Providing Mocked Services

To use the mocked service, it is provied.  Example:

    TestBed.configureTestingModule({
      providers: [
        ErrorHandlerService, 
        { provide: LoggingService, useValue: loggingService },
        { provide: HttpTestingController, useValue: httpTestingController }
      ],
    });

Full Example

We have an error handler service and we want to ensure that the message is logged by using the Error method.

import { TestBed } from '@angular/core/testing';
import { HttpErrorResponse } from '@angular/common/http';
import { ErrorHandlerService } from './error-handler.service';
import { LoggingService } from './logging.service';
import { HttpTestingController } from '@angular/common/http/testing';

describe('ErrorHandlerService', () => {
  let errorHandlerService: ErrorHandlerService;
  let loggingService: LoggingService;
  let httpTestingController: HttpTestingController;
  
  beforeEach(() => {
    loggingService = jasmine.createSpyObj('LoggingService', ['Info','Error']);
    TestBed.configureTestingModule({
      providers: [
        ErrorHandlerService, 
        { provide: LoggingService, useValue: loggingService },
        { provide: HttpTestingController, useValue: httpTestingController }
      ],
    });
    errorHandlerService = TestBed.inject(ErrorHandlerService);
    httpTestingController = TestBed.inject(HttpTestingController);
  });
  it('should be created', () => {
    expect(errorHandlerService).toBeTruthy();
  });
  it('should handle an error and log it', () => {
    spyOn(console, 'error');
    const error = new Error('Test error message');
    errorHandlerService.handleError(error);
    expect(console.error).toHaveBeenCalledWith(error);
    expect(loggingService.Error).toHaveBeenCalledWith(error.message, [error]);
  });
  
});