Unit Testing in Javascript - Part 2: Mocking

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As applications get more complex, they come with more dependencies. Mocking helps us isolate the subject under test so that our unit tests can run any external dependencies. In this example, I’d like to show a simple mocking example using Jasmine and Karma in an Angular app.

Case study: Mocking a HTTP dependency

In this example the service I want to test is called TldService. It basically gets a raw HTML from a URL and extracts some data using regex.

It looks something like this

export class TldService {

  constructor(private fetchHtmlService: FetchHtmlService) { }

  getAllSupportedTlds(): string[] {
    const rawHtml = this.fetchHtmlService.getRawHtml();
    const supportedTlds = this.parseHtml(rawHtml);
    return supportedTlds;

  private parseHtml(html: string): string[] {
    const pattern = /some-regex-pattern/g;
    const regEx = new RegExp(pattern);
    let tldList = [];

    let match = regEx.exec(html);

    while (match !== null) {
      match = regEx.exec(html);

    return tldList;

This service depends on FetchHtmlService which does the getting HTML part. Nice thing about injecting this dependency is that we can replace it with a fake one while testing. This way we can test it without even having to implement the dependency.

import { TestBed } from '@angular/core/testing';
import { TldService } from './tld.service';
import { FetchHtmlService } from './fetch-html.service';

describe('TldService', () => {
  beforeEach(() => TestBed.configureTestingModule({


  it('should be created', () => {
    const service: TldService = TestBed.get(TldService);

  it('should parse html and extract TLD list', () => {
    const fetchHtmlService: FetchHtmlService = new FetchHtmlService();
    spyOn(fetchHtmlService, 'getRawHtml').and.returnValue('<a href="registrar-tld-list.html#academy">.academy</a>');
    const service: TldService = new TldService(fetchHtmlService);

In the 2nd test above we are creating a new FetchHtmlService and overriding the getRawHtml function behaviour by using Jasmine’s spyOn method.

That’s it! Now we don’t need a network connection to make actual calls while testing our code. We can develop and test our service in isolation independent from the dependency!