Today I Learned

TIL, 2020-04-11

Introduction to Unit Testing in Angular


  • TestBed is the main utility available for Angular-specific testing.
  • TestBed.configureTestingModule is used in the beforeEach block and give it an object with similar values as a regular NgModule for declaration, provider, and import. Then, you can chain a call to compileComponents to tell Angular to compile the declared components.
  • Creating a component fixture: TestBed.createComponent. debugElement gives you access to the internals of the component fixture.
  • Wrapping the callback function of a test or the first argument of beforeEach with async allows Angular to perform asynchronous compilation and wait until the current inside of the async block to be ready before continuing.

Angular Testing: async and fakeAsync


  • async tells Angular to run the code in a dedicated test zone that intercepts promises.
  • whenStable: this allows us to wait until all promises have been resolved to run our expectations.
import { Component } from '@angular/core';

  selector: 'app-root',
  template: `<h1<</h1>

  <button (click)="setTitle()" class="set-title">
    set title
export class AppComponent {
  title: string;

  setTitle() {
    new Promise(resolve => {
      resolve('One crazy app!');
    }).then((val: string) => {
      this.title = val;
  • When the button is clicked, the title property is set using a promise. And to test the functionality using async and whenStable:
describe('AppComponent', () => {
  it('should display title', async(() => {
      .triggerEventHandler('click', null);

    fixture.whenStable().then(() => {
      const value = debugElement.query(By.css('h1')).nativeElement.innerText;
      expect(value).toEqual('One crazy app!');
  • fakeAsync: this makes things faster in the tests.
  • tick is inside a fakeAsync block to simulate the passage of time.
  • flush: Simulates the passage of time until the macrotask queue is empty. Macrotask includes things like setTimeouts , setIntervals, and requestAnimationFrame.
  it('should increment in template after 5 seconds', fakeAsync(() => {
        .triggerEventHandler('click', null);

      let value = debugElement.query(By.css('h1')).nativeElement.innerText;
      expect(value).toEqual('0'); // value should still be 0 after 2 seconds


      const value = debugElement.query(By.css('h1')).nativeElement.innerText;
      expect(value).toEqual('1'); // 3 seconds later, our value should now be 1

Server Side Rendering with Angular Universal


  • Single-page Apps (SPA) are aptly named - there is literally only on single HTML document that is served initially to a client. Any new views that are required in the app are generated solely on the client via JS.
  • SSR: You lose the ability for web crawlers to traverse your app, and there is slower performance while the app is loading.
  • Angular Universal let’s you run your Angular app on the server. This enables you to serve static HTML to the client. With Angular Universal, the server will pre-render pages and show your users something, while the client-side app loads in the background. Then, once everything is ready client-side, it will seamlessly switch from showing the server-rendered pages to the client-side app.
  • SSR with Angular Universal requires changes in both the client application and the server stack to work. For this article, we’ll assume this is a brand new Angular application.
  • Just about any server technology can run a Universal app, but it has to be able to call a special function, renderModuleFactory(), provided by Angular Universal, which is itself a Node package, so Node makes most sense.

Adding Universal to the App:

  • $ ng add @nguniversal/express-engine --clientProject

  • angular.json: has changes to dist/browser, and a new projects..architect is added, called server. This lets the Angular CLI know about our server/Universal version of the Angular application.
  • package.json: Some new scrips: compile:server, server:ssr, build:ssr, build:client-and-server-bundles.
  • server.ts: The NodeJS Express server.
  • main.ts: Modified so that the browser version of the app won’t start bootstrapping until the Universal-rendered pages have been fully loaded.
  • main.server.ts: AppServerModule, which is the entry point of the Universal version of the application.
  • tsconfig.server.json: Where to find the entry module.
  • app.module.ts: Modified to execute static method withServerTransition on the imported BrowserModule. This tells the browser version of the application that the client will be transitioning in from the server version at some point.
  • app.server.module.ts: The root module for the server version only.
  • Starting: npm run build:ssr && npm run serve:ssr

Things to Note

  • Since a Universal app runs on the server and not in a browser:
    • Check your use of window, document, or location. Actually, you should be using Angular abstractions Document or Location anyway.
    • If you really need to, then import functions isPlatformBrowser and isPlatformServer from @angular/common, injecting the PLATFORM_ID token into the component, and running the imported functions to use whether you’re on the server or the browser.
    • Don’t use nativeElement to manipulate attributes on the element, use Renderer2 and use the method there.,
    • Browser event handling won’t work, your app won’t respond to click events - however, any link generated from a routerLink will work.
    • Avoid setTimeout.
    • Make all URLs for server requests absolute. Requests for data from relative URLs will fail when running from the server.

This project is maintained by daryllxd