Angular's Dependency Injection Is Pretty Awesome For Testing

2/22/2019

As a professional software developer I have had the luxury of being able to work on successful professional projects in AngularJS, React, and now the modern Angular. To me it's so funny how similar the two frameworks are in terms of the problem they are trying to solve and even the way they do it.

A Tale of Non-DI

It wasn't until I worked on a front-end javascript project which did not use dependency injection that I really missed and appreciated it. At the time I was working on a React project, and the team values were to just stay true to regular, vanilla es6 JavaScript. We still had "services", ie helper classes that contained a few functions which did some business logic. However, we we doing a lot of instantiation. We were manually calling "new X()" in the code, or the class would call it right before module.exports, etc. Sure, it's true that coding it out in this way works, but then when you go to test it and you have MyComponent which uses MyService it's difficult to "reach into" MyService and tell it that instead of MyServiceDepencency it should use MyServiceDepencencyMock. You may even wish to not Mock MyServiceDependency, but actually it's a dependency of MyServiceDependency. To me it feels like you need to hackishly change your code to make it easier to mock things, and while it's totally possible I just feel like it's not as fun and fluid as the DI model in Angular.

Mock Anything, Anywhere

Angular's dependency injection is great because it allows you to say "For any of my services anywhere using JimHttp, just use JimHttpMock instead". All you need to do is put an object for it in the providers array when setting up your test, as in the example below:

let component: HelloComponent;
let userService: UserService;
 
beforeEach(() => {
  
  TestBed.configureTestingModule({
    declarations: [
      HelloComponent
    ],
    providers: [
      {provide: UserService, useClass: MockUser}
    ]
  });
  
  component = TestBed.createComponent(HelloComponent).componentInstance;

  // use HelloComponent with MockUser in place of UserService
});

By allowing a framework to create and instantiate dependencies we don't have to put all that in our code, and after injecting a service we just immediately start using it. This makes everything super decoupled and allows us to easily substitute in Mocks or spy on functions without even needing to know the "compositional inheritance chain" of how exactly how that asynchronous side effect class is related to each individual component or service.

Jasmine

Angular comes with jasmine out of the box as the framework used for unit testing. Personally, I like the jasmine syntax much better than mocha / chai, especially the syntax for spying on a function, etc. so for me it's a win that Angular uses jasmine, but I do know some people who don't like the syntax.

What Do You Think?

I was an AngularJS developer and then I switched over to React, but now I think I'm liking new Angular even better, simply because of the DI and how much easier it makes testing. haha. What do you guys think? Are you liking testing in Angular as well? Are you a React dev who is totally fine testing without dependency injection? Let me know! ?

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